3.7.6.4 Furnace

Furnace Capacity
Applicability	Systems with a furnace
Definition	The full load heating capacity of the unit
Units	Btu/h
Input Restrictions	As designed. Adjust the capacity if the number of unmet load hours exceeds 300.
Baseline Rules	Autosize with an oversizing factor of 25% (let the software determine heating capacity based on the building loads). If the number of unmet load hours for the baseline exceeds 300, increase the furnace capacity as described in Chapter 2.

Furnace Fuel Heating Efficiency

Applicability

Systems with a furnace

Definition

The full load thermal efficiency of either a gas or oil furnace at design conditions. 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:

(Equation 3.7.6-1)

$$E_{t}=(0.0051427\cdot AFUE)+0.3989$$

where

AFUE	The annual fuel utilization efficiency (%)
E_t	The thermal efficiency (fraction)

Units

Fraction

Input Restrictions

As designed

Baseline Rules

See the values below:

Equipment Type	Size	Efficiency	Test Procedure
Furnace	<225,000 Btu/h	78% AFUE or 80% E_t	ANSI Z21.47
Furnace	≥225,000 Btu/h	80% E_c	ANSI Z21.47
Unit Heater	All	80% E_c	ANSI Z83.8

Furnace Fuel Heating Part Load Efficiency Curve

Applicability

Systems with furnaces

Definition

An adjustment factor that represents the percentage of 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 3.7.6-2)

$$Fuel_{partload}=Fuel_{rated}\times FHeatPLC$$

(Equation 3.7.6-3)

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

Where

FHeatPLC	The Fuel Heating Part Load Efficiency Curve
Fuel_partload	The fuel consumption at part load conditions (Btu/h)
Fuel_rated	The fuel consumption at full load (Btu/h)
Q_partload	The capacity at part load conditions (Btu/h)
Q_rated	The capacity at rated conditions (Btu/h)
a, b, c	Coefficients from Appendix H

Units

Data structure

Input Restrictions

As designed when data is available, otherwise use the default values are provided above.

Baseline Rules

The defaults shall be used.

Furnace Fuel Heating Pilot
Applicability	Systems that use a furnace for heating
Definition	The fuel input for a pilot light on a furnace
Units	Btu/h
Input Restrictions	As designed
Baseline Rules	Zero (pilotless ignition)

Furnace Fuel Heating Fan/Auxiliary
Applicability	Systems that use a furnace for heating and have an auxiliary fan for combustion air.
Definition	The fan energy in forced draft furnaces and the auxiliary (pumps and outdoor fan) energy in fuel-fired heat pumps
Units	Kilowatts (kW)
Input Restrictions	As designed
Baseline Rules	Not applicable

Furnace Capacity
Applicability	Systems with a furnace
Definition	The full load heating capacity of the unit
Units	Btu/h
Input Restrictions	As designed. Adjust the capacity if the number of UMLH exceeds 300.
Baseline Building	Autosized, with a heating oversizing factor of 25%. Sizing calculations shall be based on the heating design day and cooling design day conditions, as defined in Section 3.2.3 of this document. Oversizing would be carried out at zone level where the sizing parameters would be applied to the zone design heating loads, but not the airflow rates. If the number of UMLH for the baseline exceeds 300, heating coil capacity may need to be increased along with system airflow as described in Section 2.7.2 of this document.

Furnace Fuel Heating Efficiency

Applicability

Systems with a furnace

Definition

The full load thermal efficiency of either a gas or oil furnace at design conditions. The software must accommodate input in either Thermal Efficiency (E_t) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, E_t shall be calculated as follows for both packaged and split systems:

$$E_t=(0.0051427 * AFUE) + 0.3989$$

(31)

Source: (CEC 2013)

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

E_t = E_c – 2%

Where:

AFUE = The annual fuel utilization efficiency (%)

E_t = The thermal efficiency (fraction)

E_c = Combustion efficiency

Units

Fraction

Input Restrictions

As designed

Baseline Building

This is applicable to baseline systems 3 and 9. The baseline efficiency requirement is located in Table G₃.5.5 of Standard 90.1-2019 and Table 74 of this manual. Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Table 74. Efficiency Requirements for Baseline Systems with Fossil Fuel Furnace

Equipment Type	Size	Efficiency	Test Procedure
Furnace	<225,000 Btu/h	78% AFUE or 80% E_t	DOE 10 CFR Part 430 or ANSI Z₂₁.47
Furnace	≥225,000 Btu/h	80% E_c	ANSI Z₂₁.47
Unit Heater	All	80% E_c	ANSI Z₈₃.8

Furnace Fuel Heating Part Load Efficiency Curve

Applicability

Systems with furnaces

Definition

An adjustment factor that represents the percentage of 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:

$$Fuel_{partload}=Fuel_{rated}\times FHeatPLC$$

(32)

$$FHeatPLC = \left( a + b \times \frac{Q_{\text{partload}}}{Q_{\text{rated}}} + c \times \left( \frac{Q_{\text{partload}}}{Q_{\text{rated}}} \right)^2 \right)$$

(33)

Where:

FHeatPLC = The fuel heating part load efficiency curve

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

Fuel_rated = The fuel consumption at full load (Btu/h)

Q_partload = The capacity at part load conditions (Btu/h)

Q_rate_d = The capacity at rated conditions (Btu/h)

Table 75. Furnace Efficiency Curve Coefficients

Coefficient	Furnace
a	0.0186100
b	1.0942090
c	-0.1128190
Source: COMNET 2017

Units

Data structure

Input Restrictions

As designed when data is available, otherwise defaults curves are required to be used

Baseline Building

Default curves are required to be used

*Furnace Fuel Heating Pilot*
Applicability	Systems that use a furnace for heating
Definition	The fuel input for a pilot light on a furnace
Units	Btu/h
Input Restrictions	As designed
Baseline Building	Zero (pilotless ignition)

*Furnace Fuel Heating Fan/Auxiliary Power*
Applicability	Systems that use a furnace for heating
Definition	The fan energy in forced draft furnaces and the auxiliary (pumps and outdoor fan) energy in fuel-fired heat pumps
Units	kilowatts (kW)
Input Restrictions	As designed
Baseline Building	Not applicable

Furnace Capacity
Applicability	Systems with a furnace
Definition	The full load heating capacity of the unit
Units	Btu/h
Input Restrictions	As designed. Adjust the capacity if the number of UMLH exceeds 300.
Baseline Building	G3.2 New Construction/Major Alterations Autosized, with a heating oversizing factor of 25%. Sizing calculations shall be based on the heating design day and cooling design day conditions, as defined in Section 3.2.3 of this document. Oversizing would be carried out at zone level where the sizing parameters would be applied to the zone design heating loads, but not the airflow rates. If the number of UMLH for the baseline exceeds 300, heating coil capacity may need to be increased along with system airflow as described in Section 2.7.2 of this document. G3.3 Minor Alterations Furnace heating capacity for the baseline design shall be sized proportionally to the capacities in the proposed design based on sizing runs.

Furnace Fuel Heating Efficiency

Applicability

Systems with a furnace

Definition

The full load thermal efficiency of either a gas or oil furnace at design conditions. The software must accommodate input in either Thermal Efficiency (E_t) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, E_t shall be calculated as follows for both packaged and split systems:

$$E_t=(0.0051427 * AFUE) + 0.3989$$

(31)

Source: (CEC 2013)

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

Et = Ec – 2%

Where:

AFUE = The annual fuel utilization efficiency (%)

Et = The thermal efficiency (fraction)

Ec = Combustion efficiency

Units

Fraction

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

This is applicable to baseline systems 3 and 9. The baseline efficiency requirement is located in Table G3.5.5 of Standard 90.1-2022 and Table 81 of this manual. Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Table 81. Efficiency Requirements for Standard 90.1 Section G3.2 Baseline Systems with Fossil Fuel Furnace

Equipment Type	Size	Efficiency	Test Procedure
Furnace	<225,000 Btu/h	78% AFUE or 80% E_t	DOE 10 CFR Part 430 or ANSI Z21.47
Furnace	≥225,000 Btu/h	80% E_c	ANSI Z21.47
Unit Heater	All	80% E_c	ANSI Z83.8

G3.3 Minor Alterations

All furnace fuel heating equipment included in the scope of the retrofit shall be modeled at the minimum efficiency levels, both part load and full load, in accordance with Standard 90.1 Section 6.4 Table 6.8.1-5.

Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC systems should be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

Furnaces not included in the scope of the alteration should be modeled the same in the baseline and proposed.

Furnace Fuel Heating Part Load Efficiency Curve

Applicability

Systems with furnaces

Definition

An adjustment factor that represents the percentage of 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:

$$Fuel_{\text{partload}}=Fuel_{\text{rated}}\times FHeatPLC$$

(32)

$$FHeatPLC = \left( a + b \times \frac{Q_{\text{partload}}}{Q_{\text{rated}}} + c \times \left( \frac{Q_{\text{partload}}}{Q_{\text{rated}}} \right)^2 \right)$$

(33)

Where:

FHeatPLC = The fuel heating part load efficiency curve

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

Fuel_rated = The fuel consumption at full load (Btu/h)

Q_partload = The capacity at part load conditions (Btu/h)

Q_rate_d = The capacity at rated conditions (Btu/h)

Table 82. Furnace Efficiency Curve Coefficients

Coefficient	Furnace
a	0.0186100
b	1.0942090
c	-0.1128190
Source: COMNET 2017

Units

Data structure

Input Restrictions

Where publicly accessible performance curves are available for as-designed equipment they may be used for the proposed design otherwise the default equations and coefficients given above should be used. If default curves are not used, supporting documentation is required.

Baseline Building

G3.2 New Construction/Major Alterations

Default curves are required to be used

G3.3 Minor Alterations

Default curves.

Furnace Fuel Heating Pilot
Applicability	Systems that use a furnace for heating
Definition	The fuel input for a pilot light on a furnace
Units	Btu/h
Input Restrictions	As designed
Baseline Building	G3.2 New Construction/Major Alterations Zero (pilotless ignition) G3.3 Minor Alterations Same as the proposed design.

Furnace Fuel Heating Fan/Auxiliary Power
Applicability	Systems that use a furnace for heating
Definition	The fan energy in forced draft furnaces and the auxiliary (pumps and outdoor fan) energy in fuel-fired heat pumps
Units	kilowatts (kW)
Input Restrictions	As designed
Baseline Building	G3.2 New Construction/Major Alterations Not applicable G3.3 Minor Alterations Same as the proposed design.