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New Install - Wood vs Solar & Heat Pump - Solar Wins!

Post in 'The Green Room' started by peakbagger, Oct 27, 2013.

  1. peakbagger

    peakbagger Minister of Fire

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    After the installation of my new mini split, I did some calculations and came to a fairly interesting conclusion. If someone is starting from scratch and does not have a wood stove or boiler, and are strictly considering heating cost, PV panels with a high efficiency air source heat pump is the least yearly cost to heat if the utility has net metering. Before the pitchforks get warmed up to go after the heretic, take a look at the numbers.

    I have attached a pdf of the spreadsheet and it state my assumptions. I based the sheet on my yearly wood usage. Obviously some of the assumptions are up for debate. If anything, I think the installed boiler and wood stove costs may be low as they don't really include a chimney and hearth which a new person may not have. The yearly output of the PV panels is based on the PVWatts program which is generally regarded as fairly accurate, those farther south will get more output and my installation is slightly impacted by my shallow roof angle.

    As for my wood cost assumptions, I used $220 per cord for cut split delivered green wood. I also did a run based on scrounge cost of 50%. The 50% factors in opportunity costs for things like log splitters, trailers, chain saw gas and oil, splitter gas and safety equipment. Its a swag but I expect for purposes of the spreadsheet I will use the 50%.

    Opportunity cost is the money you could get long term on a safe investment (these days its closer to 1%). This is money you would be earning if you didn't make the investment.

    Standard grid tie PV is not a suitable backup source when the power is down, although my one ton hyperheat is a 15 amp circuit at 240 volts (3600 watts) could be run by most generators.

    The mini split is also a space heater with no hot air distribution. It also locks out at -15 degrees F (where its COP is also lower). The mini split I used is 12,000 but nominal output, the output goes down as the outside temp drops towards 15 and goes up as it warms up. Someone with a high winter peak demand would need multiple units to meet a peak load. This would be a better fit for a tight home.

    Obviously everyone's situation is different but its definitely not the result I was expecting

    Attached Files:

    Last edited: Oct 27, 2013
    Where2 and woodgeek like this.

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  2. STIHLY DAN

    STIHLY DAN Minister of Fire

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    That looks good. Although the average house needs 50 to 90 thousand btu's. So you would have to multiply that by 4 to 10. They are great units. I have installed many of them. When cash is higher I will put some in my house.
  3. peakbagger

    peakbagger Minister of Fire

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    I agree although rarely does a moderate climate relatively "tight" house need 50 to 90 Mbtus. The systems are sized that large for cyclic operation while a heat pump would is going need to be left on continuously. I expect a better approach would be two or three units which would bump up the opportunity cost but wouldn't increase the electric usage so its still a winner (until it drops below -15 deg F).
  4. georgepds

    georgepds Member

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    I came to the same conclusion last summer. I've got 4.65 kw PV on the roof that puts out more energy than I use. I decided a heat pump was the way to absorb that extra energy. My calcs showed the recurring price per 100 kBTU was comparable to wood

    One thing you might want to consider is that the smaller units ( my 12k air/ 16k heat Fujitsu RLS2) just don't have enough output to cover the losses on the coldest days up here in New England

    Well that, and if you have a nice wood stove, the temptation to continue to use the stove is just too great.
  5. woodgeek

    woodgeek Minister of Fire

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    My retrofitted 1960 Split level only needs 45kBTU/h at design temp--5°F, and it is 2300 sq ft. Most of the time less than 25 kBTU
  6. peakbagger

    peakbagger Minister of Fire

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    I have been heating one entire floor of my house plus half the second floor with the one ton unit since the install, about 20 degrees at night and 40 during the day . So far no issues down to 20 degrees F. I have a stairwell between the two floors so the heat rises up to the second floor. I do have to run a ceiling fan on the second floor to move the heat around (strangely I run the fan blowing up towards the ceiling instead of down otherwise there is a draft). I have KWH meter on order so I can measure my daily power usage on the heat pump. Its a tight house with insulated blinds on the windows and 6 inch walls.
  7. Augie

    Augie Feeling the Heat

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    Even durring the economic downturn I have averaged more than that yearly, and when you look at any 10year rolling average Im closer to 10 percent, as are most of the guys I work with and my friends.....using 1 percent isnt valid IMHO
  8. peakbagger

    peakbagger Minister of Fire

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    I used 3% which is the current adjusted for inflation rate that most investment advisors recommend. I am basically comparing it to a 10 year bond which currently 2.5% (not inflation adjusted). I agree equity investors would be doing better at a higher risk. I don't think the rate really impact the results much.
  9. georgepds

    georgepds Member

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    FWIIW I use the hialeah ez read to monitor the heat pump

    I don't know anywhere else you can buy "utility grade" kwh meters (as apposed to plug in meters)


    http://www.hialeahmeter.com/siphwame.html
  10. Augie

    Augie Feeling the Heat

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    Pick and start date and then use a date 40 years later. The market as a whole(index funds) average over 11 percent year over year for a return.
    This includes the great depression Black Monday or the current economic downturn. I use 10 percent because it accounts for your own mistakes not thoes of the market.

    So basically if you invest 100 per month for 40 years you end up with 640,501.61. So......My plan is 350-500 per month, live in a smaller house, drive an older car, burn wood.... retire to a cabin in the mountains, and one in the Caribbean.


    Sorry to sidetrack back to the regularly scheduled program
    woodgeek likes this.
  11. peakbagger

    peakbagger Minister of Fire

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    Without spending a lot more time on investment strategy you are comparing apples to oranges. I agree that equity investing has higher returns but more volatility in the short term, you are preaching to the choir on this one. Of course looking at straight percent return over a long period without factoring in inflation is a dangerous game. Most standard models assumed an average 3% per year inflation so an average investment rate of return of 8% is actually 5% Bond returns are traditionally guaranteed day in day out (not so much for bond funds) . Generally when payback calculations are done they are based on bond rates. .

    My goal of these improvements is to reduce my monthly expenses to reduce the cost to keep the house i already own in the mountains.
  12. mr.fixit

    mr.fixit Member

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    I installed a 12,000 btu inverter mini-split(cheapo generic model) in my shop this summer.On AC it used considerably less kwh's than the old window AC.

    The last couple weeks I have had it as the only heat source in the 1400 sq.ft. shop.I have it set as low as it goes,62F. The main heat source is infloor radiant,but have yet to turn it on.

    I have the mini-split plugged into a kill-a-watt meter to record kwh's used.So far it has averaged 8-10 kwh's per 24 hrs.This week is expected to be colder so we will see what it uses then.

    I will try to report back when 30 days are up to see what it uses in a month.

    I am going to keep using it until it can't keep up with the colder temps.
    Also I am monitoring the concrete floor temps,if it drops to low I will fire up the infloor but so far it has stayed at 55F or so.
  13. peakbagger

    peakbagger Minister of Fire

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    I did a quick experiment this past week with the heat pump. The weather was 40 degrees day time and 20 to 30 at night. I left Monday AM and got back late on thursday/friday AM. With the heat pump T stat set at 60 I used about 60 KW so roughly 17 KW per day or btus. Using 7000 btu/lb for dry wood and 60% efficiency for my non gasifier boiler I that would be equivalent of 13 pounds of wood per day or 46 pounds for 3.5 days. I suspect that the equivalent wood usage would have been much more in the boiler for the same period.

    I was planning to use my oil as a backup but inadvertently had left it off so I am glad the heat pump worked.

    Doing a detailed performance test on the boiler would be difficult as I don't have a way to measure and record flows and Delta Ts so any direct comparison is going to be difficult. I will at some point do a run of weighing wood to see how much wood I normally use.
    Last edited: Nov 15, 2013
  14. mr.fixit

    mr.fixit Member

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    Update;after 30 days the mini-split used 310 kwh's so averaged 10 kwh's a day.At 16 cents a kwh I pay comes to $49.60 to heat the shop a month.Not as cheap as wood but still not to bad.Granted with colder outside temps it will use more kwh's but I am happy with it.
  15. peakbagger

    peakbagger Minister of Fire

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    Update 11/25. We have had a cold stretch (10 degrees daytime and 15 to 20 daytime). The heat pump is still cranking out heat although I have had to close off my second floor and run off the wood boiler. If I had a similar unit in my second floor, I probably wouldnt be running the boiler.
  16. flyingcow

    flyingcow Minister of Fire

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    I have a Mitsu hyper 18,000btu down stairs and a 12k upstairs. Just installed this summer. Very rarely will the upstairs for heat. It's mainly for A/C because I usually go to bed in the early hours of the evening, like a little A/C but detest window units.
    The downstairs unit is used occasionally. It really cranks out the heat. Unit was used alot in the shoulder season. But, i love them.

    Want to install a kw meter, just to get a closer idea how much i use and do rough figuring on wood useage. Peakbagger, what is the model of meter you're going to hook up? georgepds how do you like your meter? easy to read/understand?


    I expect to use the HP more for heat in the future.
  17. peakbagger

    peakbagger Minister of Fire

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    I bought an EKM meter off of Ebay and the enclosure kit. Its actually slightly cheaper to buy a used KWH meter on ebay and a meter base at Home Depot but the EKM takes up a lot less room. The meter keeps a running total with no way to reset it.
  18. Brick (ware)House

    Brick (ware)House Member

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    Correct. Risk-free rate of return on a liquid investment = US Treasuries. Using a speculative equity return would dramatically skew the calculations.

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