An air-to-water heat pump in IECC climate zone 5, 6, or 7 meets its hardest hour with its smallest output. Compressor capacity falls as outdoor air cools, defrost cycles remove minutes of production in the 5 to 35 F band, and a domestic hot water load rises as inlet water drops toward 40 F. These effects stack in the same evening. Storage lets the plant be sized for the day rather than for that hour, but its quantity must be derived from the derate rather than from a rule of thumb.
Key Takeaways
The method converts the heat pump's 5 F capacity derate and defrost losses into storage hours and module count, then determines when a resistance element or a retained boiler is cheaper than another tank.
The worked case uses a published capacity curve derated to 5 F, a defrost allowance, and a stated load. It converts the deficit into storage hours and 700-gallon module count for a 6-hour evening window and a 24-hour design day, priced against the 10 kW immersion element in each module at a COP of 1.0.
General buffer sizing is covered in Heat Pump Buffer Tank Sizing for Commercial Systems. Source selection is covered in Air Source vs Water Source Heat Pumps for Domestic Hot Water Systems.
In this article
Cold climates are defined by degree days, and the design hour is defined by the curve
DOE's Building America climate guide defines a cold climate as 5,400 to 9,000 heating degree days on a 65 F basis, corresponding to IECC zones 5 and 6, and a very cold climate as 9,000 to 12,600 heating degree days, corresponding to zone 7. The map was developed by Pacific Northwest National Laboratory and adopted into the IECC in 2004. Degree days size the annual energy; the design hour sizes the equipment, and in these zones it sits at or below 5 F.
Rating bodies have converged on 5 F as the low-ambient test point. ENERGY STAR's Cold Climate designation for air source heat pumps requires a heating COP of at least 1.75 at 5 F and a heating capacity at 5 F of at least 70 percent of the capacity at 47 F, confirmed under native controls.
NEEP's cold climate air source heat pump specification, version 3.1 effective August 2021, requires a COP above 1.75 at 5 F and reporting of capacity at 47 F, 17 F, and 5 F.
DOE's Cold Climate Heat Pump Technology Challenge set a target of 100 percent of 47 F capacity at 5 F with a COP of at least 2.4. DOE's 2024 field summary reports 22 completed sites, outdoor temperatures from -15 F to 65 F, and heat pump COPs of 1.5 to 4.5, falling to 1.0 to 4.5 when auxiliary heat is included.
Those are air-to-air ratings, and most air-to-water units sold for commercial DHW and hydronic duty are not listed under any of them. The reporting convention transfers: the engineer needs capacity and COP at 47 F, 17 F, and 5 F at the actual supply water temperature. Treat a unit without a published 5 F point as unrated for the design hour.
New York City's Department of Housing Preservation and Development technical requirements for heat pump water heaters state that a commercial system, heat pump and storage together, must be capable of creating and storing 140 F water at 5 F outdoor air. Storage must recover within 4 hours, electric resistance use is to be minimized, and any backup must be written into the sequence of operations.
DOE's heat pump field database hosts a 2021 to 2023 validation study of 12 variable-speed air source heat pumps in Washington, Montana, and Colorado, climate zones 5 and 6. The systems were monitored across two winters, with defrost and auxiliary heating logged as distinct operating modes.
Three mechanisms remove capacity in the same hour
Capacity derate. Vapor-compression capacity falls with evaporating temperature because refrigerant density at the compressor inlet falls. A 2025 E3S Web of Conferences paper tabulates a 20 kW nominal R290 air-to-water unit at a 55 C (131 F) supply: 22.9 kW thermal at 20 C (68 F), 20.2 kW thermal at 10 C (50 F), 17.1 kW thermal at 0 C (32 F), 14.5 kW thermal at -10 C (14 F), and 11.6 kW thermal at -20 C (-4 F). COP is 4.0, 3.7, 3.1, 2.4, and 1.8 at the same points.
The same paper lists a 40 kW nominal R744 unit at a 60 C (140 F) supply falling from 38.4 kW thermal at 20 C to 16.7 kW thermal at -20 C. Capacity at -4 F is roughly half of capacity at 68 F for both refrigerants.
Defrost. Below roughly 40 F, the outdoor coil runs below the dew point and frost accumulates, so the unit reverses or bypasses to melt it. The 1987 National Bureau of Standards field study of three residential heat pumps, NBSIR 87-3528, measured defrost at 0.34, 4.49, and 1.67 percent of compressor run time across a heating season. One unit reached 8 to 9 percent in the 5 to 15 F bins. The study found the penalty light above 40 F.
ORNL's 2018 literature review of defrost technologies describes frost as reducing airflow, adding thermal resistance, and lowering both capacity and COP. The worked case uses a 10 percent allowance against gross capacity at 5 F as an assumption. Replace it with the manufacturer's integrated defrost capacity where published. The NBS run-time shares are not capacity-loss percentages.
Load growth. A DHW load is proportional to the inlet-to-storage temperature difference. A 140 F store fed with 40 F winter mains water lifts 100 F; fed with 60 F summer water it lifts 80 F. The winter thermal load per gallon is therefore 25 percent higher before any change in draw volume.
The Minnesota Department of Commerce CARD field study of four air-to-water installations adds the hydronic equivalent: cast iron and high-temperature baseboard emitters at 160 F and above may lack cold-weather capacity and need detailed load calculations.
Integrated heat pump water heaters have a fourth limit. DOE's summary of Florida Solar Energy Center laboratory work states that compressor operation is generally limited with ambient temperatures below 50 F on those units. This article addresses split air-to-water machines with an outdoor evaporator.
What each capacity point sets in the design:
Converting a 5 F derate into storage hours
The method has five steps: state the load, derate the heat pump, subtract to find the hourly deficit, integrate over the window, and divide by usable module capacity. The worked case is a 40-unit multifamily building at 40 gallons per unit per day, a 140 F store, 40 F winter inlet, and a 6-hour evening window from 4 p.m. to 10 p.m. carrying 40 percent of the daily draw.
Q_day = V × 8.34 lb/gal × 1 Btu/lb-F × ΔT
Q_day = 1,600 gal × 8.34 lb/gal × 1 Btu/lb-F × 100 F
Q_day = 1,334,400 Btu = 391.1 kWh thermal per day, at a 100 F lift from 40 F to 140 F.
Q_win = V_win × 8.34 lb/gal × 1 Btu/lb-F × ΔT
Q_win = 640 gal × 8.34 lb/gal × 1 Btu/lb-F × 100 F
Q_win = 533,760 Btu = 156.4 kWh thermal.
P_win = Q_win / t_win = 156.4 kWh thermal / 6 h = 26.1 kW thermal average.
Heat pump output uses the E3S R290 manufacturer curve, published at a 55 C (131 F) supply. Linear interpolation between the published 10 C and 0 C points gives roughly 19.7 kW thermal at 47 F. Linear interpolation between the published -10 C and -20 C points gives roughly 13.0 kW thermal at 5 F, with a COP near 2.1. The worked electrical and cost calculations use COP 2.1 at 5 F as an assumption.
The 5 F figure is 66 percent of the 47 F figure, just below the ENERGY STAR 70 percent threshold. Output at a 140 F supply is lower than the 131 F curve shows. Substitute the manufacturer's 140 F data; the structure of the calculation does not change.
P_net,5F = P_gross,5F × (1 - f_defrost)
P_net,5F = 13.0 kW thermal × (1 - 0.10)
P_net,5F = 11.7 kW thermal.
P_elec,5F = P_gross,5F / COP
P_elec,5F = 13.0 kW thermal / 2.1
P_elec,5F = 6.2 kW electrical.
t_47F = Q_day / P_47F
t_47F = 391.1 kWh thermal / 19.7 kW thermal = 19.9 h.
The 20 kW nominal unit is adequately sized for mild weather. At 5 F it is not.
D_hr = P_win - P_net,5F
D_hr = 26.1 kW thermal - 11.7 kW thermal
D_hr = 14.4 kW thermal.
D_win = Q_win - (P_net,5F × t_win)
D_win = 156.4 kWh thermal - (11.7 kW thermal × 6 h)
D_win = 156.4 kWh thermal - 70.2 kWh thermal
D_win = 86.2 kWh thermal.
D_day = Q_day - (P_net,5F × 24 h)
D_day = 391.1 kWh thermal - (11.7 kW thermal × 24 h)
D_day = 391.1 kWh thermal - 280.8 kWh thermal
D_day = 110.3 kWh thermal.
Usable module capacity is the next correction. TEHQ publishes 108 kWh thermal for the 700-gallon Thermal Tank at a 63 F delta. The worked DHW case assumes a 140 F charge and a 115 F floor, giving a 25 F usable delta. The floor must be set by the delivery temperature the coil must hold and the exchanger approach; the recommended floor through the TEHQ dual-coil exchanger remains to be confirmed.
Q_mod = V_mod × 8.34 lb/gal × 1 Btu/lb-F × ΔT_usable
Q_mod = 700 gal × 8.34 lb/gal × 1 Btu/lb-F × 25 F
Q_mod = 145,950 Btu = 42.8 kWh thermal at a 25 F delta, or 40 percent of the 108 kWh thermal nameplate at a 63 F delta.
ΔT_usable = T_charge - T_floor = 140 F - 120 F = 20 F.
Q_mod = 700 gal × 8.34 lb/gal × 1 Btu/lb-F × 20 F = 34.2 kWh thermal at a 20 F delta.
N = D_win / Q_mod
N = 86.2 kWh thermal / (42.8 kWh thermal per module at a 25 F delta) = 2.0 modules.
N = 86.2 kWh thermal / (34.2 kWh thermal per module at a 20 F delta) = 2.5 modules.
Specify three modules. Three provide 128.4 kWh thermal at a 25 F delta or 102.6 kWh thermal at a 20 F delta. Both exceed the 86.2 kWh thermal window deficit and absorb the published standing loss of roughly 2.4 F per 24 hours per module.
Q_loss = V_mod × 8.34 lb/gal × 1 Btu/lb-F × ΔT_loss
Q_loss = 700 gal × 8.34 lb/gal × 1 Btu/lb-F × 2.4 F per 24 h
Q_loss = 14,011 Btu per day = 4.1 kWh thermal per module per day, derived from the published temperature-loss figure.
t_storage = Q_storage / D_hr
t_storage = 128.4 kWh thermal at a 25 F delta / 14.4 kW thermal
t_storage = 8.9 h.
The 24-hour design day exposes the limit. Outside the evening window, the heat pump produces Q_off = P_net,5F × t_off = 11.7 kW thermal × 18 h = 210.6 kWh thermal. The off-window load is Q_load,off = Q_day - Q_win = 391.1 kWh thermal - 156.4 kWh thermal = 234.7 kWh thermal. On a day that holds 5 F around the clock, the heat pump never recharges the tanks.
Three modules charged on the preceding milder day, providing 128.4 kWh thermal at a 25 F delta, carry the 110.3 kWh thermal design-day deficit once. On the second consecutive design day, storage is empty and the deficit lands on backup. Storage buys the first design day; it does not buy the second.
The ride-through concept in Sizing Thermal Storage for California's 4 to 9 p.m. Peak Window applies with the duration set by the weather rather than the tariff.
Backup at COP 1.0: what the immersion element costs
Each module carries an immersive electrode element rated at 10 kW. Three modules provide 30 kW of resistance backup at a COP of 1.0, controllable over the Modbus register interface from the BAS.
t_element,win = D_win / P_element
t_element,win = 86.2 kWh thermal / 30 kW thermal = 2.9 h.
E_element,win = D_win / COP = 86.2 kWh thermal / 1.0 = 86.2 kWh electrical.
t_element,day = D_day / P_element
t_element,day = 110.3 kWh thermal / 30 kW thermal = 3.7 h.
E_element,day = D_day / COP = 110.3 kWh thermal / 1.0 = 110.3 kWh electrical.
E_HP = D_day / COP
E_HP = 110.3 kWh thermal / 2.1 = 52.5 kWh electrical.
C_penalty = (E_element,day - E_HP) × r_energy
C_penalty = (110.3 - 52.5) kWh electrical × $0.15 per kWh electrical
C_penalty = $8.67 per design day.
A fourth module at the September 2026 list price of $5,798, storage line item only, adds 42.8 kWh thermal at a 25 F delta per charge cycle. If that capacity displaces resistance every time it is used, the saving uses the assumed COP of 2.1 and assumed energy rate of $0.15 per kWh electrical.
S_cycle = Q_mod × (1 / COP_resistance - 1 / COP_HP) × r_energy
S_cycle = 42.8 kWh thermal at a 25 F delta × (1 - 1/2.1) × $0.15 per kWh electrical
S_cycle = $3.36 per cycle.
N_cycles = C_module / S_cycle
N_cycles = $5,798 / $3.36 per cycle = 1,726 cycles.
N_seasons = N_cycles / cycles_per_season = 1,726 cycles / 120 cold-evening cycles per season = roughly 14 heating seasons.
On energy alone, resistance wins. The comparison turns on demand: a 30 kW electrical element array firing inside a billed interval under an assumed $15/kW-month demand charge adds C_demand = P_element,electrical × r_demand = 30 kW electrical × $15/kW-month = $450 in each month it runs. Storage that keeps the array off during the evening window recovers that charge rather than only the energy spread.
Under these stated assumptions, added storage is the cheaper answer for a deficit that recurs nightly and is billed as demand. A resistance element at COP 1.0 or a retained boiler is the cheaper answer for a multi-day design-day deficit the heat pump cannot recharge.
A hybrid is the usual answer in zones 6 and 7: storage sized to the window, elements held for the design day, and the boiler retained where one exists until the electrical service is upgraded. Thermal Storage for Commercial Boiler Electrification covers the service-capacity side of that sequence. Central Heat Pump Water Heaters for Multifamily Buildings covers the swing-tank and recirculation configuration the storage sits inside.
TEHQ's case studies index lists an Apollo EVI air-to-water heat pump project. No performance figures are published there, and none are claimed here.
Three ways to carry the 5 F deficit:
Where the storage earns its cost in a cold climate
Regulatory and market timing, as of the publish date
As of October 2026, the ENERGY STAR Cold Climate criteria of 70 percent capacity retention and COP 1.75 at 5 F are live for air source heat pumps, and NEEP's version 3.1 specification remains a common utility listing basis. Neither covers air-to-water equipment directly. Confirm with the utility which listing, if any, a program requires.
NYC HPD's 140 F at 5 F requirement is a program document, not a code, and applies to projects under HPD programs. Verify the applicable document and version with the AHJ and program administrator in any other jurisdiction.
DOE's Cold Climate Heat Pump Challenge field results cover residential ducted equipment, and the DOE heat pump field database continues to add datasets. Re-check both before citing field figures in a submittal.
Seven checks before you specify anything
Frequently Asked Questions
How much capacity does an air-to-water heat pump lose at 5 F?
A published R290 curve at a 131 F supply falls from roughly 19.7 kW thermal at 47 F to roughly 13.0 kW thermal at 5 F, a 34 percent derate before defrost. These values are linear interpolations between the published 10 C / 0 C and -10 C / -20 C points, respectively. ENERGY STAR's Cold Climate designation sets a floor of 70 percent retention at 5 F for air source units; most air-to-water units are not listed under it. The derate at a 140 F supply is larger.
How do you size thermal storage for a cold-climate heat pump?
Subtract the heat pump's net capacity at the design ambient after defrost from the load in the window, integrate over the window, and divide by usable module capacity at the actual delta: 42.8 kWh thermal per 700-gallon module at a 25 F delta against 108 kWh thermal at the 63 F rating delta.
Does storage replace backup heat in climate zone 6 or 7?
No. Storage covers a deficit only for as long as the heat pump can recharge it. On a design day that holds 5 F around the clock, the worked case produces 280.8 kWh thermal against a 391.1 kWh thermal load. Three charged modules providing 128.4 kWh thermal at a 25 F delta carry the first day, and the second falls on the element or boiler.
Is the immersion element or another tank cheaper?
At $0.15 per kWh electrical and a COP of 2.1, both assumptions, a fourth module providing 42.8 kWh thermal at a 25 F delta saves $3.36 per charge cycle against resistance and recovers its September 2026 list price of $5,798, storage line item only, after roughly 1,726 cycles. On energy alone, the element wins. Where a 30 kW electrical element array would fire in a billed demand interval, the storage that keeps it off recovers the demand charge, which changes the answer. Added storage is the cheaper answer for a nightly window deficit under the assumed demand tariff; resistance or a retained boiler is cheaper for a multi-day deficit the heat pump cannot recharge.
Can the heat pump be rated at 5 F for 140 F water?
NYC HPD requires the commercial system, heat pump and storage together, to create and store 140 F water at 5 F outdoor air. The heat pump supplies the daily energy and storage supplies the design hour. Where the heat pump alone cannot carry that design hour, storage must be sized to the deficit and recharge hours documented, including the program's 4-hour recovery requirement. This is a system-level program requirement, not an assertion of an individual unit's rating or project eligibility; verify the applicable document and version with the program administrator.
Size the heat pump for the day and the storage for the hour
Cold-climate air-to-water design depends on sizing the heat pump for daily energy and available recharge hours, with storage carrying the peak hour. In the worked case, a 20 kW nominal unit that runs 19.9 hours at 47 F leaves an 86.2 kWh thermal deficit in a 5 F evening window. Three 700-gallon modules providing 128.4 kWh thermal at a 25 F delta cover that window with margin. The 110.3 kWh thermal design-day deficit is covered by storage once and by backup thereafter.
Size the storage to the window, the backup to the design day, and the heat pump to the recharge hours in between. The client should use this information to verify with their engineering services that the proposed solution will satisfy the goals of the project.
