# Geothermal heatpump vs air heatpump



## Tarheel (Jan 24, 2010)

When I built my house 4 years ago I failed to give the geo heat pumps a fair shake in efficiencies. I have since in the last week been trying to figure if I could justify changing out my system (which I can use at another location) to a geothermal heat pump.

I have used Solar Gary's charts and have come up with the following information. The unit which I have now (air heat pump/ electric resistance backup) has a seasonal average cop of 2.3. My cost of electricity is .11 per kwh. Which equals out to around $1.40 per 1000 btu's.

The information that I have researched for geo thermal heat pumps is all over the place for the cop readings. I took the number of 3.5 as the average cop and came up with .92 cents per 1000 btu's. I also know that there are more advantages with this unit, hot water being one of the best.

I guess the question I am trying to answer for myself is how long of a payback will it take to justify changing out the unit, or is it even worth messing with.

Here in N.C. our winters are normally mild but for the last 3 years have been below average. Not uncommon to have temps like today with a 32 degree high.

A little more information - I insulated everything to the max when I built my house, walls, floor, attic, and I have a sealed crawl space. I am heating & cooling 1750 sq. feet with a 2 ton heat pump. My average usage per month is 925 kwh's.(This is for everything, heating, cooking, hotwater) Bill runs about $100 @ month.

Before you flame me for a heat pump let me say I have back up's for back up's. LOL ! Propane, wood, generator. I am just trying to think ahead and make life a little simpler (and cheaper) if something does happen to me.

Thanks in advance for your comments and opinions.


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## artificer (Feb 26, 2007)

I would check your numbers. Something is off.

$1.40 for 1,000btu's is extremely high.

At $.11/kwhr, 3.4btu/watt and COP of 2.3, I get:

1000btu/(3.4btu/kwhr) = 294watt-hrs, or .294kw-hrs
.294kw-hrs/2.3COP gives .128kw-hrs/1000btus
.128kw-hr*$.11/kw-hr = $0.014/1000btus

$1.40 should give you 99,527btus. Hmmm... you must have a typo, or dropped a couple of zeros.


Why would anyone flame you for using a heat pump? If you have to buy firewood you're not that much worse off for cost per btu. You can't beat the convenience of set a thermostat, and let it work.

As for payback... assume a cheap installed price of $5,000. At $100/month, if you ended up with free energy, it would still take 4 years to pay it back. Do you really think its going to save you $25/month? If so, then its only 16 years payback time.

You need a few more numbers to determine if it makes sense or not.

Michael


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## Tarheel (Jan 24, 2010)

artificer said:


> I would check your numbers. Something is off.
> 
> $1.40 for 1,000btu's is extremely high.
> 
> ...


Thanks for the reply, I am on the road today and will re-figure my mess tonight and update.

Thanks again !


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## Tarheel (Jan 24, 2010)

Tarheel said:


> When I built my house 4 years ago I failed to give the geo heat pumps a fair shake in efficiencies. I have since in the last week been trying to figure if I could justify changing out my system (which I can use at another location) to a geothermal heat pump.
> 
> I have used Solar Gary's charts and have come up with the following information. The unit which I have now (air heat pump/ electric resistance backup) has a seasonal average cop of 2.3. My cost of electricity is .11 per kwh. Which equals out to around $1.40 per 1000 btu's.
> 
> ...


Sometimes it helps to put your glasses on ! I left off several zeros.

On the air source heat pump that I have now it's $1.40 per 100,000 btu's

On the geothermal with a cop of 3.5, I come up with .92 cents per 100,000 btu's.

I can't make the math work out for a reasonable length of time, am I missing something ?

If I had put it in to start with (hind sight is 20/20) then I would have been ahead of the game. I really feel that I am lucky to get out with a yearly energy bill of $1200 for the square footage that I have.

I am really interested in saving energy as much as I can, so I guess I need to look in different areas.

As a side note, no cost on the wood here at the farm except for keeping up the saw etc. As you said it sure is nice just to set the thermostat and leave it alone (and) easier on the wife.


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## Nica (Oct 3, 2009)

When we built our house 6 yrs ago we put in a geo system and realy like it. I never ran the numbers, but our elect bill is very reasonable for this house. I'm for them!


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## Tarheel (Jan 24, 2010)

Nica said:


> When we built our house 6 yrs ago we put in a geo system and realy like it. I never ran the numbers, but our elect bill is very reasonable for this house. I'm for them!


If you don't mine- what size house and how much is your average bill.

Thanks


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## SolarGary (Sep 8, 2005)

Hi,
I think the reason its hard to show a good payback for a geothermal system is that you put a lot of effort into getting the heat loss for the house low in the first place -- that's a good thing!

Maybe consider:
- More insulation and sealing where you can do it.

- If you have a good site for solar, you might think about either solar water heating and/or solar space heating. If you are into building stuff, a solar water heating system can have a quite short payback, and solar space heating with air heating collectors is simple and has a short payback.
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm


- Otherwise, I'd take a look at the Half Program -- its a somewhat systematic way of looking at a range of potential energy saving projects, and picking the ones that have the best payoff:
http://www.builditsolar.com/References/Half/Half.htm

But, with 925 KWH per month for electricity and space heating, you have already done a lot.

Gary


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## Tarheel (Jan 24, 2010)

Thanks Gary for your response and all the information you make available for us here on HT. I have enjoyed reading it over the years.

Yes I was pretty vigilant when I built my house. I tried to stop all infiltration as I built it. I can see already that the extra insulation has paid off.


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## Mickie3 (Aug 28, 2010)

Tarheel said:


> Sometimes it helps to put your glasses on ! I left off several zeros.
> 
> On the air source heat pump that I have now it's $1.40 per 100,000 btu's
> 
> ...


From your numbers, it looks like you can expect to save about $412/year by going from a ASHP to a GSHP. It depends on what it would cost you *incrementally *to replace the unit after rebates as to how long it will take you to get your payback. If you can do it for $7000, the payback is almost 17 years, quite a while as the unit is not guaranteed to be working without additional costs that long (compressor failure being a prime candidate.) 

Have you looked into solar panels for electricity? If you DIY, you may be able to get a much better return on reducing the cost of the power you buy than replacement of the unit. It sounds like if you can buy and install anything in excess of 4000 KWH/year for your $7,000 investment you are $$$ ahead, plus you are not subject to rate increases on the power you generate.


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