Off Duty Heating and A/C · Granite Shoals, TX · From the truck

Steam Pouring Off the Heat Pump? It Is Probably Supposed To.

A front has come down the lake valleys overnight, and fog is still sitting on Lake LBJ. You look out the back window and a cloud of steam is rolling off the heat pump. The outdoor fan has stopped and the unit is making a loud whoosh. The first thought is fire, or at least a dying compressor.

Most likely you are watching a defrost cycle, and in damp lake air those come around more than people expect. Once you know what normal looks like, it gets a lot easier to spot the morning a heat pump actually needs help.

Why a Heat Pump Frosts Over

In winter a heat pump runs backward from summer. The outdoor coil gets colder than the air around it so it can pull heat out of that air and carry it indoors. When the outside air is cold and carrying moisture, that moisture freezes onto the coil, much like frost forming on a truck windshield overnight.

A light layer of frost is normal. A thick one is a problem, because ice blocks the air the coil depends on, and a coil that cannot breathe cannot collect heat. So every heat pump has a way to clear itself.

Close to the water, winter mornings tend to be damp as well as cold, and that pairing is when frost builds fastest. Heat pumps farther back from the lake frost up too. They just have less moisture to work with.

What Defrost Looks and Sounds Like

During defrost, the system briefly flips into cooling mode. Hot refrigerant runs through the outdoor coil and melts the ice off it. The outdoor fan usually shuts down so that heat stays on the coil instead of blowing out into the yard.

From outside, it looks dramatic. Steam rises off the cabinet, water runs out the bottom, and you may hear a sharp whoosh as the reversing valve shifts at the start and again at the finish. Indoors, the air at the vents can feel cooler for a bit, which is why most systems bring on backup heat during defrost.

Then the valve shifts back, the fan spins up, and the unit returns to heating. On a raw morning the whole routine can repeat several times. None of it means anything is broken.

Backup Heat That Will Not Shut Off

Most heat pumps around here carry electric backup heat, strips inside the air handler that help on the coldest mornings and during defrost. They are meant to be a helper, never the main source of heat.

When the backup runs constantly, something is off. The thermostat might be configured wrong, or set to emergency heat by accident. The outdoor unit might not be running at all, leaving the strips to carry the house. A low refrigerant charge or a failing defrost control can also leave the heat pump so weak that the strips never get a rest.

The house may still feel warm, which is why this one hides so well. The clues are usually a thermostat light for auxiliary heat that stays lit or an electric bill that jumps for no reason. If that sounds familiar, our heat pump repair page explains how we track it down.

When the Ice Is a Real Problem

Defrost should leave the coil clear. If the whole outdoor unit stays encased in ice, top and sides included, and a cycle does not clear it, the heat pump needs attention. So does a unit that frosts over on a dry, mild day.

Usual suspects include a defrost sensor or control board that stopped doing its job, a low charge, an outdoor fan motor that is not turning, and a reversing valve that sticks. Meltwater that cannot drain away from the base can refreeze and build ice from the bottom up, too, especially when the pad under the unit has settled.

Do not chip at the ice with anything sharp. The fins and tubing beneath it are thin, and one puncture turns a control problem into a refrigerant leak. Switch the thermostat to emergency heat, or off, and call (512) 925-3090. Someone answers 24 hours a day, and a hard front is exactly the kind of morning that phone line exists for.

Weekend owners have one more thing to watch. A heat pump at an empty house can ice up and struggle for days with nobody around to notice, while the backup strips quietly carry the load. A connected thermostat with freeze alerts shows the indoor temperature slipping from wherever you happen to be, so a stalled heat pump turns into a phone call instead of a cold surprise when you unlock the front door on a Friday night.

Lake Air and the Outdoor Coil

Heat pumps near the water face the same wear as any outdoor unit in lake air, with one difference: they work every month of the year. Humidity sits on the coil all summer and freezes onto it all winter, and over the years the fins corrode and the coil loses some of its ability to move heat.

A corroded coil frosts faster and clears slower, so defrost comes around more often and the backup heat picks up more of the load. It creeps in gradually, which is why owners tend to describe it as a house that just is not as warm as it used to be. Shrubs and leaf piles crowding the base make it worse, since they hold moisture against the lowest rows of fins, right where meltwater already gathers after every defrost.

Every fall visit includes washing that coil and checking it for corrosion and leaks. We also test the defrost controls and confirm the backup heat comes on only when the house calls for it.

Heat for the Rooms That Never Had Any

Plenty of lake cottages have a closed-in porch or an addition that never got real heat. Owners run a space heater through January and live with a cold floor.

A ductless heat pump head is a clean fix for a room like that. As a Mitsubishi Diamond Contractor, we install ductless heat pumps that warm a single space without any ducts pushed through a tight attic, and they clear their own frost on cold mornings just like the bigger systems.

If the porch room is the coldest place in the house every winter, it is worth a conversation before the next front arrives. Our Granite Shoals page shows the rest of the service we bring to homes around the lake.

Is it normal for my heat pump to blow cool air in winter?

For short stretches, yes, especially during defrost. Heat pump air also feels cooler to your hand than air from a furnace, even while it is warming the house. Air that never warms up, or a house that cannot reach its setting, points to a problem worth having checked.

Should I switch to emergency heat whenever it gets cold?

Not just because it is cold. The heat pump is designed to keep running with backup heat helping as needed. Emergency heat is for when the outdoor unit has failed or is iced solid. Leaving it on otherwise runs the whole house on the electric strips alone.

Why does my outdoor unit make a loud whoosh?

That is usually the reversing valve shifting at the start or end of a defrost cycle, and it is a normal sound. Grinding, screeching, or repeated clicking while the unit refuses to start is something else, and any of those is worth a call so a small problem stays small.

Can I pour hot water on an iced heat pump?

Better not. Water poured over the coil can refreeze in the base and around the fan, and it does nothing about why the ice built up in the first place. Set the thermostat to emergency heat so the house stays warm, then have the cause found.

How can I tell if backup heat is running too much?

Start at the thermostat. Many show an indicator for auxiliary or emergency heat. If it stays lit through mild weather, or the electric bill climbs with no cold snap to explain it, the heat pump may be leaning on the strips. A service visit can confirm it.