Heat Pump Not Heating in St. George: Reversing Valves, Defrost, and Cool Air

The first cool mornings arrive in late September, you switch the thermostat over to heat, and the air from the vents feels lukewarm. Every autumn homeowners ask the same question: why is the heat pump not heating the way the homeowner expects in St. George.

Often nothing is broken. A heat pump moves heat instead of burning fuel, and it delivers a large volume of air that is warmer than the room but cooler than your skin. Hold your hand in front of a furnace register and it feels hot. Hold it in front of a heat pump register and it feels cool, even while the house steadily warms.

The rest of the time, something real is wrong. A heat pump not heating can mean a stuck valve, a bad setting, or a leak. Here is how the machine works, so you can tell those apart.

How a Heat Pump Produces Heat

A heat pump is an air conditioner that runs in both directions. In summer it pulls heat out of your house and dumps it outside. In winter it does the reverse, pulling heat out of outdoor air and releasing it indoors.

The part that makes that possible is the reversing valve, a solenoid-operated four-way valve in the outdoor unit that redirects refrigerant flow. Flip it and the outdoor coil becomes the evaporator that absorbs heat, while the indoor coil becomes the condenser that releases it.

Even cold air holds usable heat. Refrigerant boiling at a temperature well below freezing can absorb energy from air that feels frigid to you. Capacity drops as outdoor temperatures fall, which is why these systems include supplemental heat.

Why a Heat Pump Not Heating Is Often Working Fine

Three normal behaviors get reported as failures:

• Cooler supply air. Air leaving the register is warmer than the room but well below what a gas furnace delivers. It feels cool on your hand and still heats the house.
• Longer run times. Heat pumps are designed for long, steady cycles rather than short hot bursts. A unit running for an hour is behaving normally.
• The defrost cycle. In cold, damp conditions, moisture condenses and freezes on the outdoor coil. The system briefly reverses into cooling mode to melt that frost. The outdoor fan stops, steam rises off the unit, and indoor air turns cool for a few minutes until the cycle finishes.

Defrost startles people the first time they see it. A cloud of vapor rolling off the outdoor unit on a cold morning looks like smoke, and it is the system doing exactly what it was designed to do.

Real Causes of a Heat Pump Not Heating

When it genuinely is not heating, this is what we find:

• A stuck reversing valve. The valve fails to shift or its solenoid coil fails, and the system stays locked in cooling mode. The classic symptom is cold air in heat mode with an outdoor unit that runs normally.
• Thermostat configuration. Heat pumps need a reversing valve signal, usually the O or B terminal, set correctly for the brand. A thermostat swapped without that setting heats when you call for cooling and cools when you call for heat.
• A failed defrost board or sensor. The outdoor coil ices over and stays iced, which chokes off the airflow it needs to absorb heat.
• Low refrigerant charge. A leak reduces the system's ability to move heat in either direction, and it shows up worse in heating mode.
• Failed auxiliary heat strips. On cold mornings the electric strips are supposed to supplement. When they fail, the house falls behind even though the heat pump runs.
• Airflow restriction. A dirty filter or a blocked return starves the indoor coil and drops output.

A Toquerville Morning: Cool Air After a Thermostat Swap

Picture a homeowner in Toquerville on an early October morning. Say a smart thermostat went in over the summer, cooling worked fine, and now the first heat call blows cool air that never warms up.

At the equipment, the outdoor unit runs and the compressor sounds healthy. In a case like this, the refrigerant pressures would likely show the system running in cooling mode while the thermostat is calling for heat.

A likely cause is a reversing valve setting in the thermostat configuration that is wrong for that manufacturer. That would be a settings problem rather than a parts problem, and it is worth checking before anyone discusses compressors. A proper smart thermostat installation includes verifying that setting on a heat call, not only on a cool call.

Auxiliary Heat and Emergency Heat Are Not the Same

These two thermostat terms confuse nearly everyone:

• Auxiliary heat comes on automatically to help the heat pump when outdoor temperatures drop or during defrost. The heat pump keeps running. This is normal.
• Emergency heat shuts the heat pump off entirely and heats with the backup alone. It exists for when the heat pump has failed.

Leaving a system on emergency heat all winter because the air feels warmer uses far more energy for the same comfort, since electric resistance heat produces heat rather than moving it. If your thermostat shows auxiliary heat running constantly on mild days, that is itself a symptom worth investigating.

ENERGY STAR's air-source heat pump (opens in a new tab) and heating and cooling (opens in a new tab) pages explain the efficiency logic behind heat pump operation.

Safety and What Not to Do

• Never chip or pry ice off an outdoor coil. Aluminum fins and refrigerant tubing puncture easily, and a punctured coil turns an ice problem into a refrigerant leak.
• Do not repeatedly pour hot water over a frosted unit. It refreezes and can lock the fan.
• Keep the outdoor unit clear of leaves, drifting soil, and stored items so it can breathe.
• Refrigerant work falls under the EPA's Section 608 rules (opens in a new tab) and is not a homeowner repair.

When to Call a Professional

Call for heat pump repair when supply air is genuinely cool rather than lukewarm, when the outdoor coil stays iced through more than one cycle, when the system heats in cool mode or cools in heat mode, when auxiliary heat runs on mild days, or when the unit clunks loudly and then stops heating.

A heat pump not heating is diagnosed with gauges, a thermostat check, and a defrost test. Our heating services team works through those in order instead of guessing at parts.

Frequently Asked Questions

Why does my heat pump blow cool air in heating mode?

Usually because supply air is warmer than the room but below skin temperature, so it feels cool to the touch. Compare the temperature at a supply register against the room temperature rather than judging by feel.

Is it normal for the outdoor unit to steam in the morning?

Yes. That is the defrost cycle melting frost off the outdoor coil, and the vapor is meltwater evaporating. It should end within several minutes.

What does a heat pump not heating usually turn out to be?

In early fall, two common causes are a thermostat configured wrong for the reversing valve and a system low on refrigerant. Both are diagnosed at the equipment rather than from the thermostat.

Should I switch to emergency heat if the house is cold?

Only as a short-term measure while you wait for service. Emergency heat bypasses the heat pump and uses much more energy, so it is a stopgap rather than a setting to leave on.

Does a heat pump work in St. George winters?

Yes. Winters here are mild compared with northern Utah, which is why heat pumps are a practical choice in the valley. Capacity drops as temperatures fall, and that is what auxiliary heat is for.

Get Your Heat Pump Checked Before the Cold Sets In

If you are dealing with a heat pump not heating this fall, call West Desert Plumbing, Heating & Air to schedule a heating visit in St. George. A technician can verify the reversing valve, test the defrost cycle, confirm the thermostat configuration, and check the refrigerant charge before winter.