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I have an older Tempstar 5000 furnace and a newish Honeywell wifi thermostat. The furnace works fine and the thermostat appears to be wired correctly (4 wires including C wire) but for some reason, the thermostat has no power (screen goes blank) during the fan-off delay. This furnace is equipped with a delay relay. It is during the minute after the burners turn off and the blower is still running that there is no power at the thermostat.
In the picture, the upper pair of wires is going to a humidistat that properly turns on only when burners are on. Schematics

Furnace side

Thermostat side

  • You didn't get R and C from the right places, and where you grabbed R is dropping to C's voltage, or vice versa. Photos, particularly of the furnace end of the business, will be very helpful. Hint: R and C are the two wires on the 24V side of the transformer. A schematic of the furnace would also be helpful, in case they did something odd. – Harper Aug 8 '17 at 16:49
  • I assume that you had a 4 wire system with R, G, Y, W. Did you just take G and move it to C of the new thermostat. When you do this you need to also move the wire from G to C at the control board of the furnace. Doing this will cause the circulation fan function of the thermostat to no longer work so keep that in mind. – Mnc123 Aug 8 '17 at 20:25
  • updated with photos – Chris M Aug 9 '17 at 0:08
  • Could the rollout or high-limit switches be opening during the fan delay? Also, is the fan changing speeds during the delay period compared to when the burners are firing? – ThreePhaseEel Aug 9 '17 at 1:58
  • The fan does not appear to change speeds. I wouldn't know about the rollout or high-limit switches. – Chris M Aug 9 '17 at 13:56

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