help...Nest 3rd Gen Tstat with Trane Heat Pump pressure release sound & other quirks

[asomm]

I have two 30 year old Trane XE 1000 heat pumps, one for upstairs and downstairs and just recently installed 2 Nest 3rd gen tstats for each. Everything seems to be working fine as I get cold air when calling for cooling. However, I've noticed a few quirks that I was hoping ppl on the forum might have some insight on.



1. Post installation, I noticed that somewhat frequently but not every time, when the desired temp is reached and the nest shuts off the system, the heat pumps will give off a pressure release sound. I've never noticed or experienced that sound previously so I worry that it is causing additional wear and tear on the heat pumps (whether its the reversing valve or something else)

2. I've also noticed that occasionally that in the middle of a cooling cycle, the nest will occasionally go into delay mode for 3-4 min before firing the compressor back up again and cooling. I know a the delay helps prevent the system from cycling on and off quickly, but is it odd that it is kicking into delay mode in the middle of a cooling cycle/?

3. The wiring seemed a little non-standard when I looked at the wiring at the air handler and compared to the tstat. I have photos of old tstat, nest wiring, and air handler if someone can confirm that this looks right?
Downstairs heat pump wiring



upstairs heat pump wiring




4. If I want to add a common wire, for the downstairs air handler is it simply wiring the blue wire to the B terminal at the air handler? For the upstairs air handler, the G wire for fan doesn't even seem to be wired. Would it be wiring the G wires from heat pump and tstat to G terminal at air handler, and then taking the blue wire off of T and wiring to the other blue wire on the B terminal at the air handler?

5. lastly, our heat pump for upstairs actually went out. HVAC guy came out and said that the compressor and wiring to the compressor was fried and needed replacement. I asked him if the Nest had anything to do with it and he said no, not likely. Has anyone else ever experienced or have had compressor burn out due to the Nest? I am wondering if there is a correlation to the pressure release sound I referenced in bullet 1.

[Stuff]

1. Lots of times the sound is low refrigerant in the system. Verify the O/B is set to "seasonally." Also adding a C wire keeps Nest from triggering the valve when it shouldn't.
2. This might be Nest needing to power itself (fixed with a C wire). Might be condensate line blocked and shutting things down until it drains enough to clear the sensor.
3. Trane is known to be non-standard. Your connections to Nest look correct.
4. Yes, downstairs attach blue to B at air handler after stripping the ends of the copper wire. Upstairs using T for fan is unusual. T is normally for outside temp sensor and is not supposed to be used for anything at the air handler, just a tie point so you don't need a wire nut. Something strange there as fan wire should be on G. Anyway, I would change the color to green for the fan control and then use blue for C to B common.
5. Not likely but possible. Nest acts like an on/off switch. If it cycles too frequently (multiple times each second) it can push a system to failure if it was already marginal.

[asomm]

1. Lots of times the sound is low refrigerant in the system. Verify the O/B is set to "seasonally." Also adding a C wire keeps Nest from triggering the valve when it shouldn't.
2. This might be Nest needing to power itself (fixed with a C wire). Might be condensate line blocked and shutting things down until it drains enough to clear the sensor.
3. Trane is known to be non-standard. Your connections to Nest look correct.
4. Yes, downstairs attach blue to B at air handler after stripping the ends of the copper wire. Upstairs using T for fan is unusual. T is normally for outside temp sensor and is not supposed to be used for anything at the air handler, just a tie point so you don't need a wire nut. Something strange there as fan wire should be on G. Anyway, I would change the color to green for the fan control and then use blue for C to B common.
5. Not likely but possible. Nest acts like an on/off switch. If it cycles too frequently (multiple times each second) it can push a system to failure if it was already marginal.

Thank you for the reply! Was able to get the downstairs common hooked up. O/B is set to seasonally. We'll see if common reduces the erratic behavior. Will wait for HVAC guy to replace compressor to also take a look at the wiring in upstairs air handler.