heat pump compressor stops running when heat strips activated

[RustyShackleford]

I've discussed here before the two-stage heat pump I installed a year-plus ago; it also has 8kW heat strips.

This is the first winter (only the system's second winter) where it's been cold enough to require activating the heat strips much (especially since I heat mostly with wood), so I've just now noticed that when the heat strips come on, that the compressor is not running. I've confirmed that the thermostat is energizing the G, W, and Y signals; it seems to me like the compressor should be running. What could be wrong and preventing this ?

Here is the wiring diagram:

It seems to me like the problem must be in the compressor. The Y signals go first to the air handler, but the same conductors go to the compressor; so it's unlikely the air handler is telling the compressor to shut off (it's possible it could be pulling the Y signals low, but that seems unlikely). So I'm thinking that when the compressor sees W energized, it shuts off even though the Y signals are energized. (Of course the compressor needs to be connected to W, because it energizes W when it goes int defrost mode so that there's some heat).

I guess I could disconnect W from the compressor and see what happens.

The whole thing is a little harder to investigate because it's hard to reproduce; I never quite know when the thermostat is going to decide it's time to turn on the heat strips, so I can't observe the codes that the air handler and compressor are flashing on their little LED displays (worse, you have to take the cover off the compressor's wiring area to see its LEDs, in fact I'm thinking about installing a little window in the cover). I'm wondering if I could just force heat-strip mode by shorting R to W, since after all, that's exactly what the thermostat does when it wants to call for the heat strips. Could it damage the thermostat ? I think not, because the compressor can also energize W (when it goes into defrost mode, I believe these are what a EE would call "wired-or" signals). Or, I could change the settings on the thermostat to tell it their aren't any heat strips, but to be safest I guess I should disconnect W from the thermostat.

Anyhow, mainly wondering if anyone has thoughts about why this is happening, and also about my hack to try to reproduce the behavior.

 

[TreeCo]

I though that was normal operation. There is a crossover point where it cost more in electricity to run the heat pump than it does to switch to purely electric heating elements.

Not to mention that running both at the same time may tax the capacity of the electrical circuit to the heat pump.

 

[RustyShackleford]

No, that's not how it's supposed to work. There's completely separate electrical circuits to the air handler and the compressor (both beefy 240vac circuits). Yeah, in the thermostat settings you can say don't run the heat pump if the outdoor temperature is below so-and-so, but this isn't happening when it's nearly that cold (I think I have it set to 10 degrees).

 

[EbS-P]

It could be programmed logic that overrides thermostat. But when it goes into defrost are the compressors and strips are on?

Try raising the set point 5 degrees and observe. My old Trane would kick in that strips if set point t wasn’t reached in xx number of minutes for 5 minutes.

Now with my ecobee I can set the the delta for the strips. I made it 5 degrees.

Make sure you have the AUX heat and the emergency heat functions figured out set up. . AUX runs both and EM runs just the strips. It’s seems like that should be a thermostat setting.

 

[stoveliker]

I had a similar system back in TN (before I took the strips out and installed a nat gas furnace for when the heat pump conked out below 32 F).
And the compressor stopped too - simply because the heat strips are supposed to provide the heat at that point, so no need to run the compressor?

 

[RustyShackleford]

It could be programmed logic that overrides thermostat. But when it goes into defrost are the compressors and strips are on?

Yeah, I think the system is thinking it's smarter than thermostat.
I don't think it's going into defrost mode, but not sure how to tell - probably that LED display on the compressor would say.

Try raising the set point 5 degrees and observe. My old Trane would kick in that strips if set point t wasn’t reached in xx number of minutes for 5 minutes.

Not with my ecobee I can set the the delta for the strips. I made it 5 degrees.

Make sure you have the AUX heat and the emergency heat functions figured out set up. . AUX runs both and EM runs just the strips. It’s seems like that should be a thermostat setting.

It's a very complicated Honeywell 8000-series one. It doesn't have separate AUX and EM terminals. But yeah, two different functions. But it's clearly calling for both, since Y and W both energized.

But I guess you're saying raise the setpoint to reliably get the thermostat to call for both, so I can try to observe what's going on.