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After running for some time, AC shuts but blower keeps running. NEST says no power to Rh line. Have a Rheem Criterion, have separate breakers for AC and Fan. Twice, I have turned off both and after a couple of hrs, it works ok.

Would appreciate if I can get pointers to what is happening, and what I can deal do, short of calling in someone for a simple fix.. Thanks in advance...

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  • What model # is the system, and can you post a schematic of the indoor unit (air handler)? – ThreePhaseEel Aug 27 '17 at 19:01
  • Will get to that. It was rather warm to get into the attic this afternoon. – San.D Aug 28 '17 at 5:37
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    When the A/C "shuts off", does the compressor shut off, or does it just stop blowing cold air? Is the thermostat satisfied when the A/C shuts off, or is it still calling for cool? When was the last time the system was serviced? How often do you change the filter? Does any of the equipment make any strange noises? – Tester101 Aug 28 '17 at 11:16
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    Do you know where the condensate from the unit drains? Have you noticed water leaking in any strange places around the home? It's possible that the unit has a sensor to prevent the condensate pan from overflowing, and the drain is running slowly causing the pan to fill. Check the condensate drain, and make sure it's properly plumbed and flowing freely. – Tester101 Aug 28 '17 at 11:25
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    Evaporator frozen over? I have seen systems that keep the fan running as long as the sensor says it is still cold. Low charge, blocked drain like tester101 mentioned or not enough air flow over the coil can all cause the coil to freeze up. – Ed Beal Sep 28 '17 at 23:12
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Your Rh line is Heating, RC is for Cooling , R is similar but systems might be separated and use dedicated lines.

What it sounds like is that you are hooking up your thermostat for the first time ??

Either your wiring is incorrect or there is a program mode where the fan runs. Since your question says HVAC - I am assuming AC .. however if the Unit is calling for heat Rh .. your system will have a cool down period - the Blower will run after the elements have turned off for about 5 to 10 minutes I think.

A typical color coding of wires is this : BUT DON'T BE FOOLED - Not everyone installs things correctly .. So, this is what you might expect to see in terms of colored wires and what they mean:

White – The white wire is what connects to the (auxiliary) heat on your system.
Yellow – The yellow wire connects to your compressor.
Green – The green wire connects to the fan.
Orange – This wire connects to your heat pump (if you have one).
Red – Now, there can be two separate wires for this. 
      One is for cool and the other is for heat, 
      hence the abbreviation “RH” and “RC”.
Blue – This is your common or “C” wire.

So it would be nice to see a picture of your connections in the NEST and at the Air Handler. Your wiring seems to be incorrect. especially when there is a message about Rh and you are trying to cool.

Look Here http://www.thermostatcenter.com/thermostat-wiring/

Also Here : On a thermostat, are "R" and "Rh" terminals the same thing?

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  • Thx Ken, I installed NEST 6 months ago and worked fine with heat and cold till this week. So I guess the wiring is ok. This AC (cooling) shutting off is recent. I will get to the attic for the actual model no. It is a Rheem Criterion Horiz – San.D Aug 28 '17 at 5:32
  • Rh and Rc are usually only separate when the system uses a boiler, or other non-forced air system for heat. Since the OP seems to have a gas furnace, I'd assume the furnace is also the air handler for A/C. In this case, Rh and Rc are probably not separate. – Tester101 Aug 28 '17 at 11:20
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One of Nest's selling points is something called "Airwave," which keeps the fan running after the compressor turns off in order to circulate the cool air in the chilled ducts throughout the house. You can turn this feature off in the settings.

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Some Aircond model have this function to ensure no condensation occurs on the cooling fins. Without the air flow, water condensation will occur quickly on the cooling fins. Wetness in the fins will cause bacterial growth. With the fan running for sometimes, all moist will dry off and the cooling coil will return to room temperature quickly.

With this system in place your Aircond won't produce weird odour the next time you switch it on.

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