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Heat pump calculations go wrong when they treat HSPF2 as a constant COP, assume full heating capacity below freezing, omit electric resistance backup, or compare electricity and fuel on different units of delivered heat. A better model keeps three questions separate: what the standardized DOE seasonal rating measures, how a particular unit performs at cold-weather test points, and what a household might pay under its own rates and backup controls.
Why heat pump calculations fail
A calculator can produce a precise-looking answer from inputs that do not describe the same thing. HSPF2 is a standardized seasonal metric; COP describes efficiency at a particular operating condition; and a household cost estimate depends on the home, local tariffs, equipment, and controls. Treating one as a substitute for another can distort both annual cost and the temperature at which another heat source appears cheaper.
- HSPF2 is not a point COP. It represents a modeled season with outdoor-temperature bins, building heating load, equipment performance, auxiliary electric heat, and defrost treatment.
- Cold-weather capacity matters as much as efficiency. A unit may need backup heat when its available output cannot meet the modeled load, even if it continues operating.
- Backup heat changes the bill. Electric resistance heat consumes electricity to supply the shortfall; a dual-fuel system may instead switch to a furnace or boiler.
- Fuel comparisons need a common basis. Compare the cost of delivering the same amount of heat, not a kilowatt-hour of electricity directly with a therm, gallon, or other fuel unit.
These are different modeling layers. A seasonal rating does not, by itself, predict an individual home’s bill or establish the best switchover temperature.
What is HSPF2?
HSPF2 is the DOE seasonal heating-efficiency metric for covered residential heat pumps under Appendix M1. The calculation aggregates modeled heating load and electricity use across five-degree outdoor-temperature bins, using region-specific fractional bin hours. In simplified form, it is total modeled seasonal heating load divided by the corresponding heat-pump and auxiliary-resistance electricity consumption, with the prescribed demand-defrost treatment. Its units are Btu per watt-hour.
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- Before buying, please confirm whether the product is compatible with your system, and check the size of the product to see if it can cover the traces left on the wall of the old thermostat.
- Aowel AW721 non programmable heat pump thermostat for house up to 2 heating and 1 cooling multi-stage systems.
- Compatible with Heat Pump systems (with aux or emergency heat) and Heat Pump (without auxiliary/backup heat) systems, up to 2 Heat/ 1 Cool. 【the Menu Initialize Heat Pump System】
- Compatible with most 24V conventional single-stage systems (i.e. conventional force air, central gas, oil, electric furnaces single-stage system, up to 2 Heat/ 1 Cool ). 【Note: You need to enter the menu and set it to conventional system】
- 【WARNING】1. Does not work with conventional multistage systems, mini split systems and 12v rv thermostats. ( 2. Not compatible with 110-240V electric heat systems (like electric baseboard heat, line voltage, convectors, radiant-ceiling heat).
Appendix M1 is more than a conversion of a rating into COP. The procedure includes system-specific steps for such factors as compressor speed, cycling, low-temperature cutout, and auxiliary heat. It accounts for electric resistance heat when the heat pump cannot meet the modeled load or is cut out. A heat-comfort controller can also add resistance use to maintain supply temperature.
That is why dividing HSPF2 by a constant or treating it as the heat pump’s COP at every outdoor temperature is misleading. COP is a ratio of heat delivered to electrical input at a particular condition; HSPF2 is a seasonal result from a prescribed model. Neither number alone contains the home’s actual hourly load, utility tariff, or control behavior.
DOE’s Consumer Central Air Conditioners and Heat Pumps page identifies Appendix M1 as the test procedure for SEER2 and HSPF2. It says amendments to Appendix M1 became mandatory for product testing on July 7, 2025. The page also describes Appendix M2, which introduces SCORE and SHORE; those metrics do not become mandatory until a compliance date for standards based on them. Testing requirements and compliance dates are distinct, so verify the applicable rule for a product or program rather than inferring one date from the other.
Rank #2
- Heat pump thermostat for home, non programmable, 2 heating and 1 cooling, electric or gas/oil configurable. (TIP: before purchasing, check to see if this thermostat is compatible with your old thermostat)
- Compatible With: Heat pump systems and Conventional force air, central gas, oil, or electric furnaces systems, Does NOT work with electric baseboard heat (120-240V).
- Not Compatible with 110-240V HVAC systems: like 120/240v electric baseboard heat and line voltage systems. and Not compatible with RV thermostat.
- Easy to read and use: Large digital display with large characters and green backlight, separate buttons for easy setpoint temperature, very suitable for middle-aged and elderly people to use.
- It gives precise temperature control with +/- 1 degree in Fahrenheit and Celsius for ultimate consistent control to keep your room comfortable all the time.
How does a heat pump perform below zero?
There is no single below-zero COP or capacity derate that applies to every heat pump. DOE states that heat pumps generally perform less efficiently at lower outdoor temperatures than at moderate temperatures. The size of the change depends on the equipment and test conditions, and available heating capacity can decline as well. A useful estimate therefore needs temperature-specific performance data, not a generic cold-weather percentage.
Use cold-climate test points as points, not universal promises
DOE’s 2021 Cold Climate Heat Pump Technology Challenge procedure characterizes capacity and efficiency at 5°F and -15°F, and evaluates defrost and resistance-control behavior. Those are procedure test conditions, not evidence that every product reaches a particular COP or capacity at those temperatures. Use the unit’s published or tested data where available, and label any interpolated or extrapolated values as estimates.
Do not infer a below-zero COP solely from HSPF2. Nor should a test point be extended across the entire heating season without an explicit method. A seasonal estimate needs the home’s load distribution and equipment behavior across the temperatures the home actually experiences.
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- · Not Compatible: Two transformer systems, Conventional multi-stage systems, Dual fuel/hybrid heating, Electric baseboard heat (110-240 volts), Line voltage, Convectors, Radiant-ceiling heat, Mini split systems and 12V RV systems.
- · Easy to Install and Use: Large terminal blocks with universal sub-base, can be installed within 30 minutes, clear, easy-to-read backlight LCD display and easy controls make for a user-friendly experience.
- · Dual Power Supply: Can be powered by 24VAC power or 2 AAA batteries, No common wire (C-wire) required on most systems (only required on heat only and cool only systems).
- · Simple but Powerful Control: Easy-access front-load battery compartment, adjustable temperature differential (swing) for energy optimization and maximum system life, Support temperature
- · High Compatibility: Heat pump multi-stage systems or conventional single-stage systems, Forced air (gas, oil or electric), Electric furnace, Hot water steam or gravity radiant heat, Heat only systems, Heat pump w/o or with auxiliary or emergency heat, Millivolt conventional systems, Gas fireplaces (24V), Floor or wall furnaces, Cool only systems.(TIP: Before purchasing, check to see if this thermostat is compatible with your system.) auxiliary or emergency heat, Millivolt, Gas fireplaces (24V), Floor or wall furnaces, Cool only systems.(TIP: Before purchasing, check to see if this thermostat is compatible with your system.)
Represent capacity and backup separately
At a given outdoor temperature, compare the home’s heating load with the heat pump’s available capacity. If load exceeds capacity, the difference must be served by something else to maintain the modeled indoor condition. In an all-electric scenario that may be resistance heat; in a hybrid scenario it may be fuel heat. If the compressor cuts out, its contribution is zero below the modeled cutoff. Those control choices affect energy use and costs, so they belong in the model rather than being hidden inside a single derate factor.
Build an auditable TypeScript seasonal approximation
The code below keeps the relevant quantities visible for each bin: temperature, fraction of season hours, building load, heat-pump capacity, COP, and a defrost electricity adjustment. It then estimates heat-pump electricity, resistance electricity for unmet load, and a Btu/Wh seasonal ratio. It is an explanatory approximation, not an implementation of the full Appendix M1 procedure and not an official HSPF2 rating.
Use consistent input data: the COP and capacity should refer to the same outdoor condition and compatible operating assumptions. The defrost adjustment below is an average additional electrical input in watts for the bin; do not add it if the COP input already includes the same defrost energy. The fraction values should sum to 1 across the modeled season.
Rank #4
- Installation Caution: Buyers should carefully review the detailed installation and wiring instructions in the images before making a purchase. It is advisable to proceed with caution if you do not have prior installation experience to prevent issues such as malfunction and incompatibility due to purchasing the wrong item
- Suuwer S721 non-programmable heat pump thermostat for home, up to 2 heating and 1 cooling multi-stage systems. Display with blue backlight, large clear backlit digital display, very suitable for middle-aged and elderly people
- System Compatibility: Compatible with Heat Pump systems (with aux or emergency heat) and Heat Pump (without aux) systems. Compatible with most 24V conventional single-stage systems (i.e. conventional force air, central gas, oil, or electric furnaces single-stage systems) up to 2 heat 1 cool
- System Incompatibility: Not compatible with 110-240V electric heat systems (like electric baseboard heat, line voltage, convectors, radiant-ceiling heat). Does not work with conventional multistage systems (2H/2C 3H/2C 4H/2H), and mini split systems and 12v rv thermostats
- Dual Power Options: Dual powered by 24VAC Power or 2 AAA Batteries. Features easy wire coded terminal blocks with designations R, G, Y, C, O, B, W/E, W2.
const BTU_PER_WATT_HOUR = 3.412141633;
const KWH_PER_MMBTU = 293.07107;
interface BinInput {
outdoorTempF: number;
binHourFraction: number;
buildingLoadBtuh: number;
heatPumpCapacityBtuh: number;
heatPumpCOP: number;
defrostExtraW: number;
}
interface BinResult {
outdoorTempF: number;
hours: number;
loadBtu: number;
heatPumpBtu: number;
resistanceBtu: number;
heatPumpWh: number;
resistanceWh: number;
defrostWh: number;
totalWh: number;
}
function modelSeason(
bins: BinInput[],
seasonHours: number
): { bins: BinResult[]; seasonalBtuPerWh: number } {
const fractionSum = bins.reduce((sum, bin) => sum + bin.binHourFraction, 0);
if (Math.abs(fractionSum - 1) > 0.001) {
throw new Error("Bin-hour fractions must sum to 1.");
}
const results = bins.map((bin): BinResult => {
if (bin.heatPumpCOP <= 0 || bin.heatPumpCapacityBtuh < 0) {
throw new Error("COP must be positive and capacity cannot be negative.");
}
const hours = seasonHours * bin.binHourFraction;
const heatPumpBtuh = Math.min(
bin.buildingLoadBtuh,
bin.heatPumpCapacityBtuh
);
const resistanceBtuh = Math.max(
0,
bin.buildingLoadBtuh - heatPumpBtuh
);
const loadBtu = bin.buildingLoadBtuh * hours;
const heatPumpBtu = heatPumpBtuh * hours;
const resistanceBtu = resistanceBtuh * hours;
const heatPumpWh = heatPumpBtu /
(bin.heatPumpCOP * BTU_PER_WATT_HOUR);
const resistanceWh = resistanceBtu / BTU_PER_WATT_HOUR;
const defrostWh = bin.defrostExtraW * hours;
return {
outdoorTempF: bin.outdoorTempF,
hours,
loadBtu,
heatPumpBtu,
resistanceBtu,
heatPumpWh,
resistanceWh,
defrostWh,
totalWh: heatPumpWh + resistanceWh + defrostWh
};
});
const totalLoadBtu = results.reduce((sum, bin) => sum + bin.loadBtu, 0);
const totalElectricWh = results.reduce((sum, bin) => sum + bin.totalWh, 0);
if (totalElectricWh <= 0) throw new Error("Seasonal electricity must be positive.");
return {
bins: results,
seasonalBtuPerWh: totalLoadBtu / totalElectricWh
};
}The returned seasonal ratio can help explain an assumed profile, but it is not automatically HSPF2: the calculation omits the many equipment-specific and procedural requirements of Appendix M1. Retain the bin-level results so a reader can see whether resistance use, defrost adjustment, or low-temperature performance drives the outcome. If the inputs are interpolated, extrapolated, or measured under different conditions, label that fact alongside the result.
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?
A break-even comparison is specific to the rates and equipment assumptions entered. First put both systems on a delivered-heat basis. Electricity costs per million Btu of delivered heat are the electricity tariff in dollars per kWh multiplied by 293.071 kWh per MMBtu, then divided by the heat pump’s operating COP. A furnace’s delivered-heat cost is its fuel price per unit divided by the MMBtu of fuel energy in that unit and the assumed furnace efficiency.
For example, if the fuel price is expressed in dollars per fuel unit, the fuel energy content in MMBtu per unit is an input, as is furnace efficiency as a decimal. This avoids treating a therm, gallon, or other unit as if it were a common heat unit. The resulting costs are both dollars per MMBtu of useful heat.
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interface FuelScenario {
electricityDollarsPerKwh: number;
heatPumpCOP: number;
fuelDollarsPerUnit: number;
fuelMmbtuPerUnit: number;
furnaceEfficiency: number;
}
function compareDeliveredHeat(s: FuelScenario) {
if (s.heatPumpCOP <= 0 || s.furnaceEfficiency <= 0) {
throw new Error("COP and furnace efficiency must be positive.");
}
const heatPumpDollarsPerMmbtu =
s.electricityDollarsPerKwh * KWH_PER_MMBTU / s.heatPumpCOP;
const furnaceDollarsPerMmbtu =
s.fuelDollarsPerUnit /
(s.fuelMmbtuPerUnit * s.furnaceEfficiency);
const breakEvenCOP =
s.electricityDollarsPerKwh * KWH_PER_MMBTU /
furnaceDollarsPerMmbtu;
return {
heatPumpDollarsPerMmbtu,
furnaceDollarsPerMmbtu,
breakEvenCOP
};
}At the break-even COP, the modeled operating cost per delivered MMBtu is equal under those inputs. Above it, the heat pump’s modeled variable energy cost is lower; below it, the furnace’s is lower. This is an operating-cost comparison, not a universal temperature threshold. To turn it into a switchover scenario, compare the break-even COP with the unit’s temperature-specific COP and apply the chosen system controls and building load at each temperature.
Make the scenario assumptions explicit
- Enter the household’s electricity and fuel rates, with units and effective date. DOE FEMP’s July 2024 residential air-source heat-pump purchasing guidance used 11¢/kWh as an average federal-facility electricity-price assumption for annual-cost calculations. That is not a current national household tariff.
- State the furnace’s assumed efficiency and the fuel’s energy content per billed unit. If an efficiency is seasonal, do not silently treat it as a measured efficiency at every operating condition.
- Use COP and capacity appropriate to the outdoor temperatures being modeled; identify whether each input is published, tested, interpolated, or extrapolated.
- Model resistance backup, compressor cutout, and hybrid dispatch explicitly. A furnace switchover is not equivalent to electric resistance backup.
- Say whether fixed charges, demand charges, taxes, maintenance, and equipment costs are excluded. The equations above compare variable energy cost only.
- Use a load profile or temperature-bin hours representative of the home and climate. A single design temperature cannot stand in for seasonal operation.
Keep a hybrid switch model separate from HSPF2
A dual-fuel system can use the heat pump in milder weather and switch to a fuel backup in colder conditions. DOE’s 2023 proposed-rule discussion says the described HSPF2 method does not distinguish dual-fuel operation from systems with vapor-compression or electric-resistance auxiliary heat. A custom hybrid dispatch calculation therefore answers a different question from the standardized seasonal rating.
Keep the outputs separate: report the equipment’s applicable standardized rating as the rating, then label the household-specific simulation with its tariff, load, performance inputs, switchover rule, and exclusions. The switch point may be selected to minimize modeled operating cost, but a real control decision can also reflect capacity, comfort, resilience, and equipment constraints. A cost-only parity calculation does not settle those factors.
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