HVAC question...

[GeorgeG.]

I'm helping, or at least trying to, someone with their heating system...hoping someone here is knowledgeable in this area.

They have heat but when it reaches temperature, the t-stat will switch to auxiliary heat and the unit keeps blowing air, just cold. The t-stat is set to Auto. Any ideas where to start before calling a technician?

[4.0x3.48]

What kind of thermostat? I had a digital Honeywell that used to do same thing. Ended up being configured for a multistage heat pump, but it was wired to a single stage gas furnace. Changed a setting or two and it was all good.

[juiceweezl]

Sounds like a heat pump. Confirm if so first. Aux heat is a heating coil where the blower is (usually attic). Today we had sub freezing temps and precip. If the outdoor unit ices up, the heat pump will have to defrost before it functions. The aux heat works like an old school electric heater at that point -- air blowing across hot coils. If the aux coils are bad, it will just be cold air. I had to replace them at my rental a few years ago. Have them check to see if the coils are bad.

It could also be the controller, but I'd bet the coils are bad. This is assuming heat pump which are the systems that have aux heat. This is why heat pumps aren't used often in cold climates that have harsh winters. We don't have a ton of this type of weather here, so they work well.

[Stangs 4 Life]

Aux heat is active on a Heat Pump system whenever the outside temp drops below a preset range. It is usually around 38 degrees. The heat pump loses efficiency as there is not enough exchange heat in the outside air to allow the unit to operate without risking damage.

If not heating when on aux heat, then 1 or more of the heat strips may not be activating. They are usually 5kw strips in the air handler section and are sequenced to turn on a few minutes apart from each.

[GeorgeG.]

Thanks for the info guys, this is very helpful. I've never dealt with a heat pump system so I wasn't familiar with how they work. I assumed theirs was a heat pump with the Aux heat showing. I'll look up the model numbers to confirm.

One part doesn't make sense though, and this is assuming they're describing it to me correctly. They told me that it will blow hot air till it reaches the set temperature. During this time, the t-stat does not show aux heat. Once it reaches the temp, the unit keeps running and blows cold air. Wouldn't this mean the heat pump is working and that since it reached the set temp, the blower should turn off rather than switch to aux heat?

I'll stop by and confirm what they're telling me.

[beefed88]

A heat pump it can only transfer so much heat from the outside air so once it warms the house to the maximum amount the heat pump can provide it will annunciate aux heat. The heat strips turn on but they don't heat the house as well as a typical furnace so the air may feel cold but it is actually warm just not hot.

HVAC guys in texas always push heat pumps but if you are a hot house during the winter person a regular unit is probably best.

[kpc01]

Does the fan ever shut off? If not it could also be the fan relay stuck closed. I had the same problem a couple of days ago and it was the fan relay. If you hear the thermostat "click" that is the relays inside it telling stuff to turn off, so if you switch the fan from on to auto it should click then you know the tstat is atleast telling the unit to shut off the fan. Keep in mind some have a 60 second delay on heat that will continue to run the fan to make sure it maximizes the heat already built up in the unit.

[GeorgeG.]

Thanks for the info guys. I'm still working on this. It was basically put on hold due to not having any time but I'm actually taking the day off so we can attempt to pinpoint the issue.

After watching this video, I think I have a better grasp on how it's suppose to work. Using this, I was able to make a few notes with how it's operating now.

1.) when the aux heat turns on, the condenser fan does not turn OFF as it does in the video. Also, that is the only time heat is present. When the aux heat indicator on the tstat turned off, the blower and condenser remained on but there was no heat.

2.) while messing with the tstat, we noticed that when switching to cool and EmHeat, we heard the tstat click and it turned on. It also clicked when it engaged Aux Heat. When we switched it to Heat, no click. So we took the tstat off, checked the wiring, cleaned it up cause it had leaking battery acid and put it back together.

After re-installing it, the mode buttons had quit working so we couldn't switch it to heat, cool, or On or Auto. So we figured the tstat finally gave up the ghost. Went out and bought a replacement Honeywell that supports heat pumps and installed it.

Installed the new tstat, followed the wiring instructions and for a split second, it attempted to turn on the inside unit. It never started completely and now it doesn't turn on with any mode.

Question though... In a heat pump system, does the furnace not have heating coils? Or is this what the Aux Heat is referring to?
I'm headed back over there to at least check the tstat wiring and see if we can get the tstat to at least control the hvac and rule that out. Then we'll assess whether or not a pro is needed.

I may attempt to post up what the tstat wires are wired to in the units...maybe. I know there's capacitors in there and I don't like those.

[GeorgeG.]

Here's another awesome video that explains the wiring. Just thought I'd share.

[beefed88]

Question though... In a heat pump system, does the furnace not have heating coils? Or is this what the Aux Heat is referring to?
I'm headed back over there to at least check the tstat wiring and see if we can get the tstat to at least control the hvac and rule that out. Then we'll assess whether or not a pro is needed.

Yes, there is a small set of coils that are for the aux heat on a heat pump.

[juiceweezl]

The aux heat is a set (or sets) of electric heat coils. They're working if you have heat in aux mode. There's also a controller board that is part of the outside unit. I have had it go bad before too. When mine failed, it basically made the A/C run even if the heat coils were on. Basically, it was stuck in cool mode all the time, even if aux heat was on too.