Why Is My Heat Pump Blowing Cold Air In Winter While The Heat Is On?


I have found this particular heat pump problem so many times I can’t even count. It’s easy to diagnose and it generally occurs in one of two ways.

  1. The heat pump is in the defrost mode
  2. The heat pumps compressor has failed to start

Although there can be more complicated problems, these are the two most common. Let me elaborate on these two.

Why Does A Heat Pump Need To Defrost?

I really have fun trying to make a heat pump sound simple to understand so I hope you appreciate my attempt at simplicity. So here goes.

A heat pump has two coils that air blows through. These coils are a lot like the radiator in your car. An indoor coil in your furnace and an outdoor coil in, obviously, your outdoor unit. These coils contain refrigerant (Freon). A compressor or pump moves the refrigerant through the system.

A heat pump has the capability of reversing the flow of refrigerant (Freon) in these two coils. Hence, one of the major parts in the outdoor unit of a heat pump is a reversing valve.

In the winter the indoor coil is the hot coil and the outdoor coil is the cold coil. In summer it’s just the reverse, the indoor coil is the cold coil and the outdoor coil is the hot coil.

A compressor (pump) in the outdoor unit creates really hot refrigerant that is pumped through these two coils that have air passing through them. At this point in an effort to keep this understandable you are just going to have to imagine that there are a lot of valves and bypasses that when this compressor in your outdoor unit is pumping hot gases that one coil will be hot and the other quite cold.

When your thermostat calls for heating, the reversing valve makes the indoor coil the hot coil. Now take a moment and think about this ……………………… that’s long enough.

The indoor coil is hot and warming the house. So the outdoor coil is now the cold coil, outside, in the midst of winter weather sometimes below freezing. This outdoor coil is going to be cold to begin with and now we are surrounding it with snow and ice and wind.

Now guess what? Ice begins to form on the outdoor coil and with that ice forming it becomes a blocking mat of ice that slows or stops the air flow through the coil.

To keep your heat pump efficient you have to have air flow. Dirty filters, dirty coils, and ice, they all contribute to inefficiency due to lack of air flow. You’re beginning to more about heat pumps than 90% of the people out there. So now if you’re beginning to get the picture is where the reversing valve does it’s magic.

You are about to see the first reason why your heat pump is blowing cold air when the heat is on. It’s a cold, cold day and you feel that cold air coming out of the registers. Inside the control panel in your heat pumps outdoor unit is a timer on a circuit board.

This timer regulates the reversing valve to initiate a defrost cycle. The defrost cycle reverses the mode of the heat pump just like your thermostat does when you change the mode to cool. Now for a short time the outdoor coil becomes the hot coil to melt the ice.

But, with the outdoor coil being the hot coil, the indoor coil is now the cold coil and you are feeling cold air while the heat is on. This is a reason for a heat pump to have auxiliary heat.

Auxiliary can come in different forms like electric strips, gas, or even oil, but this question leans towards electric. Heat pumps are designed to have the auxiliary heat come on while the units are in the defrost mode to temper the cold air and help defeat that cold air sensation that comes from a unit being in defrost mode.

Your furnace strip heaters may not be coming on or the air may not be tempered enough to defeat the cold sensation. Sometimes only part of the strip heat comes on for defrost. This is something a good service technician can determine.

All in all it doesn’t sound very efficient, does it? A good running heat pump can save a lot of money over a years time. They are most efficient during the spring and fall seasons and that should be considered in you calculations of overall savings. Winter bills can be high when that auxiliary heat is on.

How Do I Know If My Heat Pumps Compressor Has Failed?

When my customers call and tell me there heat pump is blowing cold air, the first thing I ask them is if their outdoor unit is running. Most the time they say yes because the outdoor fan is running. That doesn’t mean the compressor (pump) is running.

It takes a ear that is familiar with the sounds of compressors and fans to determine if the pump is pumping and even then it may take a feel of the units piping or a gauge to tell how well it’s running. So lets just assume that the compressor is dead or not pumping.

We are assuming this because it’s another reason that you have cold air coming out of your registers. But your house is staying up to the temperature or near the temperature set on your thermostat you say?

Now that you are a heat pump expert and know quite a bit more than the average heat pump owner, you should be able to comprehend this. Your fan is running all the time and the strip heat or aux heat is cycling on and off to keep the house up to temperature.

But your thermostat doesn’t know how to tell that the compressor is dead. So it just keeps cycling the electric strip heaters on and off keeping the house sort of warm, spiking up your electric bill and never shuts down completely.

My answer to this second reason for cold air blowing out of the registers while the heat is on is a lot harder to answer. I hope I have done a decent job. Your comments are welcome and I do a fairly good job at responding, so please let me know if you have questions so that I can add content.

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Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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