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heat pump compressor stops running when heat strips activated

R

RustyShackleford

New member
I recently installed a two-stage heat pump with 8kW heat strips. A friend who is not an HVAC contractor, but works on cryogenic systems at a major research university (IOW, he knows what he's doing) helped me with the ductwork and freon. So I'm kinda on my own here.

Anyhow, 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:

[Hearth.com] heat pump compressor stops running when heat strips activated


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.
 
I’m not any kind of expert on hvac, so I got a dumb question, why would the ac compressor need to run when in heat mode?
 
I have a Mitsubishi unit at one of the properties I manage, by design. It will not activate the compressor when the heat strips are active. Maybe worth checking your unit out and see if that is the case.
 
mbsieg said:
Maybe worth checking your unit out and see if that is the case.
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Pretty sure it's not. I've raised the issue with tech support at the vendor (waiting for them to get back to me after they contact mfg'er) and they didn't mention this. Also, here's a snapshot from the compressor installation instructions, which seems to suggest simultaneous operation is a normal thing ...

Screenshot 2026-02-03 at 7.38.20 PM.png
 
Your heat pump is efficient in average ambient temperature. The strips may not be an addition but a backup for when outside air temps are too low. It would depend on the unit's design.

I would suggest that the heat strips and heat Pump combined may draw more power and your installed service cable is not large enough for the amp requirements of both. What is the amp rating on the breaker?
 
FrancSevin said:
I would suggest that the heat strips and heat Pump combined may draw more power and your installed service cable is not large enough for the amp requirements of both. What is the amp rating on the breaker?
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Two separate 240vac circuits, with MCA as specified by the compressor and air handler (plus heat strip) documentation.
 
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 ? Except wouldn't the fan be running during defrost ? (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 said:
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 ? Except wouldn't the fan be running during defrost ? (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).
Click to expand...
At 45 degrees, your heat strips shouldn't be using. That'll kill your electric bill.
My strips don't come on unless it's below about 5 degrees.
 
bczoom said:
At 45 degrees, your heat strips shouldn't be using. That'll kill your electric bill.
My strips don't come on unless it's below about 5 degrees.
Click to expand...
Yeah, I think it was only because I had my set point like 8 degrees hotter than inside temp, trying to get 'em to turn on. I should probably lower my heat-strip lockout temp too.
 
There should be a relay that connects the steps to line power. Just pull the control wire off the solenoid.
 
bczoom said:
What exactly does that do and how do you do it? I've been wanting to disable in whole or part my heat strips.
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Depends on your thermostat. My Honeywell 8000-series has more settings than you can shake a stick at, and heat-strip lockout temp is one of them, as is compressor lockout temp. Also, bunch of settings (strarting on page 11) about how and when it decides to switch to higher stage (if two-stage heat pump) and backup heat (heat strips or fossil-fuel).

Failing that, yeah, pull the low-voltage wire off the relay (put some heat-shrink on it).
 

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