3.8.1 Boilers

Boiler Name
Applicability All boilers
Definition A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.
Units None
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.
Baseline Rules Boilers are only designated in the baseline model if the Baseline System is of type 1 (PTAC), type 5 (Packages VAV with Reheat) or type 7 (VAV with Reheat).
Boiler Fuel
Applicability All boilers
Definition The fuel source for the central heating equipment. The choices are:
  • Gas
  • Oil
  • Electricity
Units List (see above)
Input Restrictions As designed
Baseline Rules Same fuel as the proposed design
Boiler Type
Applicability All boilers
Definition The boiler type. Choices include:
  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater
Units List (see above)
Input Restrictions As designed
Baseline Rules The boiler type will be a hot water boiler for baseline systems 1, 5 and 7, according to the baseline system descriptions from Table G3.1.1B. All other system types do not have a boiler.
Boiler Draft Type
Applicability All boilers
Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical
Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.


Mechanical draft boilers enhance the air flow in one of three ways: 1) Induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) Forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, and 3) Balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units List (see above)
Input Restrictions As designed. Default is natural draft.
Baseline Rules The baseline boiler is always assumed to be a natural draft boiler. (G3.1.3.2)
Number of Identical Boiler Units
Applicability All boilers
Definition The number of identical units for staging
Units Numeric: integer
Input Restrictions As designed. Default is 1.
Baseline Rules The number of boilers in the baseline case is set as follows (G3.1.3.2). The baseline building has one boiler when it serves an area less than or equal to 15,000 ft². For larger service areas, the baseline building shall have two equally sized boilers.
Boiler Heat Loss
Applicability All boilers
Definition The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.
Units Percent (%)
Input Restrictions Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.
Baseline Rules Prescribed at 2% for electric boilers and heat-exchangers. Prescribed at 0% for fuel-fired boilers, since this loss is already incorporated into the overall thermal efficiency, combustion efficiency or AFUE of the boiler.
Boiler Design Capacity
Applicability All boilers
Definition The heating capacity at design conditions
Units Btu/h
Input Restrictions Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.
Baseline Rules

The boiler is sized to be 25% larger than the peak loads of the baseline building. Baseline boilers shall be sized using weather files containing 99.6% heating design temperatures and 1% dry-bulb and 1% wet-bulb cooling design temperatures.


The unmet load hours of the proposed case shall also not exceed the unmet load hours of the baseline design by more than 50 hours. If they do, then the capacity of the baseline boiler shall be decreased incrementally until this difference is less than 50 hours.

Boiler Efficiency Type
Applicability All boilers
Definition The full load efficiency of a boiler is expressed as one of the following:
  • Annual Fuel Utilization Efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal Efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion Efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.
Units List (see above)
Input Restrictions

Annual Fuel Utilization Efficiency (AFUE), for all gas and oil-fired boilers with less than 300,000 Btu/h capacity.


Thermal Efficiency (Et), for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.


Combustion Efficiency (Ec), for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Baseline Rules Same as proposed design
Boiler Efficiency
Applicability All boilers
Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either Thermal Efficiency (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:


(6.8.1-1)

$1) 75\% \leq AFUE \textless\ 80\% $

$E_t= 0.1 \times AFUE + 72.5\% $

$2) 80\% \leq AFUE \leq 100\% $

$E_t=0.875 \times AFUE + 10.5\% $

All electric boilers will have an efficiency of 100%.

Units Ratio
Input Restrictions As designed
Baseline Rules Boilers for the baseline design are assumed to have the minimum efficiency as listed in Table 6.8.1F from ASHRAE Standard 90.1-2007 or Table 6.2.1F from ASHRAE Standard 90.1-2001.
Boiler Part-Load Performance Curve 
Applicability All boilers
Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:


(6.8.1-2)

$$ Fuel_{partload} = Fuel_{design} \times F\!H\!eat\!P\!L\!C\, \left ( Q_{partload}, Q_{rated} \right )$$

$$ F\!H\!eat\!P\!L\!C = \Bigg ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left ( \frac{Q_{partload}}{Q_{rated}}\right )^2 \Bigg ) $$

where

FHeatPLC The Fuel Heating Part Load Efficiency Curve
Fuelpartload The fuel consumption at part load conditions (Btu/h)
Fueldesign The fuel consumption at design conditions (Btu/h)
Qpartload The boiler capacity at part load conditions (Btu/h)
Qrated The boiler capacity at design conditions (Btu/h)
a Constant, 0.082597
b Constant, 0.996764
c Constant, -0.079361
Units Ratio
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.
Baseline Rules The baseline building uses the default
Boiler Minimum Unloading Ratio
Applicability All boilers
Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Boiler Type Default Unloading Ratio
Electric Steam 1%
Electric Hot Water 1%
Fuel-Fired Steam 25%
Fuel-Fired Hot Water 25%
Units Percent (%)
Input Restrictions As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Use defaults.
Hot Water Supply Temperature
Applicability All boilers
Definition The temperature of the water produced by the boiler and supplied to the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 180°F for baseline boiler (G3.1.3.3).
Hot Water Return Temperature
Applicability All boilers
Definition The temperature of the water returning to the boiler from the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 130°F for baseline boiler design.
Hot Water Supply Temperature Reset
Applicability All boilers
Definition Variation of the hot water supply temperature with outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The hot water supply temperature should vary according to the following:
  • 180°F when outside air is < 20°F
  • ramp linearly between 180°F & 150°F when outdoor air is between 20°F and 50°F
  • 150°F when outdoor air is > 50°F
90.1-2007
Boiler Name
Applicability All boilers
Definition A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.
Units None
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.
Baseline Rules Boilers are only designated in the baseline model if the Baseline System is of type 1 (PTAC), type 5 (Packages VAV with Reheat) or type 7 (VAV with Reheat).
Boiler Fuel
Applicability All boilers
Definition The fuel source for the central heating equipment. The choices are:
  • Gas
  • Oil
  • Electricity
Units List (see above)
Input Restrictions As designed
Baseline Rules Same fuel as the proposed design
Boiler Type
Applicability All boilers
Definition The boiler type. Choices include:
  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater
Units List (see above)
Input Restrictions As designed
Baseline Rules The boiler type will be a hot water boiler for baseline systems 1, 5 and 7, according to the baseline system descriptions from Table G3.1.1B. All other system types do not have a boiler.
Boiler Draft Type
Applicability All boilers
Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical
Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.


Mechanical draft boilers enhance the air flow in one of three ways: 1) Induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) Forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, and 3) Balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units List (see above)
Input Restrictions As designed. Default is natural draft.
Baseline Rules The baseline boiler is always assumed to be a natural draft boiler. (G3.1.3.2)
Number of Identical Boiler Units
Applicability All boilers
Definition The number of identical units for staging
Units Numeric: integer
Input Restrictions As designed. Default is 1.
Baseline Rules The number of boilers in the baseline case is set as follows (G3.1.3.2). The baseline building has one boiler when it serves an area less than or equal to 15,000 ft². For larger service areas, the baseline building shall have two equally sized boilers.
Boiler Heat Loss
Applicability All boilers
Definition The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.
Units Percent (%)
Input Restrictions Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.
Baseline Rules Prescribed at 2% for electric boilers and heat-exchangers. Prescribed at 0% for fuel-fired boilers, since this loss is already incorporated into the overall thermal efficiency, combustion efficiency or AFUE of the boiler.
Boiler Design Capacity
Applicability All boilers
Definition The heating capacity at design conditions
Units Btu/h
Input Restrictions Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.
Baseline Rules

The boiler is sized to be 25% larger than the peak loads of the baseline building. Baseline boilers shall be sized using weather files containing 99.6% heating design temperatures and 1% dry-bulb and 1% wet-bulb cooling design temperatures.


The unmet load hours of the proposed case shall also not exceed the unmet load hours of the baseline design by more than 50 hours. If they do, then the capacity of the baseline boiler shall be decreased incrementally until this difference is less than 50 hours.

Boiler Efficiency Type
Applicability All boilers
Definition The full load efficiency of a boiler is expressed as one of the following:
  • Annual Fuel Utilization Efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal Efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion Efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.
Units List (see above)
Input Restrictions

Annual Fuel Utilization Efficiency (AFUE), for all gas and oil-fired boilers with less than 300,000 Btu/h capacity.


Thermal Efficiency (Et), for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.


Combustion Efficiency (Ec), for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Baseline Rules Same as proposed design
Boiler Efficiency
Applicability All boilers
Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either Thermal Efficiency (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:

 

(6.8.1-1)

 

$1) 75\% \leq AFUE \textless\ 80\% $

$E_t= 0.1 \times AFUE + 72.5\% $

$2) 80\% \leq AFUE \leq 100\% $

$E_t=0.875 \times AFUE + 10.5\% $

All electric boilers will have an efficiency of 100%.

Units Ratio
Input Restrictions As designed
Baseline Rules Boilers for the baseline design are assumed to have the minimum efficiency as listed in Table 6.8.1F from ASHRAE Standard 90.1-2007.
Boiler Part-Load Performance Curve 
Applicability All boilers
Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:

 

(6.8.1-2)

$$ Fuel_{partload} = Fuel_{design} \times F\!H\!eat\!P\!L\!C\, \left ( Q_{partload}, Q_{rated} \right )$$

$$ F\!H\!eat\!P\!L\!C = \Bigg ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left ( \frac{Q_{partload}}{Q_{rated}}\right )^2 \Bigg ) $$

where

FHeatPLC The Fuel Heating Part Load Efficiency Curve
Fuelpartload The fuel consumption at part load conditions (Btu/h)
Fueldesign The fuel consumption at design conditions (Btu/h)
Qpartload The boiler capacity at part load conditions (Btu/h)
Qrated The boiler capacity at design conditions (Btu/h)
a Constant, 0.082597
b Constant, 0.996764
c Constant, -0.079361
Units Ratio
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.
Baseline Rules The baseline building uses the default
Boiler Minimum Unloading Ratio
Applicability All boilers
Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Boiler Type Default Unloading Ratio
Electric Steam 1%
Electric Hot Water 1%
Fuel-Fired Steam 25%
Fuel-Fired Hot Water 25%
Units Percent (%)
Input Restrictions As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Use defaults.
Hot Water Supply Temperature
Applicability All boilers
Definition The temperature of the water produced by the boiler and supplied to the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 180°F for baseline boiler (G3.1.3.3).
Hot Water Return Temperature
Applicability All boilers
Definition The temperature of the water returning to the boiler from the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 130°F for baseline boiler design.
Hot Water Supply Temperature Reset
Applicability All boilers
Definition Variation of the hot water supply temperature with outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The hot water supply temperature should vary according to the following:
  • 180°F when outside air is < 20°F
  • ramp linearly between 180°F & 150°F when outdoor air is between 20°F and 50°F
  • 150°F when outdoor air is > 50°F
90.1-2010
Boiler Name
Applicability All boilers
Definition A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.
Units None
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.
Baseline Rules Boilers are only designated in the baseline model if the Baseline System is of type 1 (PTAC), type 5 (Packages VAV with Reheat) or type 7 (VAV with Reheat).
Boiler Fuel
Applicability All boilers
Definition The fuel source for the central heating equipment. The choices are:
  • Gas
  • Oil
  • Electricity
Units List (see above)
Input Restrictions As designed
Baseline Rules Same fuel as the proposed design
Boiler Type
Applicability All boilers
Definition The boiler type. Choices include:
  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater
Units List (see above)
Input Restrictions As designed
Baseline Rules The boiler type will be a hot water boiler for baseline systems 1, 5 and 7, according to the baseline system descriptions from Table G3.1.1B. All other system types do not have a boiler.
Boiler Draft Type
Applicability All boilers
Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical
Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.


Mechanical draft boilers enhance the air flow in one of three ways: 1) Induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) Forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, and 3) Balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units List (see above)
Input Restrictions As designed. Default is natural draft.
Baseline Rules The baseline boiler is always assumed to be a natural draft boiler. (G3.1.3.2)
Number of Identical Boiler Units
Applicability All boilers
Definition The number of identical units for staging
Units Numeric: integer
Input Restrictions As designed. Default is 1.
Baseline Rules The number of boilers in the baseline case is set as follows (G3.1.3.2). The baseline building has one boiler when it serves an area less than or equal to 15,000 ft². For larger service areas, the baseline building shall have two equally sized boilers.
Boiler Heat Loss
Applicability All boilers
Definition The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.
Units Percent (%)
Input Restrictions Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.
Baseline Rules Prescribed at 2% for electric boilers and heat-exchangers. Prescribed at 0% for fuel-fired boilers, since this loss is already incorporated into the overall thermal efficiency, combustion efficiency or AFUE of the boiler.
Boiler Design Capacity
Applicability All boilers
Definition The heating capacity at design conditions
Units Btu/h
Input Restrictions Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.
Baseline Rules

The boiler is sized to be 25% larger than the peak loads of the baseline building. Baseline boilers shall be sized using weather files containing 99.6% heating design temperatures and 1% dry-bulb and 1% wet-bulb cooling design temperatures.


The unmet load hours of the proposed case shall also not exceed the unmet load hours of the baseline design by more than 50 hours. If they do, then the capacity of the baseline boiler shall be decreased incrementally until this difference is less than 50 hours.

Boiler Efficiency Type
Applicability All boilers
Definition The full load efficiency of a boiler is expressed as one of the following:
  • Annual Fuel Utilization Efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal Efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion Efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.
Units List (see above)
Input Restrictions

Annual Fuel Utilization Efficiency (AFUE), for all gas and oil-fired boilers with less than 300,000 Btu/h capacity.


Thermal Efficiency (Et), for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.


Combustion Efficiency (Ec), for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Baseline Rules Same as proposed design
Boiler Efficiency
Applicability All boilers
Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either Thermal Efficiency (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:

 

(6.8.1-1)

 

$1) 75\% \leq AFUE \textless\ 80\% $

$E_t= 0.1 \times AFUE + 72.5\% $

$2) 80\% \leq AFUE \leq 100\% $

$E_t=0.875 \times AFUE + 10.5\% $

All electric boilers will have an efficiency of 100%.

Units Ratio
Input Restrictions As designed
Baseline Rules Boilers for the baseline design are assumed to have the minimum efficiency as listed in Table 6.8.1F from ASHRAE Standard 90.1-2007.
Boiler Part-Load Performance Curve 
Applicability All boilers
Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:

 

(6.8.1-2)

$$ Fuel_{partload} = Fuel_{design} \times F\!H\!eat\!P\!L\!C\, \left ( Q_{partload}, Q_{rated} \right )$$

$$ F\!H\!eat\!P\!L\!C = \Bigg ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left ( \frac{Q_{partload}}{Q_{rated}}\right )^2 \Bigg ) $$

where

FHeatPLC The Fuel Heating Part Load Efficiency Curve
Fuelpartload The fuel consumption at part load conditions (Btu/h)
Fueldesign The fuel consumption at design conditions (Btu/h)
Qpartload The boiler capacity at part load conditions (Btu/h)
Qrated The boiler capacity at design conditions (Btu/h)
a Constant, 0.082597
b Constant, 0.996764
c Constant, -0.079361
Units Ratio
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.
Baseline Rules The baseline building uses the default
Boiler Minimum Unloading Ratio
Applicability All boilers
Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

 

Table 6.8.1-1: Default Minimum Unloading Ratios

Boiler Type Default Unloading Ratio
Electric Steam 1%
Electric Hot Water 1%
Fuel-Fired Steam 25%
Fuel-Fired Hot Water 25%
Units Percent (%)
Input Restrictions As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Use defaults.
Hot Water Supply Temperature
Applicability All boilers
Definition The temperature of the water produced by the boiler and supplied to the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 180°F for baseline boiler (G3.1.3.3).
Hot Water Return Temperature
Applicability All boilers
Definition The temperature of the water returning to the boiler from the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Use 130°F for baseline boiler design.
Hot Water Supply Temperature Reset
Applicability All boilers
Definition Variation of the hot water supply temperature with outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The hot water supply temperature should vary according to the following:
  • 180°F when outside air is < 20°F
  • ramp linearly between 180°F & 150°F when outdoor air is between 20°F and 50°F
  • 150°F when outdoor air is > 50°F
90.1-2016 BM

Boiler Name

Applicability

All boilers

Definition

A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.

Units

None

Input Restrictions

User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.

Baseline Rules

Boilers are applicable to baseline HVAC systems 1, 5, 7, and 12.

 

Boiler Fuel

Applicability

All boilers

Definition

The fuel source for the central heating equipment. The choices are:

  • Gas
  • Oil
  • Electricity

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

For baseline HVAC systems that have a boiler, the fuel shall be gas, unless the proposed design has a boiler that uses oil or propane, in which case, the baseline HVAC system shall be the same as the proposed design.

 

Boiler Type

Applicability

All boilers

Definition

The boiler type. Choices include:

  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

Hot water boiler when the baseline building has a boiler

 

Boiler Draft Type

Applicability

All boilers

Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical

Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.

Mechanical draft boilers enhance the air flow in one of three ways: 1) induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, or 3) balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units

List (see above)

Input Restrictions

As designed. Default is natural draft.

Baseline Rules

Natural draft when the baseline building has a boiler

 

Number of Identical Boiler Units

Applicability

All boilers

Definition

The number of identical units for staging

Units

Numeric: integer

Input Restrictions

As designed. Default is 1.

Baseline Rules

The baseline building shall have one boiler for a when the baseline plant serves a conditioned floor area of 15,000 ft² or less, and have two equally size boilers for plants serving more than 15,000 ft². Boilers shall be staged as required by the load.

 

Boiler Heat Loss

Applicability

All boilers

Definition

The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.

Units

Percent (%)

Input Restrictions

Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.

Baseline Rules

Prescribed at 2% for electric boilers and heat-exchangers. Prescribed at 0% for fuel-fired boilers, since this loss is already incorporated into the overall thermal efficiency, combustion efficiency or AFUE of the boiler.

For boilers with efficiency rating prescribed as combustion efficiency, 2% jacket losses will be assumed. Therefore:

Et = Ec – 2%

Where,

Et = Thermal Efficiency

Ec = Combustion Efficiency

 

Boiler Design Capacity

Applicability

All boilers

Definition

The heating capacity at design conditions

Units

Btu/h

Input Restrictions

Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.

Baseline Rules

The boiler is sized to be 25% larger than the peak loads of the baseline building. Baseline boilers shall be sized using weather files containing 99.6% heating design temperatures.

 

Boiler Efficiency Type

Applicability

All boilers

Definition

The full load efficiency of a boiler is expressed as one of the following:

  • Annual fuel utilization efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.

Units

List (see above)

Input Restrictions

AFUE, for all gas and oil-fired boilers with less than 300,000 Btu/h capacities.

Et, for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.

Ec, for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Baseline Rules

Same efficiency type relationship with capacity as described for proposed design

 

Boiler Efficiency

Applicability

All boilers

Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either thermal efficiency (Et), combustion efficiency (Ec), or annual fuel utilization efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:

(Equation 3.8.1-1)

$$For\  75\% \leq AFUE < 80\%, E_{t} = 0.1 \cdot AFUE + 72.5\%$$

$$For\  80\% \leq AFUE \leq 100\%, E_{t} = 0.875 \cdot AFUE + 10.5\%$$

Where Ec is provided, Et shall be calculated as follows:

(Equation 3.8.1-1)

$$E_{t} = E_{c}- 2\%$$

 

All electric boilers will have an efficiency of 100%.  For applicable software, heat input ratio shall be defined as the inverse of thermal efficiency.

Units

Ratio

Input Restrictions

As designed

Baseline Rules

Boilers for the baseline design are assumed to have the minimum efficiency as listed below.

Size

Minimum Efficiency

Test Procedure

<300,000 Btu/h

80% AFUE

DOE 10 CFR Part 430

≥300,000 Btu/h and ≤2,500,000 Btu/h

75% Et

DOE 10 CFR Part 431

>2,500,000 Btu/h

80% Ec

 

Boiler Part-Load Performance Curve 

Applicability

All boilers

Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:

 

(Equation 3.8.1-2)

$$Fuel_{partload} =  Fuel_{design} \times FHeatPLC(Q_{partload}, Q_{rated})$$

$$FHeatPLC = a + b \cdot (\frac{Q_{partload}}{Q_{rated}}) + b \cdot (\frac{Q_{partload}}{Q_{rated}})^{2}$$

where

FHeatPLC

The fuel heating part load efficiency curve

Fuelpartload

The fuel consumption at part load conditions (Btu/h)

Fueldesign

The fuel consumption at design conditions (Btu/h)

Qpartload

The boiler capacity at part load conditions (Btu/h)

Qrated

The boiler capacity at design conditions (Btu/h)

a, b, c

Coefficients taken from Appendix H

 

Units

Ratio

Input Restrictions

As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.

Baseline Rules

The baseline building shall use the defaults as described above.

 

Boiler Forced Draft Fan Power

Applicability

All mechanical draft boilers

Definition

The fan power of the mechanical draft fan at design conditions.  

The software shall convert the user entry of motor HP to fan power in Watts by the following equation:

Fan Power (W) = Motor HP x 746 x 0.5                                       

Units

Horsepower

Input Restrictions

As Designed.

Baseline Rules

Not applicable

 

Boiler Minimum Unloading Ratio

Applicability

All boilers

Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

Table 3.8.1-1: Default Minimum Unloading Ratios

Boiler Type

Default Unloading Ratio

Electric Steam

1%

Electric Hot Water

1%

Fuel-Fired Steam

25%

Fuel-Fired Hot Water

25%

Units

Percent (%)

Input Restrictions

As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.

Baseline Rules

Use defaults.

 

Boiler Minimum Flow Rate 

Applicability

All boilers

Definition

The minimum flow rate recommended by the boiler manufacturer for stable and reliable operation of the boiler.

Units

gpm

Input Restrictions

As Designed. If the boiler(s) is piped in a primary only configuration in a variable flow system then the software shall assume there is a minimum flow bypass valve that allows the HW pump to bypass water from the boiler outlet back to the boiler inlet to maintain the minimum flow rate when boiler is enabled. Note that the boiler entering water temperature must accurately reflect the mixed temperature (colder water returning from the coil(s) and hotter bypass water) in order to accurately model boiler efficiency as a function of boiler entering water temperature.

Baseline Rules

0 gpm

 

Hot Water Supply Temperature

Applicability

All boilers

Definition

The temperature of the water produced by the boiler and supplied to the hot water loop

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed

Baseline Rules

Use 180°F for baseline boiler

 

Hot Water Return Temperature

Applicability

All boilers

Definition

The temperature of the water returning to the boiler from the hot water loop

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed

Baseline Rules

Use 130°F for baseline boiler design.

 

Hot Water Supply Temperature Reset

Applicability

All boilers

Definition

Variation of the hot water supply temperature with outdoor air temperature.

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed

Baseline Rules

The hot water supply temperature should vary according to the following:

  • 180°F when outside air is <= 20°F
  • 150°F when outdoor air is >= 50°F
  • ramp linearly between 180°F and 150°F when outdoor air is between 20°F and 50°F
Building EQ
Boiler Name
Applicability All boilers
Definition A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.
Units None
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.
Boiler Fuel
Applicability All boilers
Definition The fuel source for the central heating equipment. The choices are:
  • Gas
  • Oil
  • Electricity
Units List (see above)
Input Restrictions As designed
Boiler Type
Applicability All boilers
Definition The boiler type. Choices include:
  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater
Units List (see above)
Input Restrictions As designed
Boiler Draft Type
Applicability All boilers
Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical
Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.


Mechanical draft boilers enhance the air flow in one of three ways: 1) Induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) Forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, and 3) Balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units List (see above)
Input Restrictions As designed. Default is natural draft.
Number of Identical Boiler Units
Applicability All boilers
Definition The number of identical units for staging
Units Numeric: integer
Input Restrictions As designed. Default is 1.
Boiler Heat Loss
Applicability All boilers
Definition The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.
Units Percent (%)
Input Restrictions Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.
Boiler Design Capacity
Applicability All boilers
Definition The heating capacity at design conditions
Units Btu/h
Input Restrictions Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.
Boiler Efficiency Type
Applicability All boilers
Definition The full load efficiency of a boiler is expressed as one of the following:
  • Annual Fuel Utilization Efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal Efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion Efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.
Units List (see above)
Input Restrictions

Annual Fuel Utilization Efficiency (AFUE), for all gas and oil-fired boilers with less than 300,000 Btu/h capacity.


Thermal Efficiency (Et), for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.


Combustion Efficiency (Ec), for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Boiler Efficiency
Applicability All boilers
Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either Thermal Efficiency (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:

 

(6.8.1-1)

$1) 75\% \leq AFUE \textless\ 80\% $

$E_t= 0.1 \times AFUE + 72.5\% $

$2) 80\% \leq AFUE \leq 100\% $

$E_t=0.875 \times AFUE + 10.5\% $

All electric boilers will have an efficiency of 100%.

Units Ratio
Input Restrictions As designed
Boiler Part-Load Performance Curve 
Applicability All boilers
Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:

(6.8.1-2)

$$ Fuel_{partload} = Fuel_{design} \times F\!H\!eat\!P\!L\!C\, \left ( Q_{partload}, Q_{rated} \right )$$

$$ F\!H\!eat\!P\!L\!C = \Bigg ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left ( \frac{Q_{partload}}{Q_{rated}}\right )^2 \Bigg ) $$

where

FHeatPLC The Fuel Heating Part Load Efficiency Curve
Fuelpartload The fuel consumption at part load conditions (Btu/h)
Fueldesign The fuel consumption at design conditions (Btu/h)
Qpartload The boiler capacity at part load conditions (Btu/h)
Qrated The boiler capacity at design conditions (Btu/h)
a Constant, 0.082597
b Constant, 0.996764
c Constant, -0.079361
Units Ratio
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.
Boiler Minimum Unloading Ratio
Applicability All boilers
Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Boiler Type Default Unloading Ratio
Electric Steam 1%
Electric Hot Water 1%
Fuel-Fired Steam 25%
Fuel-Fired Hot Water 25%
Units Percent (%)
Input Restrictions As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.
Hot Water Supply Temperature
Applicability All boilers
Definition The temperature of the water produced by the boiler and supplied to the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Hot Water Return Temperature
Applicability All boilers
Definition The temperature of the water returning to the boiler from the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Hot Water Supply Temperature Reset
Applicability All boilers
Definition Variation of the hot water supply temperature with outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Energy Star
Boiler Name
Applicability All boilers
Definition A unique descriptor for each boiler, heat pump, central heating heat-exchanger or heat recovery device.
Units None
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans for the proposed design.
Boiler Fuel
Applicability All boilers
Definition The fuel source for the central heating equipment. The choices are:
  • Gas
  • Oil
  • Electricity
Units List (see above)
Input Restrictions As designed
Boiler Type
Applicability All boilers
Definition The boiler type. Choices include:
  • Steam Boiler
  • Hot Water Boiler
  • Heat-Pump Water Heater
Units List (see above)
Input Restrictions As designed
Boiler Draft Type
Applicability All boilers
Definition

How combustion airflow is drawn through the boiler. Choices are:

  • Natural (sometimes called atmospheric)
  • Mechanical
Natural draft boilers use natural convection to draw air for combustion through the boiler. Natural draft boilers are subject to outside air conditions and the temperature of the flue gases.


Mechanical draft boilers enhance the air flow in one of three ways: 1) Induced draft, which uses ambient air, a steam jet, or a fan to induce a negative pressure which pulls flow through the exhaust stack; 2) Forced draft, which uses a fan and ductwork to create a positive pressure that forces air into the furnace, and 3) Balanced draft, which uses both induced draft and forced draft methods to bring air through the furnace, usually keeping the pressure slightly below atmospheric.

Units List (see above)
Input Restrictions As designed. Default is natural draft.
Number of Identical Boiler Units
Applicability All boilers
Definition The number of identical units for staging
Units Numeric: integer
Input Restrictions As designed. Default is 1.
Boiler Heat Loss
Applicability All boilers
Definition The boiler or heat-exchanger heat loss expressed as a percentage of full load output capacity. This loss only occurs when the boiler is firing.
Units Percent (%)
Input Restrictions Default is 2% for electric boilers and heat-exchangers and 0% for fuel-fired boilers. If the user overrides the default, supporting documentation is required.
Boiler Design Capacity
Applicability All boilers
Definition The heating capacity at design conditions
Units Btu/h
Input Restrictions Unmet load hours shall not exceed 300. If they do, the proposed boiler capacity shall be increased incrementally until the unmet loads are reduced to 300 or less.
Boiler Efficiency Type
Applicability All boilers
Definition The full load efficiency of a boiler is expressed as one of the following:
  • Annual Fuel Utilization Efficiency (AFUE) is a measure of the boiler’s efficiency over a predefined heating season.
  • Thermal Efficiency (Et) is the ratio of the heat transferred to the water divided by the heat input of the fuel.
  • Combustion Efficiency (Ec) is the measure of how much energy is extracted from the fuel and is the ratio of heat transferred to the combustion air divided by the heat input of the fuel.
Units List (see above)
Input Restrictions

Annual Fuel Utilization Efficiency (AFUE), for all gas and oil-fired boilers with less than 300,000 Btu/h capacity.


Thermal Efficiency (Et), for all gas and oil-fired boilers with capacities between 300,000 and 2,500,000 Btu/h.


Combustion Efficiency (Ec), for all gas and oil-fired boilers with capacities above 2,500,000 Btu/h.

Boiler Efficiency
Applicability All boilers
Definition

The full load efficiency of a boiler at rated conditions (see efficiency type above) expressed as a dimensionless ratio of output over input. The software must accommodate input in either Thermal Efficiency (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et shall be calculated as follows:

 

(6.8.1-1)

$1) 75\% \leq AFUE \textless\ 80\% $

$E_t= 0.1 \times AFUE + 72.5\% $

$2) 80\% \leq AFUE \leq 100\% $

$E_t=0.875 \times AFUE + 10.5\% $

All electric boilers will have an efficiency of 100%.

Units Ratio
Input Restrictions As designed
Boiler Part-Load Performance Curve 
Applicability All boilers
Definition

An adjustment factor that represents the percentage full load fuel consumption as a function of the percentage full load capacity. This curve shall take the form of a quadratic equation as follows:

(6.8.1-2)

$$ Fuel_{partload} = Fuel_{design} \times F\!H\!eat\!P\!L\!C\, \left ( Q_{partload}, Q_{rated} \right )$$

$$ F\!H\!eat\!P\!L\!C = \Bigg ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left ( \frac{Q_{partload}}{Q_{rated}}\right )^2 \Bigg ) $$

where

FHeatPLC The Fuel Heating Part Load Efficiency Curve
Fuelpartload The fuel consumption at part load conditions (Btu/h)
Fueldesign The fuel consumption at design conditions (Btu/h)
Qpartload The boiler capacity at part load conditions (Btu/h)
Qrated The boiler capacity at design conditions (Btu/h)
a Constant, 0.082597
b Constant, 0.996764
c Constant, -0.079361
Units Ratio
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required. The software may auto-generate curves for other boiler types from descriptive type information such as boiler type, core, full or partial condensing, combustion air control, minimum unloading or staging, and return water temperature.
Boiler Minimum Unloading Ratio
Applicability All boilers
Definition

The minimum unloading capacity of a boiler expressed as a percentage of the rated capacity. Below this level the boiler must cycle to meet the load.

[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Boiler Type Default Unloading Ratio
Electric Steam 1%
Electric Hot Water 1%
Fuel-Fired Steam 25%
Fuel-Fired Hot Water 25%
Units Percent (%)
Input Restrictions As designed. If the user does not use the default curve the software must indicate that supporting documentation is required on the output forms.
Hot Water Supply Temperature
Applicability All boilers
Definition The temperature of the water produced by the boiler and supplied to the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Hot Water Return Temperature
Applicability All boilers
Definition The temperature of the water returning to the boiler from the hot water loop
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Hot Water Supply Temperature Reset
Applicability All boilers
Definition Variation of the hot water supply temperature with outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed