What triggers a heat pump's auxiliary heat to come on?

[LeeH]

I've searched all over and can't seem to find the answer to a simple question. What causes my heat pump's aux heat to come on? Mine is seemingly not working. Temps here in Phoenix got in to the low 30s last night. My thermostat is set to 70, but the house is 65.

The outside unit is running, but I think the problem is that the auxiliary electric heat is just not coming on for some reason. Funny thing is that our ancient Honeywell Chronotherm III thermostat does not have an indicator for aux heat. Everything I can find online about these thermostats suggest there is a heat pump version (with LED for aux/emergency heat) and one for standard systems. Can't image we've lived in this house for 16 years with a thermostat that's not compatible with a heat pump.

Does the interior thermostat trigger aux heat or does the outside unit have its own thermostat that triggers it? I don't want to go buy a new thermostat if that's not what's causing the problem.

[VincentVega]

Setting in the thermostat to initiate aux heat, usually configurable for desired temp.

[billybek]

You have reached and exceeded the balance point for your heat pump.
That is the available heat from the pump diminishes with a drop in outdoor ambient temperature until the heat required for keeping the house comfortable is no longer available. The unit will run continuously after the balance point.
There should be a second stage on the thermostat that will bring on your auxiliary heat be it electric strip/element or nat gas or propane.
There may be other ways of doing this too depending on the unit and its age.

[porsche4life]

Brrr! 65 in the house? I wouldn't be able to function! I'm no good at diagnosing, but if you need help swapping parts, let me know!

[LeeH]

Originally Posted by billybek

You have reached and exceeded the balance point for your heat pump.
That is the available heat from the pump diminishes with a drop in outdoor ambient temperature until the heat required for keeping the house comfortable is no longer available. The unit will run continuously after the balance point.
There should be a second stage on the thermostat that will bring on your auxiliary heat be it electric strip/element or nat gas or propane.
There may be other ways of doing this too depending on the unit and its age.

The heat pump is only a couple of years old. The interior thermostat is from 1995.

What's odd to me is that the interior thermostat has no indicator for aux. heat. The documents I can dig up from Honeywell would suggest that there was a version of the Chronotherm III thermostat specifically for use with heat pumps. Mine's not that one. We've had cold weather in the past and this hasn't been an issue. What I can't remember is if we've had cold weather since the new heat pump was installed.

[porsche4life]

It was this cold last year right after christmas, if that helps...

[billybek]

Was the entire system changed out at that time or just the condensing unit section?

[LeeH]

Originally Posted by billybek

Was the entire system changed out at that time or just the condensing unit section?

Both ends. Air handler is in the attic.

[porsche4life]

If you decide you need a pro, I've got a guy who is honest, and not ungodly overpriced.

[billybek]

Ok the product nomenclature that you have given me indicates that there is no auxiliary heat installed on the air handler. Now that doesn't mean it isn't there and was added as a kit later on.

[LeeH]

Originally Posted by porsche4life

If you decide you need a pro, I've got a guy who is honest, and not ungodly overpriced.

It's probably going to come to that.

It's a Carrier that was installed in 2011. The compressor was replaced exactly one year ago under warranty. "Under warranty" is amusing as diagnostics, labor, freon aren't part of an a/c warranty so I paid $800 out of pocket.

I'd like to upgrade the thermostat, but that's another issue. As I understand it, the new thermostats require a dedicated power line. I have an unused wire, but am confused about the proper set up. The power wire is labelled "C". Well, the terminals on my thermostat have "C" as unused, but outside, "C" seems to be attached.

[billybek]

If it is easy to access open the air handler/fan coil panel and look for the resistance heat strip. It should be quite large on the section towards the supply fan.
It could be a control issue but I would make sure that there is resistance heat there before you start chasing control problems.
It also looks like that you have a 5 ton air handler and a 3 ton condensing unit.
That is quite common in dry areas to oversize the evaporator for the A/C but it doesn't do much for the heat output.
I have an exam to run, will check back later to see how you are doing.

[LeeH]

Ok. So Bill, looks like you're correct. No aux heat installed per the guy who installed it in 2011.

He's going to come check it out shortly.

[LeeH]

Verdict is that it is low on freon due to a leak in the coil. Access to the air handler is through a scuttle that's too small for the coil so...




Really not happy with Carrier. $800 out of pocket for last year's compressor that was under warranty and now $1100 for the coil that is under warranty.

[cabmandone]

Okay, but that doesn't explain why your secondary heat didn't kick in. Your chronotherm should have a red light that illuminates when stage 2 (emergency heat) activates. You should have a swich to go between Heat, Cool, Auto and E Heat. Set it to E heat and see if the electric resistance heat is working. I'm guessing it isn't because at 5 degrees below set point your auxiliary heat should have kicked in. I installed Carrier for several years and can do some research on the system but I'm thinking yours should have an Optimizer for dual fuel (heat pump and electric resistance) but I can't recall exactly anymore. It has been several years since I've installed a heat pump with and air handler and electric resistance heat.

Correction: I think the older chronotherm had a red light on the sub base that displayed when auxiliary heat is on. On yours it might say "aux" or "E heat" when the backup resistance heat is on. Question, did your A/C work okay throughout the summer?

From the manual for the stat
STATUS LIGHTS—Lights that show system operation
settings.
SYSTEM light (yellow)—This light glows whenever
the thermostat is calling for heating or cooling
(located on thermostat).
AUX HT light [T8631R only] (green)—This light
glows whenever the thermostat is calling for operation
of the backup heater. Backup (auxiliary) heat is
more expensive to operate than the heat pump and
typically is used only when the heat pump is unable
to handle the load (located on subbase).

[LeeH]

There is no aux. heat installed on this unit. Seems lots of them get sold in AZ without them.

[porsche4life]

Come to think of it, I don't know that we have strip heat. I've never kicked it on if we do....

[cabmandone]

Originally Posted by LeeH

There is no aux. heat installed on this unit. Seems lots of them get sold in AZ without them.

Seems odd to me but I suppose the climate is mild enough that a good working heat pump would be sufficient 90% or better of the time.

BTW, I suppose the system not having any backup heat would make it difficult for the system to stage it in

Also, I would ask them to show you exactly where the leak at the coil was. I have almost NEVER seen an indoor coil leak unless the coil was in a home with sulfur water. You mentioned the system was only 2 years old or so which makes me doubt the claim of a coil leak even more. My guess is, when they brazed the lines for the evaporator coil, they didn't wrap the metering device and most have a nut with a rubber O ring that will melt if you don't keep them protected while brazing.

[Fischer_76_911S]

Sounds like it was never properly installed, not Carrier's fault. If a good vacuum was not pulled and the lines not flushed out (if it was a replacement and still using the old refrigerant lines for the flushing part) the air left in the lines will mix with the refrigerant and form an acid. That acid will slowly eat away at the epoxy coatings on the compressor windings until it fails. It will also eat away at your coils. It will do this silently and slowly bring death to your HVAC equipment

A properly installed system, almost any brand, will last at least 10 years. The good ones, like Carrier, should go for 15-20 and sometimes well beyond. These numbers are generated for my part of the country after 20 years in the biz.

Just hope you have a good tech - it is usually the ONLY thing separating you from a good system versus a money pit.

My $0.02

[cabmandone]

Originally Posted by Fischer_76_911S

Sounds like it was never properly installed, not Carrier's fault. If a good vacuum was not pulled and the lines not flushed out (if it was a replacement and still using the old refrigerant lines for the flushing part) the air left in the lines will mix with the refrigerant and form an acid. That acid will slowly eat away at the epoxy coatings on the compressor windings until it fails. It will also eat away at your coils. It will do this silently and slowly bring death to your HVAC equipment

A properly installed system, almost any brand, will last at least 10 years. The good ones, like Carrier, should go for 15-20 and sometimes well beyond. These numbers are generated for my part of the country after 20 years in the biz.

Just hope you have a good tech - it is usually the ONLY thing separating you from a good system versus a money pit.

My $0.02

I've got a General heat pump. It was a generic made by Carrier with a brown outer color and a big white star on top. I installed the thing in 1996 and it's still chugging away today. I agree, this doesn't sound like Carrier is to blame here.