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