Question Regarding Dual Fuel Heating: Heat Pump and Propane Furnace

[blues]

For those of you with a dual fuel system, where do you find that you like to cross over to the furnace from the heat pump?

I woke up last night and noticed that the heat pump was on for an extended period.

The temperature outside was 9 degrees F, and the inside temperature was a degree or two below my setting of 67*.

So, I switched the system to Emergency Heat, (which is our propane furnace), and will switch back later today when the temps rise.


Any idea of where the generally accepted changeover should take place? I realize that there are differences based upon homes, prices of electricity vs. fuel etc, but I'm speaking more from the point of view of the system laboring to keep pace with the cold and efficiency.

Thanks.

[whomever]

I think it totally depends on the particular heat pump. The newest high efficiency heat pumps (inverter drive blah blah) can work well down to the single digits. Older ones, not close to that.

For example, scroll down a couple of paragraphs:

https://www.ecomfort.com/stories/134...he-Winter.html

(note: that is for minisplits ... I think they are leading the efficiency parade right now)


Disclaimer: I'm only keeping half an eye on the high efficiency heat pump developments.

[RoyGBiv]

I would have thought that the system would be designed to auto-switch... If the heat pump is running for XX time and/or the pump is running but the house temperature is dropping, the backup heat source should just kick on... Our ancient heat pump in NC did this, but, it was a single system with backup emergency heat strips... all electric.

[Jim Watson]

I think my natural gas auxiliary kicks in when the set point is 4 degrees above the ambient... which usually only happens when I get up and turn up the thermostat. But during the present cold wave - now clear up to 20F - I set it to run the auxiliary all the time.

[0ddl0t]

Rough guide:

Gas furnace: ~60-95% efficient.

heat pump: 100-450% efficient, depending on temperature (it exceeds 100% efficiency because it pumps heat from elsewhere in addition to creating heat. The colder it is, the less heat available to pump into the house, lowering efficiency).




A modern heat pump at 10°F is around 230% efficient whereas a modern gas furnace will probably be ~85% efficient. So at 10°F, a heat pump should use 2.7 times less energy.

But electricity might cost 3 times more than gas per btu. So what is most economical might be the furnace even though the heat pump is more efficient.

This of course assumes your heat pump is large enough to keep your home warm - sizing a pump is a balancing act (too big or too small will be less efficient)

[blues]

I would have thought that the system would be designed to auto-switch... If the heat pump is running for XX time and/or the pump is running but the house temperature is dropping, the backup heat source should just kick on... Our ancient heat pump in NC did this, but, it was a single system with backup emergency heat strips... all electric.

I think the issue is that I happened to wake up, noticed that it was running continuously more than normal, noticed the temp down a degree or two, and rather than wait for it to bump on the furnace, I turned it to the furnace setting at the thermostat and went back to bed. It was about nine degrees at that time.

My main curiosity is piqued because there is such a wide divergence of opinion upon when it doesn't pay to heat the house with the heat pump vs. the furnace.

Some folks go from 40* F and below, and others say they're fine down to 0-5*.


It's all good here, as everything is operating, I'm just curious about the wide diversity of "expert" opinions out there.

[Screwball]

Some folks go from 40* F and below, and others say they're fine down to 0-5*.

Depends on the unit… my parents did a heat pump probably around the 2010 time period. That one would not work when the temperature was under 32°. Good for a chunk of the year in that area (NJ). But wouldn’t be too good up here… in northern ME. I know newer ones get down to 0°.

I’m likely going to upgrade my furnace next year… and heat pump is on the list. Main reason, we get like a week or two of hot weather up here. Not enough for air conditioning, but makes it unbearable. I have a window unit for the bedroom, but it is a pain. Being able to get the entire house cooler is the bigger benefit for me.

[ccmdfd]

I think the issue is that I happened to wake up, noticed that it was running continuously more than normal, noticed the temp down a degree or two, and rather than wait for it to bump on the furnace, I turned it to the furnace setting at the thermostat and went back to bed. It was about nine degrees at that time.

My main curiosity is piqued because there is such a wide divergence of opinion upon when it doesn't pay to heat the house with the heat pump vs. the furnace.

Some folks go from 40* F and below, and others say they're fine down to 0-5*.


It's all good here, as everything is operating, I'm just curious about the wide diversity of "expert" opinions out there.

My HVAC man told me that below 40 the heat pump is basically useless. That was back in the early 2000s. Perhaps newer units are better at that.

Efficiency is nice, but it's only one part of the equation. Technically my wife's car is a lot more efficient at fuel than my truck. However if I need to haul a trailer full of whatever, the car just isn't going to cut it.

Not as bad here on the coast but I did fire up the wood stove last night will probably try to run it for the next few days to get through the cold snap. May not be as efficient as the heat pump but if I really wanted to it doesn't take much work to make it 90° inside the house with the wood stove whereas the heat pump will never come anywhere near that.

[blues]

My HVAC man told me that below 40 the heat pump is basically useless. That was back in the early 2000s. Perhaps newer units are better at that.

Efficiency is nice, but it's only one part of the equation. Technically my wife's car is a lot more efficient at fuel than my truck. However if I need to haul a trailer full of whatever, the car just isn't going to cut it.

Not as bad here on the coast but I did fire up the wood stove last night will probably try to run it for the next few days to get through the cold snap. May not be as efficient as the heat pump but if I really wanted to it doesn't take much work to make it 90° inside the house with the wood stove whereas the heat pump will never come anywhere near that.

Mine works just fine down to 20* at the very least...and I think it was about 15 when I went to bed and the indoor temperature was at the set point.

So, clearly they will work down pretty low...but I wonder how the cost vs. the $3 / gallon for the propane furnace works out from an efficiency standpoint once the temp drops into single digits. (I'm way too lazy and ignorant to try to plot that chart.)

[BN]

We're in Ohio. We have Geo Thermal plus propane. Our propane comes on at around 20*. It was set that way when it was installed, I think. There is a sensor outside that reads the outside temp. Our thermostat automatically switches from AC to heat when needed. We set it to cool to 75* and heat to 69* and leave it there year round.