Using my heat pump in very cold weather

Messages
262
I have a Trane brand heat pump with electric furnace as back up. My HVAC guy said he turns his to emergency heat ( furnace only) when the outside temp drops into the 20s.or lower. Everything I read on the internet says never do that , that the lowest electric bill will be if you leave it on heat pump mode always, as it will automatically turn on the furnace when it can't keep up. I can see that being true if when the furnace turns on that the heat pump turned off. But since both run even if the furnace is running, how can that be more efficient??
 
Messages
5,711
Location
Atlanta,GA
Perhaps at the set point you have the system is better able to modulate the amount of heat generated by the strips by handing off some of that load onto the heat pump. In any case your system should be designed so that heating strips are rarely used during the year. I think the winter design temp is around 99 percent (hrs/per year) so your heat pump should be able to function without emergency heat for all but the coldest 1 percent (hrs/year).
 
Messages
25,976
Location
Upstate NY
My home near the shore in DE has two Trane heat pumps with what is called booster coils. The booster coils come on to supply extra heat and also to warm the air when the heat pump goes into de-ice mode (basically into A/C mode).

I have Trane thermostats connected to WIFI and from the app on my phone I can see a lot of information about what the heat pump is doing with respect to cooling or heat or emergency backup (booster coils). You can experiment and look at the data on your phone app.
 
Messages
1,237
Location
SE Ohio
Your furnace itself isn’t running while the heat pump is, the blower on it is circulating the air. My heat pump runs until it gets below 28 degrees then our gas furnace takes over.
 
Messages
59
Location
Raleigh Durham NC
I have a heat pumps in my home and I never switch to emergency heat. The heat pump does run long times when real cold, but switching to emergency heat and running the strips will cost you more. I just set it and forget it. I do have gas logs when it gets real cold.
 
Messages
1,036
Location
Virginia
A heatpump will always be more efficient than an electric resistance heater. But, as outdoor temperature decrease, a heatpump's output diminishes. An electric heater is 100% efficient in that there is no waste - all of the electricity used is turned into heat. A heatpump is greater than 100% efficient - a heatpump can use X energy and extract/transfer 2, 3, 4 times the equivalent heat, if that makes sense.

But, like I said, the capacity of heatpump decreases as outdoor temps decrease. For example, a 3 ton heatpump may produce 36,000 BTU at 50*F, but may only produce 20,000 BTU at 25*F. This effect is compounded due to the fact that your house is also losing heat quicker when it's colder outside, so the heatpump has to work harder (longer).

In my house, we have a 5 ton and a 2.5 ton heat pumps. The 5 ton has a propane furnace and the 2.5 ton has heat strips as backup or emergency heat. The 5 ton does a much better job at producing heat in its zone as the temps drop, but even then, I switch over to propane when the temp drops below 30. The propane heat is hotter and is more comfortable to my family. The 2.5 heatpump struggles below 40, so that's where I switch it over to the heat strips. Again, the strips produce a hotter heat and is more comfortable. At the end of the day, the heatpumps are more efficient when it comes down to $$$.
 
Messages
681
Location
Southern MN & Omaha NE
The weekend/early next week in NE will certainly be too cold for your heat pump to provide much value (high of 6 above and a low of -8 in Omaha). I'd expect that your system would be able to realize it can't work effectively at that temperature, and switch to the resistance strips accordingly. Or you could just lock it into em-heat yourself. It's going to be expensive, but would be functionally the same as letting the system control itself (assuming you're below the threshold that it switches to resistance heat at, and assuming that your system is "smart" enough to do so). What would be less wise is to let it spend the winter in (resistance) em-heat exclusively and not utilize the heat pump technology on more mild days. But if it's cold enough, it shouldn't matter anyway.

(My grandparents, who lived in east-central Nebraska, heated their relatively large house with an electric resistance "furnace" exclusively, to 75 degrees around the clock. They had ~$500 electric bills all winter long. That's the rough ballpark you'd be looking at if you locked out your heat pump and left it on em-heat all winter long. I have no idea why they didn't just get a heat pump instead of the central ac they had - your climate is pretty darn cold for resistance heating to make sense.)
 

Zee09

Site Donor 2021
Messages
5,189
Location
Fairhill Maryland
I have a hybrid system and cheap electric. 6.39 kwh.
That said in the winter I just put it to Aux and use oil heat.
Not my cup of tea. Here in MD. I see no use for not having propane or oil.
 
Messages
714
Location
Indiana
When you switch your thermostat to emergency heat it disables the heat pump so it wont run. I am a HUGE fan of our r410a heatpump, for every dollar of electricity we put into it you get 1.5 to over 4 times the amount in heat depending on the outside temp of course. We run ours down to zero degrees, our backup heat is electric resistance heat strips, 20kw in all! very expensive to run.

I have modified our strips controller so that you can completely disable the strips. So when its 35 degrees out and the unit goes into defrost the strips will not come on and the air temperature from the vents gets cold, this rarely bothers us and saves a considerable amount of electricity over the years. When it gets down below 20 degrees or so depending on the wind our heat pump will run almost continuously. when it gets below 15 degrees or so is when I start enabling the strip heaters heat during defrost, once we get to 5 degrees or so we need supplemental heat.

I have been studying our setup for over 10 years and I have figured when its 0 degrees or heat pump still has a COP of at least 1.5 maybe 2. I have even run it down to -10 and its still putting out heat, at that point its starts to get questionable weather its worth running it.

Remember the cooler it is outside the lower you refrigerant pressures run, less load on the compressor equals less electricity consumed. Of course you get less heat output as well.

IIRC our heat pump uses 3kwh an hour at 10 degrees.

Now if you have a natural gas furnace as back up that will change things quite a bit. And all of this depends on you prices for the various fuels available to you in your location.
 
Messages
714
Location
Indiana
I have a hybrid system and cheap electric. 6.39 kwh.
That said in the winter I just put it to Aux and use oil heat.
Not my cup of tea. Here in MD. I see no use for not having propane or oil.

I will say that when we got our air source heat pump I inquired about propane backup instead of heat strips, thankfully the installer talked me into staying with all electric. A few short years later there was a huge propane shortage and people were paying 5.50 a gallon locally if you were even able to get it, and even then I remember some placed limited you to 100 or 150 gallons.

Of course a MD winter is a lot different than our balmy Indiana winters lol.
 

Zee09

Site Donor 2021
Messages
5,189
Location
Fairhill Maryland
I will say that when we got our air source heat pump I inquired about propane backup instead of heat strips, thankfully the installer talked me into staying with all electric. A few short years later there was a huge propane shortage and people were paying 5.50 a gallon locally if you were even able to get it, and even then I remember some placed limited you to 100 or 150 gallons.

Of course a MD winter is a lot different than our balmy Indiana winters lol.
Not to one up you but we had a joker here United Propane was charging $7 a gallon.
That is why I tell people before you buy a house look into the propane situation. Never build a house and fall for free propane tank and setup if you contract for fuel.
But if you own your own tank you can also prebuy your propane.
Of course most know this.
I filled up this year for 88 cents but my best year was 67 cents.

But i was getting screwed for $4 many years ago...lol
 
Messages
4,965
Location
Lima, Ohio, USA
all i know is i went through one winter in a Townhouse with a heat pump/electric furnace combo. everything worked great until we had a big ice storm come through.

the whole unit was encrusted in anbout 1/4 inch of ice, and the furnace didn't seem to want to kick in. it was starting to get cold inside (probably mid to upper 50's)...
the only thing i could think of was to try and melt through some of the ice to "jump start" the system if you will...
luckily my roomie always kept a couple pizza stones in the oven, to I preheated it to about 350, then gloved up and carried the 350 degree slabs out the patio slider to the heat pump. once they melted their way down to the casing, the system fired up and de-iced itself, and within a couple hours the place was back to normal..

does that story help anyone here with anything? probably not. but, it's the only heat pump anecdote i've got...
 
Messages
13,092
Location
Indiana
The outdoor temp sensor is inside the heat pump apparently so when it’s sunny (the unit is on the south side of the house), it’ll give a false reading. Reads 45*F when it’s 20*F for example.

I figured this out when we bought the house and switched to gas furnace heat only. We saw no noticeable difference in our power bill so I don’t worry about it.
 
Messages
333
Location
Atlanta, GA
What kind of thermostat do you have? Some if not all of the smart thermostats should automate and optimize when to supplement or supplant the heat pump and even within that should have settings on what your priority is. Nest for example has 3 settings - Max Savings, Balanced or Max Comfort - mine is set to Max Savings so it allows a little more leeway and time to temp before the aux heat kicks on.

As I understand the more basic single speed heat pumps are sufficient down to the teens, I gather the ultra fancy and expensive variable speed units are good for temps even lower. Seeing low 20's so far this year in Atlanta I don't believe the strips have kicked in yet this year with my basic single speed Lennox heat pump and my electric bill is still trending very similar to prior 3 years ($90/month budget billing).
 
Messages
855
Location
Ozark Mountains
One thing that has not been talked about is the wear on the heat pump when they run 100% of the time when it gets very cold out as compared to running the emergency heat that runs 1/4 to 1/3 of the time and the compressor is off in emergency heat mode.

When it gets in the teens I switch to emergency heat on our heat pumps. It might use a little more energy but I like saving the equipment. For the few days a year I run the emergency heat, it does not cost much extra but it helps the equipment last longer.

Also, compared to the heat pumps running 100% of the time when very cold, the emergency heat runs very little. It seems like if a heat pump like mine runs in emergency heat 1/4 of the time of the time at 17 degrees outside and in the normal mode runs 100% of the time, in the normal mode would have to be 4 times more efficient than the emergency heat.

It is a lot warmer feeling in the house when running the emergency heat than the normal heat pump 100% of the time.
 
Messages
714
Location
Indiana
One thing that has not been talked about is the wear on the heat pump when they run 100% of the time when it gets very cold out as compared to running the emergency heat that runs 1/4 to 1/3 of the time and the compressor is off in emergency heat mode.

When it gets in the teens I switch to emergency heat on our heat pumps. It might use a little more energy but I like saving the equipment. For the few days a year I run the emergency heat, it does not cost much extra but it helps the equipment last longer.

Also, compared to the heat pumps running 100% of the time when very cold, the emergency heat runs very little. It seems like if a heat pump like mine runs in emergency heat 1/4 of the time of the time at 17 degrees outside and in the normal mode runs 100% of the time, in the normal mode would have to be 4 times more efficient than the emergency heat.

It is a lot warmer feeling in the house when running the emergency heat than the normal heat pump 100% of the time.
I would say the efficiency comparison would require to know the cost and amount of the backup "fuel" source used in an hour and if its electric strips the size of your heat strip package.

I had all this info plotted down at one time for our particular situation, I will see if I can find it, I found it to be interesting.
 
Messages
3,507
Location
Cincinnati, OH
X2 the system will activate EM-Heat when it’s needed to maintain desired temperature. It’s much cheaper than forcing the furnace to run EM-heat. Money would be best spent on a blower door test and air sealing your home to help retain temperature longer and “help” the system.
 
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