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Heat pump calculations go wrong when they treat HSPF2 as a constant COP, assume the unit keeps its mild-weather capacity below zero, or compare electricity and fuel on different units of delivered heat. A credible model keeps three questions separate: what the DOE seasonal test measures, how the equipment performs at specific outdoor temperatures, and what a particular home might pay under its own tariffs and control strategy.
Why heat pump calculations fail
A single efficiency number cannot answer every operating-cost question. HSPF2 is a standardized seasonal rating; COP describes performance at a particular operating condition; and a household cost estimate depends on local prices, the home’s heating load, backup operation, and equipment controls. Mixing these quantities can make a model look precise while answering the wrong question.
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- Using HSPF2 as COP: HSPF2 is expressed in Btu per watt-hour and aggregates modeled heating over outdoor-temperature bins. It is not the heat pump’s COP at a chosen temperature.
- Ignoring cold-weather capacity: A heat pump may deliver less heat as outdoor temperature falls. If the model assumes it can still meet the whole load, it understates backup use or overstates comfort and savings.
- Leaving out resistance heat: Electric backup can add substantial electricity consumption when the heat pump cannot meet the modeled load or is cut out. A controller that calls for resistance heat to maintain supply temperature can add further use.
- Comparing unlike units: A price per kWh of electricity and a price per therm of gas are not directly comparable. Convert both to the cost of the same quantity of useful heat.
- Calling a custom dispatch simulation an HSPF2 rating: Standard seasonal ratings and a household-specific heat-pump/furnace switch scenario are different calculations.
What is HSPF2?
HSPF2 is a DOE seasonal heating-efficiency metric under Appendix M1. In simplified terms, it divides modeled seasonal building heating load by the electricity used to provide that heat. The calculation uses five-degree outdoor-temperature bins and region-specific fractional bin hours, and accounts for equipment performance, cycling, low-temperature cutout, auxiliary heat, and demand defrost treatment. Its units are Btu/Wh.
That makes HSPF2 useful for comparing products under a standardized procedure, but it is not a prediction of one home’s bill. A home’s climate, heat loss, thermostat settings, installation, electricity tariff, and backup controls can differ from the conditions represented by the rating procedure. Nor can a seasonal rating tell you the COP at a specific outdoor temperature: it combines performance across many modeled conditions.
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How Appendix M1 treats backup heat
Appendix M1 includes electric resistance heat when the heat pump cannot meet the modeled building load or when the system cuts out. A control that uses resistance heat to maintain a desired supply temperature may also increase auxiliary consumption. The seasonal result therefore reflects more than compressor efficiency alone.
DOE’s central air-conditioner and heat-pump guidance identifies Appendix M1 as the procedure for SEER2 and HSPF2, and says amendments to Appendix M1 became mandatory for product testing on July 7, 2025. The same DOE page describes Appendix M2 and its SCORE/SHORE metrics; those metrics do not become mandatory until a compliance date for standards based on them. Regulatory dates can change, so check DOE’s current guidance and the applicable rule before relying on a compliance date.
How does a heat pump perform below zero?
There is no single sub-zero COP or capacity value that applies to all heat pumps. Performance depends on the equipment and operating conditions, and DOE states that heat pumps generally perform less efficiently at lower ambient temperatures than at moderate temperatures. Available capacity matters alongside COP: a unit can still operate efficiently per unit of heat while no longer supplying all the heat the home needs.
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DOE’s 2021 Cold Climate Heat Pump Technology Challenge procedure characterizes capacity and efficiency at 5°F and -15°F, and also evaluates defrost and resistance-control behavior. Those are test points, not a promise that every product achieves a particular COP or capacity at either temperature. Use published or tested values for the equipment where available. Label any values between test points as interpolated, and values outside them as extrapolated; do not quietly extend a mild-weather rating into colder conditions.
Keep three kinds of evidence separate
- Seasonal rating: HSPF2 under the applicable DOE test procedure.
- Cold-weather point performance: capacity and electrical input at a stated outdoor temperature and operating condition, preferably from equipment-specific test data.
- Household scenario: a model using the home’s load, local tariffs, backup rules, and temperature distribution. This is an estimate, not a DOE rating.
Build an auditable bin model in TypeScript
A useful explainer model makes each assumption visible. The example below represents each temperature bin with the building load, hours, heat-pump capacity and input, and separate defrost adjustments. It assigns unmet load to electric resistance backup. It is deliberately an approximation, not an Appendix M1 implementation: it does not implement the full procedure’s equipment-specific steps for compressor speed, cycling, cutout, bin-hour distributions, or demand defrost.
In this model, capacity and load are in Btu/h, electrical input is in watts at the stated bin condition, and hours are actual hours represented by that bin. The input value is assumed to correspond to full available heat-pump output after the explicit defrost adjustments; partial-load behavior is treated as proportional. Those assumptions should be replaced with equipment-specific data and control logic for a real estimate.
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type Bin = {
outdoorF: number;
hours: number;
buildingLoadBtuPerHour: number;
heatPumpCapacityBtuPerHour: number;
heatPumpInputW: number;
defrostCapacityFactor: number;
defrostInputFactor: number;
};
const BTU_PER_WH = 3.412141633;
function modelBin(bin: Bin) {
const adjustedCapacity =
bin.heatPumpCapacityBtuPerHour * bin.defrostCapacityFactor;
const adjustedInputW = bin.heatPumpInputW * bin.defrostInputFactor;
const heatPumpHeatBtuPerHour = Math.min(
bin.buildingLoadBtuPerHour,
adjustedCapacity
);
const resistanceHeatBtuPerHour = Math.max(
0,
bin.buildingLoadBtuPerHour - heatPumpHeatBtuPerHour
);
// Proportional part-load input is an explicit simplification.
const heatPumpW = adjustedCapacity === 0
? 0
: adjustedInputW * heatPumpHeatBtuPerHour / adjustedCapacity;
const resistanceW = resistanceHeatBtuPerHour / BTU_PER_WH;
const electricWh = (heatPumpW + resistanceW) * bin.hours;
const deliveredBtu = bin.buildingLoadBtuPerHour * bin.hours;
return {
outdoorF: bin.outdoorF,
deliveredBtu,
heatPumpWh: heatPumpW * bin.hours,
resistanceWh: resistanceW * bin.hours,
totalElectricWh: electricWh
};
}
function seasonalApproximation(bins: Bin[]) {
const rows = bins.map(modelBin);
const deliveredBtu = rows.reduce((sum, row) => sum + row.deliveredBtu, 0);
const electricWh = rows.reduce((sum, row) => sum + row.totalElectricWh, 0);
return {
rows,
deliveredBtu,
electricWh,
seasonalBtuPerWh: electricWh === 0 ? null : deliveredBtu / electricWh
};
}The returned value is a simplified seasonal ratio, not certified HSPF2. The code preserves bin-level results so you can inspect how much electricity comes from the compressor and resistance heat instead of seeing only one seasonal total. For a useful implementation:
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- Use equipment-specific capacity and electrical-input data at each relevant condition. Preserve the source and test condition for each value.
- Define defrost adjustments from the data and method being modeled. Do not apply a generic factor without evidence.
- Represent cutout and control behavior explicitly. If the heat pump is disabled in a bin, model its capacity as unavailable and assign the load to the actual backup system.
- Keep auxiliary heat separate from compressor use so resistance consumption is visible and auditable.
Appendix M1 has additional system-specific calculations, so reproducing its rating requires following the applicable procedure and using certified equipment data. Do not label a simplified bin calculation HSPF2 merely because it uses temperature bins.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When is a heat pump cheaper than a gas furnace?
Compare the cost of delivering the same useful heat. Let electricity cost e dollars per kWh, the heat pump’s operating COP be COP, gas cost g dollars per therm, and furnace efficiency be η as a decimal. One MMBtu of delivered heat requires 293.071 kWh of heat-pump electricity divided by COP, or 10 therms of gas divided by furnace efficiency.
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So, under those assumptions:
- Heat-pump cost per delivered MMBtu =
e × 293.071 ÷ COP. - Furnace cost per delivered MMBtu =
g × 10 ÷ η. - Break-even heat-pump COP =
e × 29.3071 × η ÷ g.
Below the break-even COP, the furnace has the lower modeled fuel cost for delivered heat; above it, the heat pump does. This is an operating-energy comparison, not a complete ownership-cost comparison. It excludes fixed charges, demand charges, taxes, maintenance, and equipment costs unless you add them explicitly.
Calculate the threshold with declared inputs
function breakEvenCop(input: {
electricityDollarsPerKWh: number;
gasDollarsPerTherm: number;
furnaceEfficiency: number;
}) {
const { electricityDollarsPerKWh, gasDollarsPerTherm, furnaceEfficiency } = input;
if (gasDollarsPerTherm <= 0 || furnaceEfficiency <= 0) {
throw new Error("Gas price and furnace efficiency must be positive.");
}
return electricityDollarsPerKWh * 29.3071 * furnaceEfficiency
/ gasDollarsPerTherm;
}Enter prices in the stated units and use the same price period. State whether the gas price includes delivery charges and whether the electricity rate reflects the household’s actual time-of-use or tiered tariff. DOE’s Federal Energy Management Program used 11¢/kWh as an average federal-facility electricity-price assumption in July 2024 purchasing guidance annual-cost calculations. That is a federal-facility assumption, not a current national household tariff, and should not be substituted for a reader’s local rate.
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Model hybrid systems separately from HSPF2
A hybrid system can use the heat pump in milder weather and switch to a fuel-fired backup in colder conditions. A custom dispatch model should calculate heat-pump and furnace costs by bin, apply the chosen switch rule, and report the resulting household scenario separately from the product’s HSPF2.
DOE’s 2023 proposed-rule discussion says its described HSPF2 method does not distinguish dual-fuel operation from heat pumps using vapor-compression or electric-resistance auxiliary heat. That is a modeling boundary: a reader-specific fuel-switch simulation is not a distinct official HSPF2 seasonal calculation. Keep the rating, equipment point data, and hybrid scenario as separate outputs.
Quick Recap
Checks before trusting a result
- Metric: Is the number HSPF2, a COP at a stated condition, or a household scenario?
- Cold-weather data: Are capacity and input tied to the specific equipment and outdoor condition, with interpolation or extrapolation identified?
- Backup: Does the calculation include resistance or fuel backup when capacity is insufficient or the heat pump is cut out?
- Common basis: Are both systems compared for the same delivered heat using compatible energy units?
- Tariffs and assumptions: Are the location, date, rate units, furnace efficiency, and excluded costs stated?
- Controls: Does a hybrid switch rule match the modeled system rather than being mistaken for part of a standard rating?
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