Tempstar Furnace at Church

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Old 11-03-11, 12:48 PM
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Tempstar Furnace at Church

Hello All: I can use some help diagnosing an issue with a Tempstar Furnance/Air Conditioner combo unit (NTC6) at my church. The unit was functioning fine for the last heating season. We have not used the airconditioning that much. The unit is a Tempstar NTC6 Horizontal Series installed in the attic. The compressor unit for the AC portion is outside the building.

When we tried to turn the unit last week during the cold spell in NY, the unit would not turn on. The thermostat appeared to be working fine (Honeywell PRO TH4000 series). I did some basic diagnostic work and found that the furnace control module was not receving 24v between the C and W terminals. The 24 volt trasformer for this unit is located in the compressor unit outside the building. I checked the transformer and it is fine (26.4V). However, the connections from the transformer is passing through two "Dleay on Break Timers" and one High Pressure (HP) Switch and one Low Pressure Switch. When I bypass these componets and use a shunt to connect directly to the termostat outputs, then the furnace is working fine. The only difference I can see is that the airhandler motor shuts down inmmediately after the heat call goes off or when the thermostat is turned off. Previously there was a delay of few minutes after the heat call was off for the airhandler fan to turn off.

I am stumped here and would really appreciate any help to guide me solve the problem for good.

I have posted several pictures of both the furnane control module and the contactor panel in the compressor unit along with the connection diagrams.

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Old 11-03-11, 07:12 PM
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I'm sure an HVAC guy will have some better info for you but here is what my electrician mind knows.

At the furnace, you should get 24v between C and R. C is your common and R is your "hot". W=heat G=fan, Y=cool. So when your stat is calling for heat you should then get 24v between C and W. If you are not then I would suspect the stat is faulty. You can jumper between R and W and it should start heating. Jumper R to G and the fan should run. If that all checks out then again, I would suspect the stat.
 
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Old 11-05-11, 05:27 PM
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You have two transformers. One in the furnace & another in the outdoor unit. At the thermostat you should have separate wires connected to Rh & Rc. Check Rh to W & you should get 24 volts AC. If not, you need to check the transformer in the furnace.
 
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Old 11-14-11, 11:51 AM
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Tempstar Furnace at Church

Grady - Thanks for your reply. I checked the setup at the church. There is only one transformer for both the cooling and heating side. The Rc and Rh are joined by a jumper at the thermostat. Some additional troubleshooting tells me that the 24v at the compressor unit located outside is passing throgh two pressure switches before getting to the airhandler control board. There is no continuity in the pressure switches. (the wiring diagram included in my original post shows this clearly as LP and HP switches) Either they malfuntioned or is defective or there is a condition in the compressor that activated the pressure switches. I bypassed both the pressure switches using a piece of wire and then the heating works fine. I am not able test the A/C because of the cold conditions outside. I tried to turn the AC on. However, all I hear is a humming sound from the compressor. The fan is not coming on. Since the heating is now now working (with the presssure switches being bypassed) I am going to wait till the summer to fix the issues with the pressure swich. Can these pressure switches be replaced without loosing the freon in the cooling loop? It is tough to get to the switch due to its location. So I am not sure whethter these are meant to be replaced. Could it be (lack of continuity thrrough the Low pressure and high pressure switches) an indication of a problem with the compressor?
 
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Old 11-14-11, 05:53 PM
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I still think something is wired incorrectly If you have two transformers (one each for heating & cooling), the hot (red) from each should go to it's respective terminal on the stat (Rh for heat & Rc for cooling). Another thing that bothers me is 2 compressors & only one fan (which appears to be a single speed motor). Honestly, I'm completely baffled on how the system works in the cooling mode.

Regarding the high & low pressure switches:
It makes no sense to me as to why you don't get continuity thru either one unless the high pressure switch is bad AND you have lost refrigerant pressure causing the low pressure switch to open. Usually one would have to remove all of the refrigerant in the system to change the switches. If you can get me a good clear picture of them, I can probably tell you for sure it that is the case.
 
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