# no gas furnace back up on hybrid(heat pump)system



## tsl2166 (Apr 20, 2018)

Working on a Carrier heat pump/gas backup with visionpro 8500 tstat. The furnace will not come on when backup heat is needed below 35 deg. outside ambient in normal heat mode. Furnace runs when set in emergency heat mode on tstat but not in heat mode. the tstat shows the outside temp. correct and shows its in emergency heat mode after the heat pump shuts off but furnace will not kick on. The heat pump works in heating and cooling. Never encountered this at any service call.


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## bpalfy523 (Apr 30, 2018)

If it's dual fuel with had backup. The furnace will not kick on and run as backup heat with the heat pump. Unlike electric strip heat as your backup,that runs with the heat pump. Running the furnace with the heat pump running would cause high pressure issues. You need to set the tstat to lockout the heat pump at your 35 degrees or whatever temp you desire and bring on the gas furnace alone. Usually don't even defrost with a gas furnace because itll get to hot. Have to make sure your settings are all correct in the tstat and it's a 3h/2c tstat. Hopefully this helps 
Good luck

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## thekux (Jul 29, 2018)

Sounds to me like your thermostat has a gas heat lockout set point of lower than 35. If you can do it you go into the program part of the thermostat check the lockout temperatures and if you want to race them I would raise them. It should not be lower than 35 in my opinion to lock out any backup heat on a heat pump. Once you get below 35 outside the pump will struggle no matter how good it is. I set mine to 40

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