3.7.6 Heating Systems

• Hot water
• Steam
• Electric resistance
• Electric heat pump
• Gas furnace
• Gas heat pump (optional feature)
• Oil furnace
• Heat recovery (for preheat coils in proposed designs)

(6.7.6-1)

$ 1) All\ Single\ Package\ Equipment$
$ E_t = 0.005163 \times AFUE + 0.4033 $
$ 2) Split\ Systems, AFUE \leq 83.5$
$ E_t = 0.002907 \times AFUE + 0.5787$
$ 3) Split\ Systems, AFUE \textgreater 83.5 $
$ E_t = 0.011116 \times AFUE - 0.098185$

where

(6.7.6-2)

$$ Fuel_{partload} = Fuel_{rated} \times F\!HeatP\!LC$$

(6.7.6-3)

$$ F\!HeatP\!LC = \left ( a + b \times \frac{Q_{partload}}{Q_{rated}} + c \times \left( \frac{Q_{partload}}{Q_{rated}} \right )^2 \right )$$

where

[table title="Furnace Efficiency Curve Coefficients" id="furnace-efficiency-curve-coefficients"]

• Electric resistance
• Gas furnace
• Oil furnace
• Hot water
• Other

The heating efficiency of a heat pump at ARI rated conditions as a dimensionless ratio of output over input. The software must accommodate user input in terms of either the Coefficient of Performance (COP) or the Heating Season Performance Factor (HSPF). Where HSPF is provided, COP shall be calculated as follows:

(6.7.6-4)

$ 1) All\ Single\ Package\ Equipment$
$ C\!O\!P = 0.2778 \times H\!S\!P\!F + 0.9667 $
$ 2) All\ Split\ Systems$
$ C\!O\!P = 0.4813 \times H\!S\!P\!F - 0.2606$

For all unitary and applied equipment where the fan energy is part of the equipment efficiency rating, the COP shall be adjusted as follows to remove the fan energy:

(6.7.6-5)

$$ C\!O\!P_{adj} = \frac{\frac{H\!C\!A\!P_{rated}}{3.413} - B\!H\!P_{supply} \times 0.7457}{\frac{H\!C\!A\!P_{rated}}{C\!O\!P \times 3.413} - B\!H\!P_{supply} \times 0.7457}$$

where

Refer to building descriptor Supply Fan BHP.

A curve or group of curves that represent the available heat-pump heating capacity as a function of evaporator and condenser conditions. The default curves are given as follows:

(6.7.6-6)

$$ Q_{available} = C\!A\!P\_FT \times Q_{rated}$$For air-cooled heat pumps:

(6.7.6-7)

$$ C\!A\!P\_FT = a + b \times t_{odb} + c \times \left. t_{odb}\right. ^2 + d \times \left. t_{odb}\right. ^3 $$

For water-cooled heat pumps:

(6.7.6-8)

$$ C\!A\!P\_FT = a + b \times t_{db} + d \times t_{wt}$$

where

[table title="Heat Pump Capacity Adjustment Curves (CAP-FT)" id="heat-pump-capacity-adjustment-curves-(CAP-FT)"]

A curve or group of curves that varies the heat-pump heating efficiency as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.7.6-9)

$$ P\!LR = \frac{Q_{operating}}{Q_{available}\left ( t_{db},t_{odb/wt}\right )}$$

(6.7.6-10)

$$ E\!I\!R\_F\!P\!LR = a + b \times P\!LR + c \times P\!LR^2 + d \times P\!LR^3 $$

Air Source Heat Pumps:

(6.7.6-11)

$$ E\!I\!R\_FT = a + b \times \left ( \frac{t_{odb}}{t_{db}} \right ) + c \times \left ( \frac{t_{odb}}{t_{db}} \right ) ^2 + d \times \left ( \frac{t_{odb}}{t_{db}} \right )^3$$Water Source Heat Pumps:

(6.7.6-12)

$$ E\!I\!R\_FT = a + b \times t_{wt} + d \times t_{db}$$

(6.7.6-13)

$$ P_{operating} = P_{rated} \times E\!I\!R\_F\!P\!LR \times E\!I\!R\_FT \times C\!AP\_FT $$

where

[table title="Heat Pump Heating Efficiency Adjustment (Curves)" id="heat-pump-heating-efficiency-adjustment-curves"]

• Hot-gas defrost, on-demand
• Hot-gas defrost, timed 3.5 minute cycle
• Electric resistance defrost, on-demand
• Electric resistance defrost, timed 3.5 minute cycle

The effectiveness of an air-to-air heat exchanger between the building exhaust and entering outside air streams. Effectiveness is defined as follows:

(6.7.6-14)

$$ H\!R\!E\!F\!F = \frac{\left ( E\!E\!A_{db} - E\!L\!A_{db}\right )}{\left ( E\!E\!A_{db} - O\!S\!A_{db}\right )}$$where

• Reheat coils
• Water heating