Author Topic: When to change over dual fuel heat pump  (Read 1023 times)

intellectsucks

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When to change over dual fuel heat pump
« on: January 29, 2018, 08:13:05 AM »
I have a dual fuel heat pump as part of my HVAC. Main furnace is oil heater.
My thermostat has three settings, cool, heat and emergency heat.  At what temperature (if ever?) Should I switch the setting to emergency heat?

Bill76

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Re: When to change over dual fuel heat pump
« Reply #1 on: January 29, 2018, 09:30:07 AM »
Mine is a dual fuel system; electric heat pump with natural gas furnace.

I normally just rely on the electric heat pump, but the gas heat comes on automatically if the temperature in the house is 3-4 degrees below the thermostat setting. Otherwise, I would only switch the system over to the emergency heat setting if the electric heat pump wasn't working for some reason (either a malfunction or just because it's too cold outside for it to adequately warm the house). Being in the south, I don't end up using the gas furnace much.

sol

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Re: When to change over dual fuel heat pump
« Reply #2 on: January 29, 2018, 09:44:49 AM »
I did this calculation when we installed a heat pump with a backup gas furnace last year.  Comparing my local cost of power against my local cost for natural gas, the efficiencies of both heat types, and making the decision based solely on what is the most cost-effective way to heat my home, I decided that we should use the heat pump down to at least 15 degrees F.  Below that the efficiency of the heat pump starts to drop off, requiring more power to make the same amount of heat.  Fortunately, it hardly ever gets that cold here.

Your breakpoint is probably even lower, given that you're using oil heat as the backup and I expect it is more expensive than natural gas is right now.

But cost is not the only factor here, obviously.  The heat pump heats more slowly than does an oil heater, and depending on the insulation level of your house it's possible that the heat pump won't be able to keep up on its own, if your house is poorly insulated and the temperature outside is too cold.  In that case, the heat pump would run continuously and your house would not get any warmer (because it is losing heat faster than the heat pump can add it), and you'd want to switch over to your backup heat source, which heats much more quickly even though it costs more.

Without knowing what you pay for power or oil, or what kind of house you have, it's hard to know for sure.  But in general the heat pump should be more cost effective under most conditions.

boarder42

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Re: When to change over dual fuel heat pump
« Reply #3 on: January 29, 2018, 09:49:02 AM »
@sol do you have a spreadsheet where we could copy this and enter our own data?

sol

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Re: When to change over dual fuel heat pump
« Reply #4 on: January 29, 2018, 09:53:50 AM »
@sol do you have a spreadsheet where we could copy this and enter our own data?

No, I don't.

It depends on the temperature rating curve of your heat pump, for one.  I had to look mine up in the manufacturer's information online.  It looks like most modern heat pumps are better than 100% down to well below the freezing point, which kind of surprised me. 

And it depends on what kind of alternative heat source you have.  Electric baseboards are easy, they're guaranteed 100% efficient on straight up resistance heating.  Older gas furnaces are typically 85%, but some of the newer ones are 95%.  Then you have to consider that even your gas furnace uses some elecricity, to run the blower. 

And you have to know what you pay for gas vs power, of course.

Then I just converted everything into $total per effective BTU, basically.  There was some unit conversion involved.

intellectsucks

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Re: When to change over dual fuel heat pump
« Reply #5 on: January 29, 2018, 10:37:17 AM »
Sol, before running calculations, could I guesstimate based on comfort level inside? As in if it feels chilly in the house or the temp is holding let the heat pump handle it? Or should I grab the calculator and start crunching?

boarder42

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Re: When to change over dual fuel heat pump
« Reply #6 on: January 29, 2018, 10:40:18 AM »
Sol, before running calculations, could I guesstimate based on comfort level inside? As in if it feels chilly in the house or the temp is holding let the heat pump handle it? Or should I grab the calculator and start crunching?

if you start crunching numbers make a spreadsheet and share it here.

sol

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Re: When to change over dual fuel heat pump
« Reply #7 on: January 29, 2018, 10:45:02 AM »
Sol, before running calculations, could I guesstimate based on comfort level inside? As in if it feels chilly in the house or the temp is holding let the heat pump handle it? Or should I grab the calculator and start crunching?

The heat pump will always be slower, so comfort levels don't really help.  If you turn the heat up, expect to wait 30 minutes to notice the difference.  This doesn't mean it is more cost effective to use your oil heat, just that oil heat will raise the temperature more quickly.

If you want it warm NOW, and don't care about cost, use the oil heater.  If you want to be cost efficient, try to be patient and let the heat pump work.