HVAC blower running but outdoor unit off

I just noticed that my air handler is blowing warm air in heat mode but the outdoor unit is not running. The temp outside is 38 degrees and my thermostat is set for 74 degrees. Out of curiosity I turned on the air conditioner and the outdoor unit kicks on as expected. Is this normal? I recently replaced my old thermostat with a new programmable digital thermostat and I’m wondering if, perhaps, the handyman connected something wrong. Anyone familiar with this?

What type of heat do you have?

It sounds like you have a heat pump/AC. With a 38 degree outdoor temperature many of these units switch to resistance heating. Be prepared for a high electric bill.

Sometimes, if the humidity is high enough outdoors, and the temperature is low enough, the outdoor unit will ice up. I am not sure if every brand handles that situation the same way. Seems like the units have a special defrost cycle, but I don’t remember what the blower does while it is doing that.

And they are not kidding the backup resistance heating is expensive. OMG!

Yes, it is a heat pump/AC. If my bill from last month is any indication I know that resistance heating is expensive! But, before I switched out my thermostat, the outdoor unit was running constantly. Now, at least when I have checked today, it is not running at all outside. Is it normal for a heat pump to just stop working and let the resistance heat do all the work?

No ice on the unit…it’s plenty dry out there.

What is the outside air temperature?

If it is cold enough out side the compressor shuts off and resistance heat is used for heating. Sorry I do not know the setings. I live in Calif so the units I have seen do not have resistance heat. That and most of the heat pumps I work with are water source heat pumps in buildings.


There is a tremendous amount of heat in 38° ambient.

At that temperature I would expect very few heat pumps would need electric back up heat.

You’re not giving us enough information to give you much more than a WAG.

Do this:

Run your stat set point down to the actual temperature in the house; so that the stat is satisfied. Everything should shut off, including the indoor blower.

Now, increase the stat set point so it is only 1° above the actual temperature in the room. (as indicated by the stat) Essentially you’re creating a call for heat.

What happens?

It is possible that the OP noticed exactly what you’re describing. The OP may have seen the unit when it was in defrost cycle.

Not all thermostats will work with heatpumps. If you have the directions for yours, read them carefully. If not., check the manufacture’s website.

Sorry, I really don’t know enough about HVACs to know what information is important. I did what you requested and, no, the outdoor unit does not come on after I set the temp to RT and then 1 degree over RT. The air handler comes on and is moving warm air. I have checked the outdoor unit numerous times during the day and it has not been on once. As a side note, both of my neighbor’s outdoor units are humming away.

As I noted earlier I switched to AC mode and set the temp below RT and the outdoor unit kicked on. So, the unit is not completely out of service and, at some level, my thermostat can connect with the outdoor unit. Before I had the thermostat switched out the outdoor unit ran (as far as I can recall) every time the heat kicked on.

That is an excellent point. I learned that while shopping for my thermostat. This particular one does work with heat pumps.

Here in Central Texas, the temp got down below 20F, with snow, and both of my heat pumps (central units, one upstairs, one down) continued to function. We have electric heating strips, but they have very rarely been used in the ten years we’ve been in this house.

Is your thermostat the old mercury switch style? My older heat pump unit uses a mercury switch thermo. It has two levers on it, one for heat and one for cool. You might try taking the cover off and blowing any dust out of there. Maybe the contacts are dusty on the heat side. If your 'stat is a fancy electronic one (like our newer one upstairs), it uses electronic magic to cycle the unit on and off. No user serviceable parts in there.

Since you made that comment I went to double check the manual. It DOES work with heat pumps but DOES NOT work with electric baseboard heat (does not apply to me) or multi-stage heating/cooling systems. I know the air handler contains heat packs that act as backup heat. Is that considered a multi-stage heating system? If so, I’m screwed.

Heat pumps can be difficult to diagnose in person let alone over the internet, but heres my hunch:

The thermostat hasn’t been properly configured yet for heat pump use.

Heres why I suspect that:

A heat pump stat will energize the “Y” terminal in both heating and cooling. The “Y” (yellow) terminal is the compressor. In heating, the Y, and G (green/fan) terminals are energized. (In cooling an additional terminal, “O” is energized to create cooling vs heating but thats probably not an issue at the moment)

The fact that the compressor comes on when you set it to cooling, and not for heating, suggests to me that the Y terminal is only being energized on a call for cooling. If thats the case, its likely that the stat hasnt been yet set up for heat pump use.

Try this:

Set it to cooling and run the stat down to call for cooling. There are 2 copper lines that run from the indoor unit to the outdoor unit. The larger of the 2 lines has some black foam insulation over it. Set the stat to call for cooling. After 60 seconds or so, feel the larger of the 2 copper lines. (you’ll need to feel at a point where theres no insulation, or cut a slit in the insulation to get a couple fingertips right onto the copper)

That larger line will be distinctly warm/hot, or distinctly cool/cold.

Which is it?

You do need to check the configuration of the thermostat as well. My new one leaves the fan running for 3 minutes after the outside unit shuts off, and also cycles on for 5 minutes or so every 20 minutes to keep the air stirred up in the house.

I’ll check as soon as I get home from work and report back. I have a feeling that the new thermostat is wired incorrectly. I’m going to check that ASAP as well.

Its possible its wired incorrectly, but from what you’re describing, I suspect the way its set up.

Okay, I did what you said and the larger copper line is cooler in this mode. As a side note the smaller one is much colder.