# How does a heat pump system work with a woodstove?!



## tickbitty (Jan 30, 2010)

Part of the reason we got a woodstove this season is because our oil furnace cr@pped out and we were using a heat pump.  The furnace was in the partial basement and had heated through ductwork under the house.  It was a very old system.  The heat pump is part of the AC system we had installed into the attic a few years ago, it has ceiling ducts and intakes.    We were not keen on the electric bills or the heat pump heat.  It seemed to be running all the time and using auxiliary heat more than we wanted.

So now we have the woodstove which is doing a good job, we need some work with fans and etc to get the heat elsewhere in the house, but all in all it is OK.  Because we are not home during the day and etc, we don't keep the stove running 24/7 so we still need the heat pump on to keep the temps even-ish in the house.  

Should the heat pump be left on set at the desired temp?  Or off with just the fan on?  Or several degrees less than the thermometer is reading, as long as it is warm enough for us in the house?  Will it help to move the heated air around?

I just don't really get how a heat pump works I guess.  With the furnace, you just set it and it comes on if the house gets cold, and that's that.  This thing doesn't seem to work that way at all.  The first night we had the woodstove going all night, the auxiliary heat was even coming on.  

The thermostat is around the corner and down the hall, so it is quite a bit cooler than the area where the stove is.  So I would think we could just set the thermo where we want it and expect it to come on whenever the woodstove is not keeping up, but it just doesn't quite seem to work that way.


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## Pagey (Jan 30, 2010)

We have 2 heat pumps - a smaller one for the upstairs and then a little larger one for the downstairs.  You can read up on them, but basically, from Wikipedia: A heat pump is a machine or device that moves heat from one location (the 'source') to another location (the 'sink' or 'heat sink') using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink.[1] Common examples are food refrigerators and freezers, air conditioners, and reversible-cycle heat pumps for providing thermal comfort.

Believe it or not, there is heat in the cold air, and that heat pump pulls that heat out of the air and sends into your home.  When its too cold for that to work effectively, the auxiliary heat comes on, and this is what is so expensive.

My wife is home to feed the stove most days, so our units don't run much unless she's gone.  I set the downstairs to 65, and on all but the coldest of nights it doesn't kick on until around 2am or so.  The upstairs is set to 70, and the timing is about the same.  When we are home to run the stove, I keep them at 70 and 65, but the house is warmer so they don't run.  The fans they use are very poor circulators of warm air, in my humble opinion.  The ductwork just won't keep the air warm on its long journey. 

We incorporate the heat pumps into our overall heating plan.  In milder weather, they can be quite efficient and effective at heating/cooling a home.  Ours used to run all day every day, now we might run them 4 or 5 hours out of 24 during really cold weather (for this house, teens or less).

The blower on the stove has really helped to move the convection air, so it's staying warmer longer in the house right now, and that seems to have cut the time the heat pumps would run.  I figure it's a bit of a wash, though, as the blower is electric.  :lol:


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## hyperion (Jan 30, 2010)

Actually, a heat pump doesn't work much different from a furnace, i.e. it comes on when the thermostat calls for it. However, it is different in the way it heats. A heat pump doesn't generate heat, it extracts heat and transports it and thus it is quite efficient until the outside gets to around 35F or below. Another marked difference is that a heat pump doesn't put out the same amount of heat as a furnace. The air that comes out of the vents are around 90F, which feels cool compare to what a furnace provides. Thus it is designed to be on a lot and maintain a certain temperature. We learned that with a heat pump, it's actually better to set a temperature and leave it (i.e 68F in winter, and 75F in the summer for us), then trying to turn down or up when we are not in the house, like you do with a furnace. 

For us, the thermostat is kind of close to the wood stove and when the stove is on the heat is never on, which is actually a problem because we have a floor plan with living area upstairs and bedroom downstairs (typical hillside construction). The stove is upstairs in the living area. Sometimes we have had to turn the thermostat to 80F so that the heat pump would come on and heat the downstairs.

Just saw Pagey's post above me, which appeared when I was typing this post. I wish we had a two-level heat pump system like you do. Would solve all of our woodstove/thermostat problems. Right now, even if we rig the heat to come on, we never know when to turn it off. We also have to close all the vents upstairs. A few times we have forgotten to open the vents back up, and it wasn't pretty.


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## dougand3 (Jan 30, 2010)

The heat pump tstat works like your furnace did...when set temp is reached - hp shuts off. In temps above 40F, the hp grabs heat out of outside air and TRANSFERS heat into your house. Below 40F, you'll have aux electrical resistance strips BURN TO PRODUCE heat (big energy hog). HPs just aren't good in cold climes. Don't make big swings in temp setting when in heat mode (winter). Normally, greater than 2 deg increase causes elec strips to burn. So, they are more of a set temp and forget.
If you have your Lopi blazing and hp inside blower is running - that heat will go in the return and be sent out to your registers. I don't understand why the tstat wouldn't register this increase and shut hp down. Bad tstat? Air doesn't get to tstat - weird place? single digit temp outside? need refrigerant recharge in hp? So many variables here.
Get some $1 thermometers from discount store and place around house - stove room, hall, BR, etc and see what temps are.


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## SlyFerret (Jan 31, 2010)

As has been mentioned, it's been too cold outside for a heat pump to work effectively, which is why the resistance heating is coming on.

Try setting up some box fans (on the lowest setting)  to blow the cool air in the house towards the stove room.  Creating this air movement will encourage the warm air to take its place.  You might be able to keep your thermostat from kicking your backup system on.

Heat pumps can be great though.  My propane furnace is only about 13 years old, still in good shape, and hardly ever gets used, so it will be quite some time before it gets replaced, but... 

When it is time to replace it though, I will be putting in a geothermal heat pump.  The nice thing about the geothermal unit is that it pulls heat out of the ground  instead of the air.  Since the ground is always about 55 degrees, it's always working efficiently.  I can't think of a better combination that my wood burner for primary heat, and a geothermal heat pump for a backup.

I expect that I'll probably only spend $100 or so on propane this year, maybe slightly more depending on how the spring shoulder season goes.  If I could do that with a heat pump, it would only cost me a fraction of that for the little bit that I use my backup central-heat.

-SF


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## Corey (Jan 31, 2010)

The main difference with a heat pump is they work best in short bursts.  It is literally an air conditioner running backward, so just like you get condensation draining off the inside coil in the summer, you get condensation on the outside coil in winter.  *Except* this outside condensation usually forms frost/ice on the coil.  The problem with the frost is it insulates the coil and cuts efficiency and can form an ice block in the worst case. So if you can run the heat pump in short ~ 10 minute blast and shut off to let the coil defrost, then another 10 minute blast , defrost, etc. That is the most efficient way.  If the heat pump has to run for a long time, it will either kick on electric defrosters or run in reverse to thaw the outside coil.  In that instance, you're basically pumping heat outside which is wasteful.

So what ever method you run your heat pump, it's best used to maintain an even heat as opposed to heat up the house several degrees.


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## tickbitty (Jan 31, 2010)

TY for all the information and suggestions.  They do help.  WHen the system seemed to be struggling the most was when we tried to have the thermostat set just a bit higher than the stove could do.  I kinda thought that the stove could get it a good part of the way, and the heat pump could help get it warmer in the rest of the house that the stove doesn't do as well, and it seemed to have a hard time with that.  I actually wondered if the thermostat became confused because the intakes were pulling in hotter air than the thermo was registering or something?  
But anyway, thanks for the explanations, they are helpful.


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## begreen (Jan 31, 2010)

dougand3 said:
			
		

> The heat pump tstat works like your furnace did...when set temp is reached - hp shuts off. In temps above 40F, the hp grabs heat out of outside air and TRANSFERS heat into your house. Below 40F, you'll have aux electrical resistance strips BURN TO PRODUCE heat (big energy hog).



This really depends on the efficiency of the heat pump. Our unit will keep on eeking out heat from the air until around 25 degrees before switching to the resistance coil heaters. If you are getting a new system, go for a serious heating efficiency rating.

We use both the heat pump and the wood stove. Ours is a multistage unit that has dc, variable speed blowers. It works out well when the woodstove is going because if it does kick on when the stove is burning, it's usually at low speed. The duct system is fully insulated and going through a conditioned space that stays around 60 in the winter so it loses very little heat. That keeps the house temps exceptionally even.


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## Green Energy (Jan 31, 2010)

Heat pumps can be a great compliment to wood heating.  As many previously said heat pumps work best when its above 40 F outside.  Where I'm at, we get a good number of days in the 40s with nights in the 20s, like later this week.  In that weather, I run a fire in the evening and refresh it in the earlier morning, but it dies down during the day.

A key goal for me is to avoid the auxiliary resistance heat coming on by increasing my wood heat as it goes below 35 F.  When it gets into the 50s during the day, and 40s at night, I let my stove go out until it gets into a colder pattern, gives me a break, and conserves wood.  The past month and a half has not since much of that, so I burn quite a bit of wood.  I hope to see more of this later in February.  March is definitely my transition month.


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## mitchinpa (Feb 1, 2010)

My heat pump is 3 years old.  My house is also 3 years old and insulated pretty good.  The heat strips wont turn on till it's around 12 degrees out (also depends on how windy it is).  I do have the fireplace going most nights in the winter, so if the heat does come on, it's usually not till the next day when I leave for work and the fire goes out.  

Doug is correct in that heat pump owners should not change the t-stat on a daily basis, as any swing in temp more than 2 degrees will activate the heat strips and affect your electric bill.

When the fireplace is going, my heat pump is set to 67 degrees.  Usually when I go to bed, the house is around 75 or so.  The upstairs is around 72, as I do use the "fan only" setting on the heat pump to move the air a bit.  I'll leave the fan on for about 20 minutes each time, and I do this once every 2 hours or so.


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## pyper (Feb 1, 2010)

tickbitty said:
			
		

> Should the heat pump be left on set at the desired temp?  Or off with just the fan on?



I set our thermostat at about 65 when we go to bed, and when it gets cold the heat pump comes on. I turn it off in the morning, and set up a fire so the stove is ready to go when someone gets home. Ours does that defrost thing. It really bugs me to see steam rising outside! 

Instead of dollar store thermometers, for $15 or so you can get a digital meat thermometer. I've got one of these:

http://www.target.com/Taylor-Compac...rmometer/dp/B001G97F7A/ref=sc_qi_detailbutton

I just walk around with it to see the temperature in various places. It adjusts fast, and it's accurate. Mainly I use it for meat, but it's great for room temperatures too.

I've run our heatpump in "fan only" mode a few times. Our return is in the room with the stove. If it's 90F in that room and 53F in the back of the house, then the fan-only will do some good. I get a big temperature drop through the ducts though, so if the stove room is a normal 75F or so, then it doesn't really help.

If I insulated the crawlspace it might make a difference -- maybe it would help to just get the ducts up off the ground. Give it a try and measure the air coming out of your ducts to see if it's helping. If I have 75F air blowing into a 53F room, after 4 or 5 hours it will be a 60F room. I've been told it costs about $1 a day to run the blower (but i don't know if that's true).


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## begreen (Feb 1, 2010)

Nice Mitch. Sounds like a very efficient unit. What make and model is it?

Our heatpump system is set to 63 at night, 68 during the day. I don't touch the thermostat unless it's mild outside and I want to bump it up to 70 at night. If I am burning wood, the heatpump will stop running once the hallway temp exceeds the thermostat setting. This usually takes about an hour in the morning from a fresh stove start. If the stove is running 24/7, the heatpump rarely kicks in unless it's very windy outside or very cold.


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## Pine Knot (Feb 1, 2010)

My heat pump and stove work well together. My house is a split level with the stove on the lowest level, and the thermostat and cold air return on the midlevel. I set the air distribution fan  to run all the time, day or night. This keep the air moveing through out the house and results in a more even temp. room to room. When the stove dies down the heat pump picks up the load, it's almost seamless, except of course the electric meter runs faster


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## mitchinpa (Feb 2, 2010)

BeGreen

My heatpump is a Lennox Elite series.  Not sure which model, as I am at work at the moment, but when I bought it, it was their "top of the line".  I believe they have an even better model now.  

My house has an open foyer as well, and that really helps get the heat to the second floor.


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## daleeper (Feb 2, 2010)

tickbitty said:
			
		

> Part of the reason we got a woodstove this season is because our oil furnace cr@pped out and we were using a heat pump.  The furnace was in the partial basement and had heated through ductwork under the house.  It was a very old system.  The heat pump is part of the AC system we had installed into the attic a few years ago, it has ceiling ducts and intakes.    We were not keen on the electric bills or the heat pump heat.  It seemed to be running all the time and using auxiliary heat more than we wanted.
> 
> So now we have the woodstove which is doing a good job, we need some work with fans and etc to get the heat elsewhere in the house, but all in all it is OK.  Because we are not home during the day and etc, we don't keep the stove running 24/7 so we still need the heat pump on to keep the temps even-ish in the house.
> 
> ...



To answer the topic title, here is how we use our heat pump with the wood stove.  Our stove is in the basement, and I have an office in that room that I work in 1-2 days of each week, and am gone through the day the rest of the week.  I keep the wood stove going 24/7 when it is 30-35 degees F. or below outside, and burn mornings and evenings till it is about 40, then the stove goes out.  I have a nice programmable thermostat that controls both our heat pump and gas furnace (and a/c).  We keep the thermostat set at 68 during the day, and during sleep hours, 72 when we are getting ready for the day, and evening time.  Our heat pump has a thermostat on it that sets the lowest temperature that it will run, and ours is adjustable and is currently set at around 32.  When it is below 32 outside, then the gas furnace kicks on.  I noticed during some mild weather last week that during warmup in the morning the gas furnace kicked on, and then that afternoon for warmup when the girls get home from school that the heat pump kicked on.  If I keep the stove hot, it will keep either unit from kicking on, but it gets hot in my office sometimes, and I let the fire go down, and then the upstairs thermostat calls for heat.  I guess I really don't mind that either kick on to supplement heat in the house to keep it comfortable.  I have found that we are only burning 50 gallon of lp gas a year to keep the house comfortable, and I can live with that.  The heat pump will not run much in Dec and Jan, and adds a little to the electric bill the shoulder months, but not too bad, we live with it, but if I had to choose between the heat pump and the gas furnace, I would rather have gas as our backup heat.

It sounds like you have a couple of things going on that you need to consider.  One is what temperature your auxiliary heater is kicking on at, find out what temperature it is set to kick on at, and if your goal is to prevent it from coming on, then fire that wood stove up hotter on those days it is colder outside.  I am assuming that your auxiliary heat is resistant electric heat.  You may need to get with your heating/cooling person to adjust that setting one way or another, or shut it off if you don't want it to ever kick on.  The heat pump will run more and more as it gets colder, just as the a/c will run more and more as it gets hotter in the summer.  At some point it will not be able to keep up with your heating needs if it is not an over sized unit for your house.

Another item is that your heat pump ducts are in the ceiling.  This is not bad for a/c, but not so good for pushing heat, unless the ducts are well insulated, you are loosing heat into the attic.  Is it well insulated?  If it is not, then that may be contributing to the auxilary heat strips kicking on more frequently, trying to heat your house faster than the heat pump alone can do.  Also, your heat pump may be low on charge, and not be pumping most efficiently.  If you haven't had the furnace person clean and check the system in a while, it might be a good time to do so.

If you are not serious about running a wood stove 24/7 when it really gets cold outside, then you need to consider a high efficiency gas or oil fired furnace, and run it instead of that heat pump.  Heat pumps are very good for the shoulder season, but will run out of efficiency when really cold outside, most likely when your wood stove really needs help keeping the house warm.  If you don't mind the house cooling off at times, and you refuse to pay the gas/electric man, then shut off the auxilary heat, let the heat pump run as best it can, and load that stove up and let her rip.


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## Lumber-Jack (Feb 2, 2010)

Seems like most of you guys have your heat pumps set so the aux heating kicks in below 32 F  (0 C), That seem far too high to be getting the most out of them, especially if your back up is electric resistance.
Typical heat pumps performance show that even at -8.3 C  ( 17 F) they will still transfer 2.3 (kWh) of heat for every 1 (kWh) of electricity supplied. 

 Check out this link  then scroll up just a tad.


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