View Full Version : furnace vs heat pump on new construction in nashville
07-09-2006, 04:39 PM
Hoping I can get some advice on a spec house we are considering buying that is about 50% finished. It is outside Nashville and is about 5057 sq ft--1200 in a finished basement, 2400 main floor and approx 1457 on second floor (in addition, another 410 sq ft on the second floor may be finished at a later date). The builder's plan is to have one gas furnace for the basement and first floor and a heat pump for the second floor (13 SEER). (Don't know any other info on the units but can find out if I know what to ask.) I do know that they plan on three AC units. My question is about whether a heat pump is adequate for that area for heating--a search listed the average low in the winter as 28 degrees.
07-09-2006, 05:07 PM
A hybrid heat split system would be best for your house from I gather reading at Carrier's web site. The heat pump would be on during mild cold and the gas furnace would start around 50 degrees.
Here is the link:
Hope this helps.
07-09-2006, 05:20 PM
Assuming both systems are sized correctly along with a properly designed ductwork system for both zones,I would suggest a high eff heat pump system for the basement/main floor as well or at least a hybrid system(high eff heat pump paired with a backup gas furnace=dual fuel). And you should insist upon a var speed blower for both zones. Don't let builder snooker you with low end builder grade equipment.You should be looking at a heat pump with minimum 14 SEER cooling and 9+ HSPF heating ratings.
Keep in mind your electric rates are some of the most favorable in the country and nat gas is no longer a value fuel for winter heating.
07-09-2006, 09:35 PM
Am I correct that you believe that a quality heat pump could heat the second floor, which could wind up being ~1900 sq feet (if we finish the optional room)? Would a dual system be better in cooler temps like in January when it dips to 28 degrees, or are we better off with the electric coils on the heat pump that would kick in at such temps given the lower electricity in that region? Years ago I had a hybrid, it was designed to kick in the gas unit when outside temps dropped to 32 or below. But gas was cheaper than electricity then.
BTW, the heat pump will be located in an attic space on the second floor, so blower will not have to push air from ground level up.
07-10-2006, 06:23 AM
heat pump for both areas --
ducts in conditioned space --
have space between door & window frames foamed --
plastic under slab?
drains at foundation level or just below all around outside?
insulation under slab, or at least at perimeter?
insulation outside of bsmt walls?
read at BUILDINGSCIENCE.com!!
Nvl has more heating DD than cooling.
spec houses are of "average" quality --
check builder by looking where the vapor barrier is installed -- report back.
07-10-2006, 10:03 AM
You might need a small auxiliary heater to raise fresh air's temperature. This will help in preventing water freezing at humidifier.
Load calculation is essential.
07-10-2006, 02:14 PM
You all sound knowledgeable--however, I am not. So, we are thinking of hiring a HVAC guy to look at the planned symstem and ductwork and give an opinion. Is anyone familiar with a company called Comfort Engineered Systems in Nolensville, TN? (found them from the ACCA website)
07-10-2006, 03:38 PM
go with a gas furnace and a heat pump youll be very happy
07-11-2006, 05:34 PM
As you seem to know, the heat output of a heat pump drops off as outdoor temperatures get colder. They are very efficient heat sources, though, so in areas with decent electric rates like yours they are economical to run. Since in cold weather their power consumption drops almost as fast as their output drops, in themselves they are economical down to very low temperatures. The question is how to keep warm when the heat pump doesn't have enough output to keep up with your demand for heat.
North of Miami or so, heat pumps always need to have an auxiliary heat source to help them out in cold weather (and during their brief defrost cycles). The auxilary heat source always ends up being a more expensive-to-run heat source- otherwise you'd use that for your primary heat source!- but in moderate climates you don't use it very much, so the numbers work out ok.
With an all-electric heat pump, you use electric resistance heaters in cold weather only to cover the *shortfall* between how much heat you need and how much heat the heat pump can supply. The heat pump just runs and runs and runs, and the electric resistance elements cycle on and off as needed to maintain your temperature setting. So in an area like yours, the heat pump still does 90% of the work, and the amount of electric heat you need to cover the shortfall is reasonably small.
When you pair a heat pump with a furnace for backup, you can only operate one unit at a time. So you run the heat pump as long as it can keep up. As soon as you need more heat than that, you shut down the heat pump completely and just use the furnace. So the heat pump goes out of the picture at about 35 degrees, even though the heat pump plus electric backup combination would probably run cheaper down until <20 degrees.
In your case, gas-only heat (for either floor) will be the most expensive to run in your application. Heat pumps should be the primary heat source one way or another- your heating bills will be halved or less compared to gas-only heat. The only question is what to use for your backup heat source. The operating cost between electric and gas backup will be reasonably close. I'd have to see your utility rates (rather than just general knowledge of Tennessee rates) to give a firm answer.
If you are going to have gas around anyway (stove, fireplace, water heater, etc.) it wouldn't be a crying shame to use it. If you want to be able to heat the house when the power is out using a generator, definitely use gas. I'd have no hesitation about building an all-electric house anywhere south of the Mason-Dixon line, though.
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