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# Heat pump / furnace question

Member Posts: 13
My heat pump gives 100 % rated heat until the outdoor temperature reaches -25 c , so I was planning on using a Johnson controller so when the outdoor temperature reaches - 24 c ,this would shut off the heat pump . Then the 2 stage natural gas furnace will start working. I was going to have W1 and W2  split through contacts connected in the relay . So if the heat pump is working the furnace can't turn on . Does that sound ok?

• Member Posts: 6,761
edited September 25
Yes but your 100% number is ambiguous. A heat pump is rated with something called a coefficient of performance (COP). Basically electric resistance heat is 100% efficient. All the electricity is converted to heat energy with no loss up the chimney or out of a vent because there is no vent.

The COP tells you that when there is a given quantity of electricity applied to a compressor (and other heat pump components) that same electricity will yield more heat than that of electric resistance. For example, when the outdoor temperature is 10°C the COP might be 4.0 meaning that the heat pump is converting the electric energy into 4 times the heat of a resistance heater.
When the outdoor temperature is only 5°C that COP might drop to 2.0 meaning that the same amount of electricity will only produce 2 times the amount of heat that a resistance heater will provide.

Now you need to compare the cost of the fuel for your furnace to produce a constant amount of BTU heating energy with the cost of electricity to produce that same amount of BTU heating energy. That number may not be 0°C. It may be at -5°C or +2°C. Once you determine that number, that is where you should set your changeover to fossil fuel.

There are cost of fuel conversion charts available online. You will also need to look at the performance chart of your particular heat pump to see what the COP is at different outdoor conditions.

As far as selecting that temperature and having a control switch that functions automatically, I do not believe that the thermostat with different heat stages is the best chance of operating a relay for that purpose. You will need an outdoor sensor probe of some sort connected to a relay that will change the thermostat signal to operate one or the other.

That said, if your heat pump and furnace are of the same manufacturer and have been installed as a matched set, there is a fossil fuel kit available for some systems that will do that job for you. You don't need to reinvent the wheel on this. What furnace model and heat pump model are you operating? You may already have what you need.

Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 4,390
Total said:
My heat pump gives 100 % rated heat until the outdoor temperature reaches -25 c , so I was planning on using a Johnson controller so when the outdoor temperature reaches - 24 c ,this would shut off the heat pump . Then the 2 stage natural gas furnace will start working. I was going to have W1 and W2  split through contacts connected in the relay . So if the heat pump is working the furnace can't turn on . Does that sound ok?
• Member Posts: 13
Last winter season ,the heat pump did perform great but not at colder than -25 c .
• Member Posts: 6,761
Total said:

Last winter season ,the heat pump did perform great but not at colder than -25 c .

what brand of heat pump, what brand of furnace, are they connected together to the same thermostat or separate units with separate thermsostats?
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 13
It is a Bosch heat pump , and a heil furnace , seperate thermostats . I was going to connect to a Honeywell thermostat, going to set the heat pump to primary , and the furnace to secondary , but the thermostat change over could only be set to -9 c max  .
• Member Posts: 13
That Johnson control does have a outdoor temperature probe .
• Member Posts: 13
thanks for the help everyone , I'm tracking down the cop data to figure out the best temperature for the switch over .
• Member Posts: 5,106
but the thermostat change over could only be set to -9 c max

Do you mean "-9c min"?

I would get a better thermostat then that lets you put the switchover where you want, rather than have to install some whole other system to manage it.
NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el