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Air source Heat Pump

Post in 'The Green Room' started by 73blazer, Sep 13, 2010.

  1. 73blazer

    73blazer Member

    Joined:
    Jan 29, 2008
    Messages:
    164
    Loc:
    Birch Run, MI
    Over the summer, my Air Conditioner crapped out. So instead of simply replacing it, I replaced it with an air source heat pump. I have a 2 year old 95% efficient forced air propane furnace I put in 2 years ago, and also of course my nice North Star fireplace. I did some cost projections, it seems since gov't forced everyone to the new R410A refrigerants, everyone had to redesign all their equipment anyway, since R410A requires higher pressures among other differences. The heat pump versions only cost roughly $500-700 more than their air conditioner counterparts. Using some nice spreadsheets supplied by HVAC manufacturers, it suggested I would save about $350/year over straight propane. The wood heat really isn't a factor, I don't fire it up much when it's above 35 degrees anyway, which is the lockout point of the heat pump anyway (meaning, heat pump doesn't run below 35 degrees because it becomes rather inefficient below those temperatures). I know reality is different than a spread sheet, but even if I get half the $350 projected savings, that's $175/year, that's only a 4 year payback, that's almost a no-brainer. I put in a nice 16seer unit. I figured there's a good 3-4 months of the year of 35-60 degree days where heat pump could be used instead of gas and too warm to fire up the north star (it'd blow me out of the house).
    I cannot believe how much quieter that unit is than my 20 year old unit that crapped out. I can hardly hear it run.

    This past week I finally had a chance to fire up the heat pump side of the unit and see how it works. I'm actually quite impressed. It's blowing 95 out the registers while it was 48 outside. That's pretty darn good, better than I expected. I know alot of people put in heat pumps and are disappointed because they blow luke warm air. I knew heat pump air would be far lower than gas fired furnace air, I really expected it to top out around 85 or 90. 95 is really good.

    I think alot of people are disappointed because they have the wrong expectations, or perhaps a bad install(er) where they didn't set it up right. They'll never blow 115-120 degree heat and they aren't a replacement for a gas furnace in a cold climate. You have to couple it to a good thermostat with an outdoor air sensor. My t-stat allows you to set heat pump lockout below a certain outdoor temp (a must here in Michigan), and also it allows for "droop" control which says if the heat pump is running and the temp in the house actually drops x degrees, switch to your gas furnace. That's nice in case there's an issue with your unit or it goes into defrost mode (meaning it's not making heat anymore while it's defrosting). The t-stat and heat pump unit has a feedback circuit anyway for defrost mode, which will send it immediately to gas furnace if the unit kicks in to defrost mode. The installer said he would not expect it to go to defrost mode unless I was running the unit in below freezing temps for long periods of time. Also it has control that says if heat pump is running and it takes more than X amount of time (you set X, I use 0.5hrs) to get to set temperature, than switch to forced air. Nice if your on vacation or something and you come home to a 50 degree house, the heat pump will take quite some time to get it up to 70 from that starting point. I think also heat pumps get a bad rap in colder climates because of just plain lack of knowledge on the installers part. My uncle is a retired HVAC installer. I asked his opinion on it, he said no way here in Michigan, complete waste of money. BUt my research and now real world usage has really proven him wrong. I think he formed his opinion back in the 70's when heat pumps started to become available to residential customers, but they were horribly inefficient and I could see them not working so well here in cold climates. I think he formed his opinion then when he installed a few and just never gave them a chance since.

    So far, I'm pretty happy. Of course I've only run it a couple of times. I like having the 3 ways to heat my home now. Electricity, propane and wood. I always have the wood which is nice, but kinda not so worth using the wood and/or choking off the wood stove to the point where it's smoking up the flue alot. Plus I'm gone alot and the wife just doesn't really like running the wood stove (she's somewhat afraid of it, i've been working on her, she's come along way, but I think another couple years at least before I think she'd run it 24/7 while I'm on a business trip). Plus with this setup, If propane price spikes, I can always set the heat pump lockout point lower, or if electricity spikes, I can set a higher lockout point. If they both spike, I can simply just stock up more wood. I have a basically free supply, but it's the work to cut, haul, split, stack, move.

    For those that are considering a heat pump in a colder climate, don't let an installer talk you out of one. Do your own research and come to your own conclusion.

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  2. woodgeek

    woodgeek Minister of Fire

    Joined:
    Jan 27, 2008
    Messages:
    2,524
    Loc:
    SE PA
    I am a big fan of my HP too, esp with my milder climate and during shoulder season. With my choice based on $0.10 wind power or fuel oil, I run the HP down to 15°F. (We prob spend less than 100 hours/yr below that).

    Nice spreadsheet, but not completely satisfying either. With my elec rates in PA set to double in the next couple years, I am going to switch my dual fuel to a higher balance point. I am not sure the ss will help me with that. I can get nom COP versus temp for my unit, but no one tabulates the value including defrost demand or less than dry air. Maybe I'll mod the spreadsheet....
  3. 73blazer

    73blazer Member

    Joined:
    Jan 29, 2008
    Messages:
    164
    Loc:
    Birch Run, MI
    Shh...your not supposed to know about that defrost cycle, nothing to see here......move along now.....

    Yeah, that's not factored in anywhere.
    How often, when you run it at 15 or 20 degrees, does yours actually go into defrost, praytell?
  4. begreen

    begreen Mooderator Staff Member

    Joined:
    Nov 18, 2005
    Messages:
    46,003
    Loc:
    South Puget Sound, WA
    The newer systems can work quite well if everything is fined tuned and the ductwork well insulated. We are seeing 105-110 at the register depending on the stage running on the heat pump.

    We run our system on the heatpump down to about 25 degrees and yes, at that temp it is really working hard. Below that it switches to the resistance coils. The system will go into defrost cycling when it has been running a lot and the temp is below about 32. (It runs more when it is both cold and windy, natch.) But normally we're burning during the cold weather so we don't notice this too much.
  5. woodgeek

    woodgeek Minister of Fire

    Joined:
    Jan 27, 2008
    Messages:
    2,524
    Loc:
    SE PA
    My cheap (single stage Goodman) 14 SEER HP has a simple timer control that defrosts every 60 minutes of run time when the coil is below 35°F (which works out to air temps <40F). The cycle runs until the coil gets heated to something like 50°F, which takes only a couple minutes if the coil is ice-free, up to 6-8 minutes in worst case (60 run minutes in freezing rain/mist). The defrost seems to take less time/energy as it gets colder and dryer outside, worst case is def wet/snowy weather 25-34F.

    The energy use for defrost wouldn't be that bad if we let it suck heat out of the house to heat the coil, we could get the heat back later at a high COP. But it would give a 'cold blast' out the registers that no one wants, so we run a strip during defrost, 15 kW in our case. I suspect this drops our overall COP by 20% from tabulated values below 35F.

    Of course, the installer set the defrost timer to 30 minutes (causing double the defrost loss). I flipped the DIP switches to 60 minutes, and watched it closely across different weather conditions to verify that it didn't ice up excessively (it didn't). The installer, of course, doesn't pay my bills, and wants to avoid a service call, so plays it safe on the defrost timer.

    FYI, using the HP for 90% of my winter heat (running it all season) here near Philly, I am getting a seasonally averaged COP or SCOP of ~2.1. YMMV.

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