Heat Pumps in a Bay Area Winter: Normal or a Problem?

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A heat pump behaves differently from the gas furnace most Bay Area homes grew up with, and a lot of first-winter worry is about things it is designed to do: steam from the outdoor unit, a fan that stops, air that feels warm rather than hot. This page sorts normal from not normal, explains defrost, aux heat and emergency heat, and covers ductless mini-splits in heat mode. It is part of our Bay Area Winter Heating Help Center.

Normal or not? The quick version

Usually normal

• Steam or fog rising from the outdoor unit on a cold, damp morning.
• The outdoor fan stopping for a few minutes while the compressor keeps running, then a "whoosh" when it switches back.
• A thin layer of frost on the outdoor coil that clears on its own.
• Air from the registers that feels warm but not hot, and long, steady run times.
• A mini-split that blows little or no air for the first few minutes after you turn it on in heat mode.
• Water dripping from the outdoor unit in winter.

Not normal: time to call

• Ice that keeps growing, or a coil encased in ice that never clears.
• Air at the registers that stays at or below room temperature.
• "Aux" or "Emergency" showing on the thermostat most of the day in ordinary Bay Area weather.
• The outdoor fan not running while the unit hums, outside of a defrost.
• Grinding, screeching or a breaker that trips again after one reset.
• A burning smell or smoke from either unit. Turn the system off at the thermostat and the breaker.

The winter your heat pump actually faces

Most of our service area is mild. NOAA 1991-2020 climate normals put the average January low at 41.8°F in San Jose, with about three nights a year at or below freezing. The inland valleys are colder: Livermore averages a 37.8°F January low and about 19 freezing nights a year. There is no normals station for Walnut Creek or Morgan Hill, so the table uses Concord and Gilroy as the nearest stand-ins.

For comparison, ENERGY STAR certification tests air-source heat pumps' performance down to 5°F (ENERGY STAR). A properly sized, working heat pump has the capacity for a Bay Area winter, so most winter complaints here come down to defrost, settings, airflow or installation. Cool, damp mornings in the 30s and low 40s are also exactly when outdoor coils frost up, so expect defrost cycles most often then, particularly in Livermore, Pleasanton, Dublin, Gilroy and Morgan Hill.

Defrost: steam, a silent fan and a whoosh

In heating mode the outdoor coil runs colder than the outdoor air, so moisture freezes on it. Every heat pump has a defrost cycle to melt it. The Bryant owner's manual says: "During defrost, you may see steam or fog rising from the outdoor unit, which is normal." Newer Bryant manuals add that at the start and end of defrost "you may hear a 'whoosh' sound" (Bryant Evolution manual).

Here is what happens, per the Carrier/Bryant service manual: the control shifts the reversing valve into its cooling position and "turns the outdoor fan off". Defrost ends when the coil warms to about 65°F "or automatically after 10 minutes." On many systems the control also turns on backup heat during defrost so the air inside does not turn cool. So a few minutes of steam, a stopped fan and a brief change in the air at your registers is the system working as designed. Meltwater dripping from the unit is normal too.

Ice that does not clear

Trane puts the line in the right place: periodic frost "is perfectly normal", but "if you find the ice on your heat pump is not going away, or is growing, it likely means a bigger issue is at play," such as "low refrigerant levels, faulty defrost controls, or a faulty sensor" (Trane). Before you call, check the things that cause ice from the outside:

1. Look up. A gutter overflowing or dripping onto the unit keeps re-freezing water on top of it. Redirect it.
2. Look down. The unit should sit on a pad above the ground so meltwater drains away. A unit sitting in a puddle, or with leaves packed around the base, can freeze from the bottom.
3. Clear leaves, fence boards and stored items from around the unit, and never cover it for the winter. It needs free airflow in heat mode.
4. Do not chip, pry or hose off hard ice. The fins and tubing are thin and easy to puncture.
5. If the coil is encased, turn the system off and call. If you have backup heat, the emergency heat setting (below) can keep you warm meanwhile.

After an atmospheric river, also check that the unit was not sitting in floodwater or buried in debris; see our power outages and storms page.

"The air feels lukewarm"

A furnace delivers short blasts of hot air. A heat pump delivers a steadier, cooler stream. The Bryant owner's manual explains that a heat pump "delivers a constant flow of air at around 90°F (32°C) to about 105°F (41°C)", which "may feel cool if it is slightly below your body temperature, however it is warm enough to keep your home comfortable." Hold a kitchen thermometer in a register after the system has run for ten minutes: air in that range is normal. Air close to room temperature for long stretches is not. Common causes are a thermostat set to Cool or Auto, a reversing valve that is stuck, a failed outdoor fan or compressor, or low refrigerant from a leak. Our guide to how technicians diagnose heat pump problems goes deeper.

Aux heat and emergency heat

Many ducted heat pumps have backup heat, usually electric heat strips in the air handler, or a gas furnace in a dual-fuel system. Aux heat is when the control adds backup heat automatically: during defrost, when the house is far below the setpoint, or in very cold weather. The Bryant Evolution manual says that below 10°F outdoors the control "may automatically shut the heat pump off and energize auxiliary heat" (Bryant Evolution manual), which is far colder than the average lows in the table above.

Emergency heat is a setting you choose. It shuts the heat pump off and runs on backup heat only. Use it only when the outdoor unit has failed or is iced over and you are waiting for service. Electric heat strips cost more to run: ENERGY STAR says a heat pump can deliver "up to three times more heat energy to a home than the electrical energy it consumes" (ENERGY STAR), while a heat strip turns electricity into heat one for one.

If the thermostat shows Aux every morning on a 45°F night, something is off. The U.S. Department of Energy's consumer guidance has said energy codes require controls that prevent unnecessary heat strip operation, and called missing ones "one of the most commonly violated installation practices for heat pumps". A technician can check the thermostat's staging and lockout settings and the strip wiring.

Thermostat setbacks work differently

Deep night setbacks save money with a gas furnace. With a heat pump they can backfire, because recovering from a big drop on a cold morning is exactly when aux heat kicks in. ENERGY STAR is direct: "Unlike a furnace or boiler, heat pumps do not save energy by turning it down when you're away or asleep. For best use, pick a comfortable temperature and leave it there." It also advises against Auto mode, so the system does not switch modes on unseasonably warm or cold days (ENERGY STAR). DOE's consumer guidance said the same about setbacks in heating mode. If you want a schedule, keep the setback small, or use a thermostat made for heat pumps, which uses algorithms to limit backup heat. Our lower winter heating bills page has more.

Mini-splits in heat mode

• Check the remote. Heat mode (often a sun icon), not Auto, Dry, Cool or Fan. Batteries matter too.
• Give it a few minutes. Mitsubishi's "cold air prevention control" stops the indoor fan or runs it very slowly while the unit starts or defrosts (Mitsubishi Electric service manual). Other brands do something similar.
• Aim the louvers down. Warm air rises. Pointed at the ceiling, the heat stays up there.
• Clean the filters. The washable mesh filters behind the front panel clog faster than most people expect. Rinse, dry fully and refit.
• Read the blinking light. A flashing indicator is usually an error code. Note the pattern before you call.

Our common mini-split problems page and mini-split repair service cover the rest. Mini-splits are also how we add heat to homes without ducts, including Eichlers; see our radiant heating and Eichler page.

What a technician checks

We test the defrost control and its sensor, the reversing valve, the outdoor fan motor and capacitor, and the refrigerant charge (our technicians are EPA Universal certified, and a low charge means a leak we find and repair rather than just topping off). We also check airflow, heat strip staging and lockout, and the thermostat wiring. Every repair carries a 1-year parts and labor warranty. Still deciding what should heat your home? Read heat pump vs furnace in the Bay Area.

Book heat pump service

Frequently asked questions

Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.