I woke up early this morning to the smell of the aux. heat strips running full blast. Thermostat showed house at 3 degrees below set point and aux heat on. Climatemaster TT038 system with CXM board, about 8 years old. Checked system, found steady green light on cxm board, calls for stage 1 & 2 and fan, but compressor contactor was open. Suspecting a loose electrical connection, i started lightly tapping some of the components. When i tapped the top of the compressor relay on the cxm board, the contactor immediately closed and the compressor started normally. Compressor ran for around 10 minutes and then the contactor opened, again no lock out code and thermostat still calling for stage 1 & 2 heat. Tapping the compressor relay got things going again. Unit has since run for several hours without and more trouble and has cycled on/off several times. I'm thinking bad/pitted contacts in the compressor relay on the cxm board? Anyone have similar experience?
So I ordered a new cxm board and installed it yesterday. The new board looked identical to the old but I noticed the part number on the new board was 17B0001N03, while the old board number was 17B0001N01. The new board seems to function ok, but has a what seems to be a consistent 2 minute delay between a "Y" call and the compressor contactor closure. The old board didn't have this delay. Any Climatemaster experts out there know what difference(s) there are between the 17B0001N01 & the 17B0001N03 boards?
Sounds normal. Heat pumps and A/C units normally have a built-in delay of about 5 minutes before energizing the compressor. This protects the compressor from damage due to short cycling.
I contacted Climatemaster today to see if I could get this explained. Here is the story: The new cxm board has a guaranteed 7 min. of compressor off time programed in. My thermostat is set for min. 5 minutes of compressor off time and max. 3 cycles per hour for 1st and 2nd stages. The old cxm board had 5 min. compressor off time (matching the thermostat setting) It just so happens that at the current heat load for my house, the system was cycling at 3 cycles per hour (5 min. off and 15 min. on per cycle). So the new cxm board was not "honoring" the "Y" call when it came in after only 5 minutes of off time, instead waiting the extra 2 minutes. The problem is that my current thermostat (Honeywell Vision Pro) won't support more than 5 min. of compressor off time. Does anyone know of a good 3H/2C thermostat that allows 7 minutes of compressor off time? Thanks to all for the help on this!
I don't see what the problem is. If you can set the TSTAT to 0 min off time, then do that. Then the cxm board will provide 7 minutes.
The problem seems to be that because the thermostat doesn't know that the cxm board is keeping the compressor off, it is expecting to see some temperature rise after powering the "Y" call. When it doesn't see any temp. rise, it starts upstaging quickly. I've observed the system for some time and the following sequence plays out: 1) T'stat places "G" call 2) T'stat places "Y1" call 3) fan starts in response to "G" call (ECM blower, begins to ramp up) 4) comp. contactor does not close 5) after ~ 1 minute, T'stat places "Y2" call 6) fan ramps up to high speed in response to "Y2" call 7) after ~ 1 more minute, compressor contactor closes 8) compressor runs on stage 2 for ~ 5 minutes 9) T'stat down stages to "Y1" call 10) fan and compressor ramp down to stage 1 11) after ~10 minutes, T'stat removes "Y1" and "G" call, system stops 12) after ~ 5 minutes, sequence starts at #1 again ... I will try to set the T'stat to a lower number of cycles per hour and see if this helps. Thank you again for your help on this.
How quickly is your space losing heat? After 5 or 7 minutes of a cycle ending is the need for more than a minimum (1F?) temp rise?
There is nothing technically wrong about the sequence you describe. However, it indicates your house is dropping enough to engage Stage 1 in 5 minutes and then Stage 2 in another 1 minute. Doesn't sound right unless the doors and windows are open and the TSTAT has a supply register blowing on it. If starting in Stage 2 bothers you, you can look at the differential setting for engaging Stage 2. If it's set to "comfort" the TSTAT will use Stage 2 to keep the inside temperature within 1 degree of setpoint. Setting the differential to a larger value might allow the unit to start before the temp drops enough to engage Stage 2.
I think the main cause of the 5 minute off is that the heat load of the house is now around 95-100% of the stage 1 capacity. I've measured my house heat constant and it is around 650 btu/hr -F and it is in the low thirties currently around here. T'stat is set for 68F so: (68-32)x650= 23,400 btuh. The HP is good for around 22,000 btuh in stage 1 and 30,000 btuh in stage 2. So I'm in that area where the system will cycle and jump between stage 1 &2 to try to satisfy the T'stat. With the T'stat set for 3 CPH, it will try to cycle as much as 3 times an hour to most tightly control the temp. around the set point. The irony is that if it was very cold, it wouldn't be cycling at all and I probably wouldn't have noticed the "problem". I also think that the T'stat "learned" the behavior of the system and expected some heat output immediately after a "Y" call and that has now changed to a 2 minute delay after the call. I'll try your idea on changing the droop for stage 1 to 2 from the current "comfort" to 1F and report back. That should also I think reset the T'stat learning algorithm which may help as well. Thanks for staying with me on this!
Update: staying on stage 1 much longer so less cycles off. But when it does cycle off the same issue still happens. I'm going to also try reducing the CPH from 3 to 1 but I am starting to think the only real solution is to get a thermostat that supports compressor off time greater than 5 minutes. So far. I've only found the Ecobee which can be set for 7 minutes.
Further update: reducing CPH from 3 to 2 did not seem to make any difference at all. When I went down to 1 CPH, things got better. I now have both stages 1 & 2 set for 1 CPH, stage 2 droop set to 'comfort' and aux. droop set to 2F with a 60 min. max. run time for stage 2 before aux. come on. It is now around 40F outside and the HP is cycling at around 8 min. on/15 min. off. It does go right to stage 2 when it starts, but I can live with that. As the outside temp. drops, I would bet that the off time is going to go down to 5 min. eventually because after watching the thermostat perform over several days, it seems like the programing is designed around the min. compressor off time value. I can see why the compressor relay contacts did wear out after several years of around 6 CPH operation. Also might explain the compressor contactor bad contact I have a couple of years back ... I think I'm going to install an ecobee3 lite thermostat where I can set guaranteed compressor on and off times, just to be sure. Moral of my story - check up on your thermostat performance now and then. Don't assume as I did that if you set it for 3 CPH, it will actually run 3 CPH. Also, if you have an older system without communicating thermostat, be aware that conflicts between the HP control board logic and the thermostat logic can lead to short cycling and other problems. Thanks to all for your feedback on my problems - this forum is the best!
