# 3.7.6.4 Furnace

90.1-2016 BM

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