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I just switched my upstairs and downstair thermostats for smart thermostats. The C wire was not connected so I opened the furnace and connected it. Both units deliver 27v to the thermostat. I read where 20-30v is ok. The upstairs thermostat functions just fine with both A/C and Heat. The downstairs thermostat is fine with A/C, but when on heat the unit repeatedly blanks out losing power and the clock time. I swapped the upstairs and downstairs thermostats to see if I had a bad thermostat, but the problem stays with the downstairs unit.

I suspect that when the Bryant gas furnace kicks on that the power must be unstable therefore the thermostat doesn't have sufficient power. The Heat stays on and runs and the unit will get power again while its running, but the clock gets reset every time. Therefore its only able to work in hold mode and not with a schedule.

I did verify the cables all have secure connections.

[Update 1]

Voltages measured at the furnace.

28.5v idle

28.1v when it first kicks on

24v blip when the blower kicked on

25v for a bit

14v

I don't know all the stages it goes through (stages is probably not the right word, it's not a multi-stage unit), but something kicks in a few moments after everything is running to drop the voltage.

[Update 2]

Replaced transformer, but no change.

[Update 3]

Discovered that when the voltage drops to 14v and the thermostat goes out, that the control board is also blinking error code 33. Tested the blower capacitor and it was below the 6% threshold. Replaced capacitor, but no change.

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    Losing power when the furnace is on is exactly what is supposed to happen without a C wire... Except the smart stat is supposed to have a hefty battery to carry it across those times. Does your stat have a battery? Is it any good? Did you jump straight in immediately, or did you give it 24hrs to initially charge? – Harper Feb 26 '17 at 23:08
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    Can you post photos of how the thermostats are wired at both ends? – ThreePhaseEel Feb 26 '17 at 23:24
  • Is there a zone controller involved, or do you have separate heat/cool for each floor? – Tester101 Feb 27 '17 at 0:20
  • The stat does not have a battery, but it does have the C wire. It's had more than 24hrs to try and charge if it had some sort of internal battery. No pictures available right now. No zone controller. Two separate units for each floor. – getSurreal Feb 27 '17 at 3:27
  • What's the make and model of the heating equipment that's having the problem? How are the thermostat wires connected to the equipment? – Tester101 Feb 27 '17 at 12:56
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The symptoms sound to me like the transformer isn't capable of supplying enough power. Check what the VA rating is on the transformer, and compare to the draw required by the furnace and two thermostats (divide VA by volts to get amps).

If it's undersized, you'll have to setup an external transformer.

  • The transformer is 24v 40VA. I don't know what the draw of the furnace is or how to find out, but is it more likely that some component is the problem by drawing more than it's supposed to. How could I check for that? – getSurreal Mar 2 '17 at 2:50
  • @getSurreal -- is the transformer on your furnace a separate thing, or part of the control board? – ThreePhaseEel Mar 2 '17 at 2:52
  • The transformer is separate from the control board. – getSurreal Mar 2 '17 at 3:03
  • @gregmac -- would swapping out the transformer for one with a higher VA create any other issues? – getSurreal Mar 3 '17 at 22:27
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Error code 33 on the control board is causing the thermostat issue, but the root cause for error code 33 is poor airflow.

There are two returns for this unit, but one of them is not getting good airflow. We had a duct cleaning service clean our ducts and apparently there was some damage done to the downstairs return line as it has almost no suction.

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Same problem here so I looked at the schematic.

There is a safety circuit on the r terminals wiring path to its side of the 24v power. When the heating has been achieved it opens the limit switch circuit and shuts of 24 v power temporarily. It is on the schematic as LS or limit switch. There are 3 other safeties in the circuit. My Smart thermostat from Honeywell loses power in the heating cycle with the fan still running then comes back on with no memory loss.

I was previously using a 2 wire tstat off the W and R terminals. I now am using the c and g as well as w and r. The g does fan only for hot days. The common is not that useful since it cuts out as part of the cycle but at least we know what’s going on.

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