Heat pumps in a cold snap
What a heat pump does on the coldest night of the year.
The Energy Code sizes a heat pump for each area's design low, and it limits when backup heat may switch on. Here's what that means when the frost comes.
The numbers
What the code plans for
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31°F
San Mateo's design low
The heating winter median of extremes the Energy Code's design data give San Mateo.
Source: California Energy Commission, 2025 Reference Appendices, JA2 Table 2-3 Design Day Data (CEC-400-2025-010), last checked 2026-10-22.
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Lockout
for backup heat, at no more than 35°F
Heat pumps with backup heat must lock it out whenever it's warmer outside than that setpoint.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), last checked 2026-10-22.
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90 minutes
defrost delay, at least
On heat pumps with an adjustable defrost delay timer, it has to be set to at least 90 minutes.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), last checked 2026-10-22.
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Down to 5°F
ENERGY STAR's cold test
Cold climate heat pumps carry third-party verified performance at low temperatures.
Source: ENERGY STAR, Air Source Heat Pumps, last checked 2026-10-22.
Normal or not
Backup heat on a cold night
When the Energy Code lets a heat pump's backup heat run, and when it shouldn't.
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Backup heat on a freezing night
Below the lockout setpoint, backup heat is allowed to run.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), checked 2026-10-22.
Normal
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Backup heat during defrost
The controls may run it during defrost, whatever the temperature outside.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), checked 2026-10-22.
Normal
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Emergency heat you switched on
Choosing emergency operation is the other time it may run in milder weather.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), checked 2026-10-22.
Normal
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Backup heat carrying the house
It's meant for defrost, morning warmup and emergencies, not for meeting the design heating load.
Source: California Energy Commission, 2025 Single-Family Residential Compliance Manual (June 2025), checked 2026-10-22.
Book a visit
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Strip heat coming on often
Silicon Valley Clean Energy says strip heat should be avoided here, by air-sealing the house and sizing the system and ducts properly.
Source: Silicon Valley Clean Energy, Electrification Best Panel Practices, checked 2026-10-22.
Look at sizing
Tell us what it's doing. We'll take it from there.
Call and describe the problem: the noise, the smell, the room that never warms up. Or book a visit online.
