Heat pump on gas furnace.

[kevind158]

Need advice,

I have a 3 Ton split system gas furnace. My compressor failed after 15 years of service. I recently purchased a new 3.5 Ton heat pump condenser as a replacement. I would like to take advantage of the heat pump technology and use the gas furnace as backup heat. My question revolves around the specifications of the new replacement evaporator coil. I noticed that most heat pump condenser units are married to an air handler with electric backup heat, instead of an evaporator coil box on a gas furnace. I understand that gas heat is possible as backup with a heat pump, but I'm confused about what modifications need to be made to the evaporator coil TXV. Please advise if there are special TXV for heat pump evaporator coil configuration and if there is something that needs to be done to the furnace.

Thanks
Kevin

[Admin]

It probably would've been cheaper in the long run to just replace the coil when you replaced the outdoor unit.  If the existing coil is a 3 ton and the outdoor unit is a 3.5 ton  you could run into problems.  Usually the indoor coil would be the same size or larger than the outdoor unit. 

Chances are there's a TXV kit that can be added to the existing coil but we would need to know the model number and it's probably concealed inside the supply plenum.

And with the right thermostat and controls you should be able to set up a dual fuel system.  Most people in my area would run a heat pump until it reaches -12°C outside and then it would use the gas furnace.  Although unless you're using propane it's probably cheaper to use a natural gas furnace than the electric heat pump.  Also using a TXV with the indoor coil is not always required with a heat pump. 

With a Lennox heat pump you could use a TXV or non TXV coil at the furnace.  With their dual fuel thermostat you usually have to install an outdoor sensor and possibly a defrost control at the indoor coil to set up a dual fuel system. 

It sounds like you have a cased coil now but it's cheaper to buy an uncased coil and just install it inside the supply plenum.  It may be a cheaper install if you just purchased another case coil with the same dimensions though.

[Sergroum]

What kind of heat pump is it? Some of them have EXVs inside. In which case, your coil  should not have a txv at all.