NEST Thermostat causing compressor to slam?

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  #1  
Old 04-20-12, 11:37 PM
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NEST Thermostat causing compressor to slam?

Hi all!

To begin with....my system is as follows:
  • Trane XE-900 Heat Pump outside and Air Handler in the attic.
  • Thermostat termination:
    • Red - R
    • White - O/B
    • Yellow - Y
    • Green - G

OK, just installed the NEST thermostat here not too long ago. These have a feature they call "Airwave" where they essentially turn off the compressor early and let the fan run to bring the house down that last degree or so.

I notice that as soon as it goes into Airwave mode and turns the compressor off, the compressor slams like it never used to. It's pretty jarring while I'm sitting in the house. It's almost like it's de-energizing the reversing valve the same time it's shutting off the compressor. Does this make sense? What else could the banging be?

Thanks!

Tony
 
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  #2  
Old 04-20-12, 11:42 PM
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Just realized that there is a Thermostatic Controls forum. My apologies.
 
  #3  
Old 04-21-12, 08:28 AM
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SO, you have no back up heat source? I can see that you might not in AZ. It very well may be dropping the O signal when airwave is utilized.

Airwave is NOT good technology to begin with.... When you shut the compressor off before the end of the cooling cycle and only run the fan you essentially just turned on a humidifier. This is because the evaporator coil is wet and the drain pan has water in it. By only running the fan some of the water then evaporates and puts the moisture right back into your home. That is not what you want to do!

You can verify your assumption about the reversing valve by checking for 24VAC between O and B/C at the outdoor unit when this sound happens.
 
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Old 04-21-12, 09:53 AM
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Thanks for the reply!

No backup heat source. This is the most common installation here in AZ. Very rarely does it get down in the low 30's at night. Even then, during the day it's up in the upper-60's - lower 70's with not a cloud in sight so the home heats itself very well from the greenhouse effect.

The Airwave function has a humidity interlock that ensures it only operates when the humidy inside and out is below a certain level. Right now it's 29% @ 77deg with the windows open. I shut the unit off last night right before I posted to try and make sure I wasnt damaging anything. If I close the windows and turn the unit on the humidity drops to ~22%.

If my assumption is correct and it's de-energizing the reversing valve, can this cause any damage?
 
  #5  
Old 04-21-12, 09:55 AM
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it should not.... But it will be loud!
 
  #6  
Old 04-21-12, 09:59 AM
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And it is! Scared the crap outta me. It's a ~21 Yr. old unit and I dont feel like dropping 3-5K on a new unit this year
 
  #7  
Old 06-01-12, 04:35 PM
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Did you ever figure out how to fix it?

I installed a Nest thermostat and am having the exact same problem. The loud noise it makes can't be good for the compressor. Also, I'm sure it won't be long before the neighbors start filing noise complaints (yeah, it's that loud). I'm sure this has to be a common problem since my A/C system is common here in the Phoenix area.

If I can't figure something out soon, I'm returning it and going with something else. It's too bad because I really wanted this to work.

-Brian
 
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Old 06-09-12, 11:47 PM
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Move the wire connected to O to R. (energize reversing valve continuously)

If it doesn't make the same noise, call the manufacturer.

I think the nest stat is a very new product so the firmware might have bugs. (sounds like they didn't do much testing on heatpumps) It's probably more trouble than it's worth.
 
  #9  
Old 06-14-12, 08:35 PM
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You know, I was wondering about that. I guess I could do that and just leave the wire de-terminated when I need heat.

I've been dealing with Nest customer support over the last few weeks. They wanted me to add a common wire to C at the thermostat. I was able to find my splice in the attic and jumped on to the common running between the Air Handler and Compressor. Terminated to C at the thermostat to no avail. Same symptoms. I called Nest support with my findings. Went through some voltage readings, etc... He put me on hold and found one of their resident heat pump experts and explained the situation to him. The first question out of his mouth was "Is it a Trane?" Apparently, they dont have a fix in place for these units yet and are adding them to the "incompatible list" as we speak. He mentioned that I could install a control relay to seal-in the circuit at the compressor. This is leading me to believe that other units besides the Trane must have this seal-in capability from the outset, only requiring a momentary output from the thermostat to seal the circuit.

I'm not sure if I'm going to go this route. I just reinstalled my old Honeywell RTH8500D and it's working flawless. Waiting on Nest support to call me back with my return authorization as I'm outside the 30-day return period.
 
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Old 06-14-12, 10:13 PM
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If the stat is de-energizing the reversing valve right after each call for cooling, the problem is not limited to trane systems.

The 8500 is a nice product.
 
  #11  
Old 06-24-12, 08:19 AM
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@Tony - I've got the same exact problem. The previous thermostat that was installed never caused this slamming noise to happen. It sounds like a school bus air brake releasing. I haven't contacted Nest yet, as I felt their support is iffy. Did you find a solution to this problem?
 
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Old 06-24-12, 09:07 AM
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The solution is to NOT use the NEST. There are much better more appropriate thermostats out on today's market. The problem is that most of those thermostats require professional installation unless you can follow directions. Heat pumps require more wiring and programming of their thermostats than conventional systems. However, heat pumps are less expensive to operate than conventional systems when they are setup and controlled properly. The NEST is not a good thermostat for a HP system because you will typically spend more money in utility costs trying to recover from the setback temperatures than you would if you just set the thermostat and forget it.
 
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Old 06-24-12, 10:51 AM
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^Excellent advice.

The OP's original stat is a very good one. I don't see the point of the nest, especially on heatpumps with electric backup.
 
  #14  
Old 10-25-12, 02:59 PM
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Was there ever a solution to this problem, other than "Do not use Nest"?
 
  #15  
Old 10-25-12, 03:17 PM
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The solution is there is an option to turn off Airwave if you do not want to use it...
 
  #16  
Old 10-25-12, 03:24 PM
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Even without airwave, it does it. I live in FL, so the humidity was too high for Nest to use air wave. It's when it turns the compressor off, that it makes the sound.
 
  #17  
Old 10-25-12, 04:31 PM
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Do you have the latest sofware update? If so, then call NEST. You may not like their answer though...(not that I know what they will say)
 
  #18  
Old 10-29-12, 01:04 AM
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"We have researched this issue, and it is a expected product behavior, and your system should be able to function perfectly fine with this behavior."
 
  #19  
Old 10-29-12, 04:31 AM
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It is not a proper behavior for a thermostat. Nest doesn't understand that it should NOT do that after every call for cooling.
 
  #20  
Old 10-29-12, 06:50 PM
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The nest really sounds like a pos - an expensive pos, but a pos nevertheless.

The best fix is to remove it and install a honeywell or white rodgers stat.
 
  #21  
Old 11-27-12, 07:28 PM
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They fixed this issue a little while back, just in case anyone wanted to know that.
 
  #22  
Old 04-17-13, 02:35 PM
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I only learned (the loud compressor noise) was a problem when I asked about the feature in the o/b menu with regards to the difference between "each cycle" and "seasonally". I was told it was to address some units that made a loud noise when the compressors shut off.

Neither my old unit or my new unit (replaced due to leaking evap coil) have the problem and require the work around (Each Cycle is the norm).
 
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