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.

 

[EbS-P]

I don't think it's going into defrost mode

That’s an issue. I can hear ours. Stops for 30 -60 seconds then compressor and air handler only for 3-6 minutes the. The condenser fan comes on. The steam is fun to watch. It’s 3 feet from my head in my bedroom

 

[Pinesmoke]

Sounds like probably due to the outside temps it's running in emergency heat mode. That normally has the condenser off and just on the coils. When it's single digit cold I run eheat mode since the condenser running is just a waste of electrons.

 

[begreen]

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).

That's the way our 2 stage system worked when outdoor temps dropped below the set threshold. The system switched over to emergency heat. It was one or the other, not both.

 

[RustyShackleford]

This is happening when it's not that cold outside.

 

[EbS-P]

This is happening when it's not that cold outside.

Above 20F or above 30

 

[Pinesmoke]

Rabbit hole time rusty

Lots of setup in this stat revolving around how it changes over. I assume 2c/3h stages are what you set up from back when you installed this system. The outdoor sensor option caught my attention. If you don't have it, assuming not, there will still be a change over set to drop the condenser and run HS only. This, one coffee in, seems to be the rabbit hole to drop jnto.

Did you write down the setup parameters you used?

 

[EbS-P]

Rabbit hole time rusty

View attachment 345999
Lots of setup in this stat revolving around how it changes over. I assume 2c/3h stages are what you set up from back when you installed this system. The outdoor sensor option caught my attention. If you don't have it, assuming not, there will still be a change over set to drop the condenser and run HS only. This, one coffee in, seems to be the rabbit hole to drop jnto.

Did you write down the setup parameters you used?

But the Tstat was sending two calls. I agree this will take some digging in the heat pump manual and the Tstat.

 

[Pinesmoke]

But the Tstat was sending two calls. I agree this will take some digging in the heat pump manual and the Tstat.

I seem to remember when rusty put this in the "dual signal" was normal to trigger certain modes. If memory serves he got that info from the manufacturer or hvacdirect.

 

[RustyShackleford]

For purposes of addressing my issue, it doesn't matter how the thermostat is set. The bottom line is that the thermostat is asserting Y (or Y and Y2) and W, and therefore the compressor and heat strips should both be running - but they're not. Make sense ?

Seems the only way this could be happening is the compressor is seeing Y and W and shutting itself off.

 

[Pinesmoke]

Rusty to sort this out,aside from what the stat install says, is a function of the command encoding in the system. Talk to techs to understand what the firmware in the system is doing. All I have to work with is the stat manual, which isn't specific to your system (a very good stat just not tailoredto the system)

Don't take this wrong rusty but "For purposes of addressing my issue, it doesn't matter how the thermostat is set." ties us into a control issue that can only be cleared up with deep system information as to how the system interprets the stat commands.

 

[EbS-P]

For purposes of addressing my issue, it doesn't matter how the thermostat is set. The bottom line is that the thermostat is asserting Y (or Y and Y2) and W, and therefore the compressor and heat strips should both be running - but they're not. Make sense ?

Seems the only way this could be happening is the compressor is seeing Y and W and shutting itself off.

There could be a situation where if the compressor is running you could over heat the strips. When my strips kick in it also sets blower to max speed. My thinking was/is that to keep the strips from overheating.

 

[begreen]

There could be a situation where if the compressor is running you could over heat the strips. When my strips kick in it also sets blower to max speed. My thinking was/is that to keep the strips from overheating.

Our system was the same.

 

[begreen]

This is happening when it's not that cold outside.

At what temperature? Our system crossover was set at 25ºF. The switching temp should be set to when the output of the heatpump is less than the heat loss of the house.

 

[RustyShackleford]

Don't take this wrong rusty but "For purposes of addressing my issue, it doesn't matter how the thermostat is set." ties us into a control issue that can only be cleared up with deep system information as to how the system interprets the stat commands.

No problem, you are entirely correct.

 

[RustyShackleford]

There could be a situation where if the compressor is running you could over heat the strips. When my strips kick in it also sets blower to max speed. My thinking was/is that to keep the strips from overheating.

It definitely kicks into a higher fan speed when the strips come on.

 

[RustyShackleford]

At what temperature? Our system crossover was set at 25ºF. The switching temp should be set to when the output of the heatpump is less than the heat loss of the house.

Yes, but again, the thermostat is calling for compressor AND strips and they're not both on. I think it's what @Pinesmoke said above in post #16. It's something about the way the system is interpreting the thermostat's commands, most likely the compressor.

 

[EbS-P]

It definitely kicks into a higher fan speed when the strips come on.

Do you have a manual for the unit?

 

[RustyShackleford]

But ... news flash ... I gave up on waiting for the thing to kick into compressor+heatsrip mode and hard-wired it. That, is, I disconnected the 'W' wire from the thermostat and shorted it to 'R' and everything worked fine (compressor and heat strips both running). Then I put everything back to normal and a bit later noticed the thermostat calling for compressor+heatstrips and again everything seemed correct.

So I dunno what the heck. It's much warmer now (about 45 degrees) than when I noticed the problem. Maybe it was defrosting when I noticed the problem ? According to "google AI overview" the fan doesn't run when in defrost mode, which I guess makes sense. (The reason I thought the compressor wasn't running was looking out the window and seeing the fan standing still - didn't notice if I heard the "growl" of the compressor itself or not).

 

[RustyShackleford]

Do you have a manual for the unit?

Yep, attached 'em here (surprised the site allowed it) if you're spirit is feeling strong enough to actually look at them. I have, and can't really find anything that would explain what I saw.