Questions on a Hybrid System

15 years ago

I'm a first-timer here, so bear with me.

We live on a windy mountainside in Western North Carolina, and I want to replace a propane furnace and electric air conditioner with a heat pump and a propane furnace. I've gotten three estimates that have left me comparing apples and oranges. Only one contractor has done anything approaching a Manual J heat calculation, but one of the others says he'll do one after I sign a contract and before he brings the equipment.

All three estimates treat the heat pump as an add-on to the propane furnace, and I think it ought to be the other way around. Two of the three are proposing 92% AFUE two-stage, variable speed furnaces; the third is proposing a 92% AFUE one-stage, single-speed, but it appears to be a builder's model.

All are quoting 13 SEER heat pumps, but one also is offering a 15 SEER HP for only about $300 more. I could get that difference back from the 2009 federal tax rebate.

What I'm proposing to do is to ask all three to give me estimates on a 15 SEER, 9 HSPF 13 EER HP, which qualifies for the federal rebate, and a 92% one-stage furnace. My thought is that the furnace would kick in at an outside temperature (35 degrees or so) where it would have to run at or near full tilt anyhow, so the lower stage wouldn't be of much benefit.

Does this make sense, or am I off base?

Thanks for your input.

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Comments (5)

tigerdunes
15 years ago
double tree

what is your cost for propane? cost of electricity?

I would not have a furnace backup without a var speed blower.

what size is existing AC? does it handle cooling OK?

the contractor that says he will provide a load calc after you sign a contract should be sent on his way. forget him.

IMO
double_tree
Original Author
15 years ago
Last propane fill-up, about three weeks ago, was at $2.499 a gallon. Electricity runs 9.4 cents per KwH.

Can I get a variable-speed blower in a one-stage furnace?

I think the A/C is a 4-ton unit. It handles the cooling OK. We don't get a lot of hot weather here, and even then most evenings cool off enough that we can sleep without it. Our master bedroom is on the first floor of a two-story, and the airflow through the windows is good.

The first summer we were here, in 2001, we only ran the A/C about four days, and that was only because we had company sleeping in an upstairs guest room. We did run it about three or four weeks last summer because the humidity had kicked in.

Not like where we came from, where the A/C went on about Memorial Day and stayed on until after Labor Day...
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tigerdunes
15 years ago
double tree

"Can I get a variable-speed blower in a one-stage furnace?"

no. why would you want to?

as far as cost to produce BTUs, straight electric backup is cheaper than propane. run the numbers on attached fuel comparison calculator. still propane backup may be better considering Western NC Mtns do have occasional heavy snow/ice storms.

I would look at a Carrier Infinity Hybrid system.

you need a professional HVAC dealer to perform a good load calculation.

IMO

Here is a link that might be useful: Fuel Comparison Calculator
double_tree
Original Author
15 years ago
The Fuel Comparison Calculator shows I'm paying $3.02 per 100 Kbtus for propane at $2.50 a gallon, while 100 Kbtus out of a heat pump at 9.1 cents a Kwh would cost only $1.57.
tigerdunes
15 years ago
double tree

using 95% eff on a propane furnace and a COP of 2.75 for a high eff HP, these are the figures.

Cost per 100,000 btu of useable heat

Electric baseboard: $2.40
Heat pump: $0.97
Propane: $2.88

obviously a HP is very attractive vs propane furnace, but the point I wanted to make and for you to note is that straight electric resistance backup is less expensive than propane.

IMO

Questions on a Hybrid System

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Questions on a Hybrid System

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