I have a combination medium efficiency gas furnace with added air to air heat pump for heating/cooling. When it is cold outside and the gas furnace is doing the heating, the furnace comes on, gas ignites and the gas/blower runs for about two minutes and then the gas suddenly goes off. At the same time the thermostat (Honeywell T5) loses power - the display goes black and unresponsive. The blower continues to run until the plenum cools down. At some point the thermostat comes back on and has to go through a power on reset cycle, so the delay between heating cycles is 5 minutes or more. When it is cold, this means that it takes a long time to heat the house. I have checked the limit switch and it stays closed during the cycle so the plenum is not overheating (I have also checked the plenum temperature with an infrared thermometer). Can anyone explain how this happens? It seems to me that the gas heat should run a lot longer than 2 minutes and there is no reason for the thermostat to lose power.
source https://hvac-talk.com/vbb/showthread.php?2225944-Thermostat-loses-power-during-heating-with-gas-Why&goto=newpost
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