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  4. 3.7 HVAC Secondary Systems
  5. 3.7.6 Heating Systems

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 (%)

Et

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% Et

ANSI Z21.47

≥225,000 Btu/h

80% Ec

ANSI Z21.47

Unit Heater

All

80% Ec

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

Fuelpartload

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

Fuelrated

The fuel consumption at full load (Btu/h)

Qpartload

The capacity at part load conditions (Btu/h)

Qrated

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

 

90.1-2019

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 (Et) or Annual Fuel Utilization Efficiency (AFUE). Where AFUE is provided, Et 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

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% Et

DOE 10 CFR Part 430 or ANSI Z21.47

≥225,000 Btu/h

80% Ec

ANSI Z21.47

Unit Heater

All

80% Ec

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)

 

$$FHEATPLC=(a+b\times \frac{Q_{partload}{Q_{rated}}+c\times (\frac{Q_{parltoad}}{Q_{rated}})^2 )$$

(33)

Where:

FHeatPLC    =    The fuel heating part load efficiency curve

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

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

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

Qrated             =    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