Why Does My Furnace Fan Keep Running On My Heat Pump?

Your furnace fan keeps running because there is a problem with your heat pump. The thermostat thinks the heat pump can’t keep up so it turns on the auxiliary heat until the house warms then back to the heat pump that’s not working and the fan keeps running while the compressor produces no heat.

Invariably, the answer falls upon a problem with the outdoor unit or the heat pump itself.

But, the simplest one reminds me of a story which happened years ago that was several miles away for the technician to drive and when he arrived the customer explained the problem, which is our topic here.

His fan wouldn’t shut off.

This guy must have been 6 foot 5 inches and the thermostat was about five feet off the floor. The switch for the fan was under, on the bottom, of the stat.

It was more than a simple effort for him to look at the underside of the thermostat, because if he had, he would have seen the fan switch was on.

I’m sure you are beyond that. At least now.

The settings for the fan on the thermostat are usually “on” and “auto”.

Some thermostats have another option called “circ” or circulate. On the circ setting your fan will cycle on and off about every ten minutes.

Some folks like that setting to filter the air more often and not have the fan on continuously.

Check out the list below and then continue to follow along for some in depth answers to get your fan under control.

  • Problem with thermostat
  • Fan switch on
  • Breaker to outdoor unit tripped
  • Outdoor unit failure
  • Stuck relay
  • Shorted thermostat wire

By following along and doing a few simple checks you could save some embarrassment of service call that really wasn’t necessary.

Knowing just a few simple things makes you a much smarter consumer and you are much less likely to have some wool pulled over your eyes.

Is My Thermostat Keeping The Fan Running?

The first thing most people think is wrong when almost anything malfunctions with a heating or cooling system is that thermostat is broken.

That is hardly ever the case. But, it does happen.

Next, check all your breakers. If your not familiar with how a breaker trips it can be simple to check. Locate your panel and open the door. If all breakers are on they will each be turned in the same direction top to bottom.

If one is sort of in the middle and by moving it slightly if feels like a loose tooth, you have a tripped breaker. Switching that breaker to the off position and then back on will reset the breaker.

If the breaker continues to trip, you have a short in the system and you should call a technician.

Sometimes the breaker itself could be defective.

Will Understanding Heat Pumps Help Me Solve My Fan Problem?

This article is about heat pumps, so let’s understand a few things to make it clear what we are talking about.

Because your heat pump both heats and cools, the terms evaporator and condenser are not used anymore. An AC only unit has an evaporator (indoor coil) and a condenser (outdoor coil).

Furnace and AC systems can be referred to as indoor and outdoor units to be clear. It just makes talking about your system much more clear.

An example of when this does not apply would be when discussing a water source heat pump.

Water source heat pumps have an indoor unit only and the outdoor unit is replaced by a water source.

Coils are the things we refer to that the air blows through. So we refer to the heating and cooling coils as indoor and outdoor coils or units.

Between these two coils are two copper refrigeration lines. One, normally 3/8 inches in diameter and the other larger. The larger line is usually, in most residential homes, between 3/4 and 1-1/8 inches and covered in pipe insulation, hopefully.

You have a fan pulling air through the coil in your outdoor unit and a fan blowing air through a coil in your indoor unit.


In the winter the large line should be very hot, almost to the point you can’t hold your hand on it for long. And that temperature will depend on the outdoor or ambient temperature.

The colder the outdoor temp the less heat will be created by your heat pump.

In the summer the large line should be pop bottle cold as it cools the indoor coil to keep you home nice and frigid.

These tests are fly by the seat of your pants type tests. Not accurate when it comes to efficiency.

This switching of temperatures in the coils is achieved by what’s called a reversing valve in the outdoor unit. See how you know where that is because I said outdoor unit. Look down inside through the fan, when it’s off, and you can see 4 copper lines all connecting together on that reversing valve. Some units, like Trane, you won’t be able to see the valve unless you remove a panel.

Now we’re able to address the major reason why the fan keeps running.
With the system calling for heat, go to the outdoor unit and listen and feel. So much can be determined with just our natural senses.

There should be two motors running, a fan motor and a compressor. Listen for them both. The fan noise is primarily air being moved through the coil. The compressor noise is very distinctive, like the compressor on a refrigerator. Probably an older refrigerator.

If you can hear that compressor, then reach down on the outside of the unit and feel that large copper line. Make sure it’s had a little run time so it can heat up. If it’s getting pretty warm then our fan problem is elsewhere.

Skip on down in this article to the next bold line if the large copper line on your heat pump is nice and hot.

If the line stays cool and your not sure if you can hear that compressor then we’ve found what could be our reason for a constant indoor blower fan.

First stage on your heat pump thermostat is to run the outdoor unit. Second stage is to run the supplementary or auxiliary heat.

When no heat is being produced by stage one then stage two takes over to heat your home. When the temperature setting is satisfied, then stage 2 shuts off and the fan keeps running with stage 1, which is still producing no heat.

This is when you use that mode on your thermostat that says “EM HEAT”. That expensive electric strip heat stuff that you’ve been using anyway and didn’t know it.

With your system in emergency heat mode your furnace will cycle properly and the fan will shut down between cycles.

Outdoor Unit Runs Normal But Fan Still Runs Constantly

The system checks we’ve performed so far are touch, feel and listen. Pretty easy stuff for the typical home owner. Digging deeper requires some troubleshooting skill and a familiarity with electricity.

If you have those skills, there’s little sense in me going further. You’re going to find a stuck relay or a shorted thermostat wire.

Without those skills, you’re at the point where a technician is required. But, now you’re more knowledgeable and you’ve become familiar with system to the point that you will not be that 6 foot 5 guy who had to pay a technician for 2 hours labor to cover his travel time to flip a switch.

Hopefully you have a good technician on the hook that has the skill and parts in the truck to service your equipment in a reasonable time for a reasonable amount of money.

Don’t quibble over the price of a service call too much. It’s so expensive these days to stock a truck, buy insurance so just pay the man. By paying his price you insure he stays in business to service you the next time around. It needs to be reasonable.

Finding and knowing you have a good technician is a topic for another complete article. All companies make mistakes. It’s the good company that stands up to those mistakes and puts you, the customer first.

And when the customers wrong, he’s still the customer.

Chad Peterson

Chad Peterson is a veteran of the HVAC industry since 1977. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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