Thread: Calculating heating costs

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Calculating heating costs

How do I calculate heating costs:
Here are the numbers:
Gas Furnace:
Gas: \$1.3479 /therm
AFUE:94.1

Heat Pump:
HSPF: 9.3
Elect.: 0.0736 /kwh

How would I calculate the cost of these two heating methods?
Last edited by The_Pickle; 03-31-2007 at 08:31 PM. Reason: fat fingered it before done with post

2. amd
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It's practically impossible with a heatpump, since actual coefficient of performance greatly varies based on operating conditions and no one can tell you how much backup heat your house will be required.

With the furnace it's a little bit easier, but you still need a full heat loss calculation (per degree day) and the average number of heating degree days for your climate.

You can try to do a rough comparison (&#37; between the two) but it won't be accurate. The heatpump moves several units of energy (in the form of heat) per unit of energy consumed above 32F and the furnace adds 0.94 units of energy (in the form of heat) to the house per unit of energy consumed.

Dual-fuel is (heatpump in conjunction with gas furnace) ideal in borderline climates where the heatpump costs less to operate in all but one or two months each year.

Provided that I'm correct, electricity costs \$2.16 per therm with your rate. Given that most heatpumps have a coefficient of performance of 3-4 in mild weather, it could potentially cost you only \$0.72 per therm, excluding backup heat and defrost cycles. The gas furnace will cost you \$1.44 per therm since 6% of the heat produced is lost with the exhaust.

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More numbers:

House size: 1254sq ft.

Manual J (using HVAC Calc):
Sensible Gain: 10,783
Latent Gain: 875
Total Heat Gain: 11,658
Total Heat Loss: 20,556

I am not sure what the actual degree days numbers are, but the winter average temps are around 35-45 with usually about 2 weeks of temps below 32. (Coldest I remember it getting was high teens, but that was only at night. And a few years ago, we had about a week where it never got above freezing) but it is usually does not get that cold.

4. amd
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What's are the design temperatures?

Here's what you need to do...

- Take the total BTUs/Hr heat loss and multiply it by 24 hours
- Divide total BTUs over one day at design conditions by 100,000 to calculate therms per day at design conditions
- Subtract outdoor design temperature from 64F or 18C (depending on what you're using) to calculate how many "degree days" per day there are at design conditions
- Divide therms per day at design conditions by "degree days" per day at design conditions to calculate therms per degree day (will probably be less than one)
- Multiply therms per degree day by average heating degree days per year
- Multiply total heating degree days per year by \$/therm

There might be a quicker way - What I posted is just based on logic and not some formula.

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Design conditions

I believe they were 90/23 and for inside 74/72.
I found a web site: http://www.energycap.com/weather/ that gave me some degree days numbers you asked for:
Balance
Point Heat Cool Total
65 4335 419 4754
60 3034 943 3977
55 1934 1668 3602

Im making my decission early next week and really need to decide now which is the cheaper way to go (19 seer 9.2 HP) or "high" seer, 93&#37; gas furnace. Gas+HP would get a little spendy.
Last edited by The_Pickle; 03-31-2007 at 11:52 PM. Reason: add detail

6. amd
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With those numbers a dual fuel system doesn't make sense. You won't be relying very much on backup heat.

Just note that if you get the HP, it will have to be sized for heating to be economical. (Heat strips running half the time would be very expensive) Two-stage is advisable so that it doesn't short-cycle in cooling mode.

Even the smallest furnace available is way too large 20k heat loss. (Again, two stage advisable)

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the most accurate way is to use about 3y of past billings --
get the degreeDays for the BILLING period
subtract the fuel used in May or whichever billing period has the least heating & cooling days.
now compute the btu/dd.

else, just hold up a wetted finger & pick a number -- or thermometer --

our elec co lists the DD on each bill.
oil & propane delivery companies use DD to know when to fill each customer's tank.

we can get 18m of billing info from elec co data site.

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All electric it is then!

I am going after the Lennux XP19 with VS AH. I think (will have to verify) that it has 2 stage electric heat backup and since electric backup can run with the HP, all the better...

9. amd
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For your climate that's a good decision.

Originally Posted by cem-bsee
the most accurate way is to use about 3y of past billings --
get the degreeDays for the BILLING period
subtract the fuel used in May or whichever billing period has the least heating & cooling days.
now compute the btu/dd.

else, just hold up a wetted finger & pick a number -- or thermometer --

our elec co lists the DD on each bill.
oil & propane delivery companies use DD to know when to fill each customer's tank.

we can get 18m of billing info from elec co data site.
Remember to consider existing equipment efficiency. (like if you have a 60\$ Eff gas furnace, divide by 0.6 to get total heat added to the house)

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I made a spreadsheet a while back that compares fuel costs. At about 7 cents per kw-hr, gas would be cheaper if it was less than \$1.10/therm. Since yours is higher than that, electricity will be cheaper. Like others said, it depends on the HP COP, but this is assuming a COP of 2. You should be over 2 almost all the time.

The bad thing with Vancouver-Portland is the amount of time the HP will spend in defrost. Here in the Seattle area, I've set mine to 60 minutes as that's all I can get away with. This is a real tough one to calculate, but the next choice was 30 minutes and that seems too wasteful.

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Originally Posted by suemarkp
I made a spreadsheet a while back that compares fuel costs. At about 7 cents per kw-hr, gas would be cheaper if it was less than \$1.10/therm. Since yours is higher than that, electricity will be cheaper. Like others said, it depends on the HP COP, but this is assuming a COP of 2. You should be over 2 almost all the time.

The bad thing with Vancouver-Portland is the amount of time the HP will spend in defrost. Here in the Seattle area, I've set mine to 60 minutes as that's all I can get away with. This is a real tough one to calculate, but the next choice was 30 minutes and that seems too wasteful.
Can you post that spreadsheet?

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Pick

I assume you have a small home or condo. I crunched the numbers using the fuel comparison calculator on www.warmair.com with a heat pump COP of 2.75 which gives you a rough comparison. Conservatively, you should save over 50% vs nat gas in normal heat pump mode.

Cost per 100,000 btu of useable heat

Electric baseboard: \$1.94
Heat pump: \$0.79
Natural gas: \$1.39

For your area/climate, I don't think defrost mode would be a big deal but I would suggest that you find out whether the XP19 has time or electronic demand defrost. What size heat strip module has your dealer recommended-a 5 kw?

I would check with your dealer on available Lennox rebate and the Fed Energy Eff tax credit of \$300.

My opinion
Good LucK!

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Thanks

Originally Posted by tigerdunes
Pick

I assume you have a small home or condo. I crunched the numbers using the fuel comparison calculator on www.warmair.com with a heat pump COP of 2.75 which gives you a rough comparison. Conservatively, you should save over 50% vs nat gas in normal heat pump mode.

Cost per 100,000 btu of useable heat

Electric baseboard: \$1.94
Heat pump: \$0.79
Natural gas: \$1.39

For your area/climate, I don't think defrost mode would be a big deal but I would suggest that you find out whether the XP19 has time or electronic demand defrost. What size heat strip module has your dealer recommended-a 5 kw?

I would check with your dealer on available Lennox rebate and the Fed Energy Eff tax credit of \$300.

My opinion
Good LucK!
Yes, the Lennox 19xp does get the fed tax credit, (with 3ton VS AH using 2 ton HP). About 1/3 of that credit is getting eaten by the larger AH but I am still get some credit, plus a little more effeciency. THe XP19 also has a demand defrost.

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