Air Conditioner or Heat Pump?

Tarheel13March 26, 2014

I just got my AC and furnace checked up a couple of days ago. Ours is a 2,500 sq ft. two-story house built in 2000. The current heating cooling equipment include a 2 -ton Carrier AC and a natural gas furnace for downstairs (coil and furnace in the crawlspace), and a 2.5 -ton AC and natural gas furnace for upstairs (coil and furnace in the attic). The natural gas furnace is probably 60-70% AFUE. The HVAC person told me that everything seems OK except the coils, which have got rusted, and that they may go out anytime. When that time comes, his recommendation was to replace them with AC and 80% AFUE furnace. Going to a higher AFUE furnace would require making changes and putting PVC pipes for exhaust (costly changes according to him). I asked him if I should go with Heat Pumps (instead of ACs) and 80% AFUE furnace. He said I should stay away from the heat pumps as they are noisy. My question is, which is a better option: heat pump + 80% AFUE furnace (dual fuel system), or AC + 80% AFUE furnace? My understanding is that most of the heating will be done by the heat pump except the very cold days when the furnace would kick in. Since the furnace will be used minimally, a 80% AFUE furnace is adequate. So heat pump plus furnace is a better option than AC plus furnace. Are the heat pumps noisy and unreliable compared to ACs. But I have no knowledge about these things and would appreciate your advice. The house is located in Chapel Hill, NC.

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I don't agree with noise being a consideration with heat pumps nor added cost of labor for higher effeiency gas furnace. Why the contractor raised those concurns escapes me. My knee jerk says high effecience heat pump and affordable furnace since your annual temps aren't extreme. To boil it down based on cost,all the numbers plus local climate must go into a program desighned to show options. I would suggest getting bids for different combos from 2 or more contractors when you are ready to replace one or both systems. Give those to your electric and gas supplier asking them to prepare a spread sheet for you.

    Bookmark   March 26, 2014 at 4:15PM
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The statement about noise for HPs compared to straight AC condensers is pure unadulterated nonsense. Simply not true.

What is your electric and nat gas rates?

Because of your location and relatively mild winter weather and depending on your electric breaker capacity, I would drop the furnace and install a quality straight 15 SEER heat pump system. If breaker service requires upgrade, then go straight AC and furnace.


This post was edited by tigerdunes on Thu, Mar 27, 14 at 7:55

    Bookmark   March 26, 2014 at 4:40PM
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Living in Chapel Hill, also, I think that dual fuel systems are an unnecessary expense. I'd go with a straight A/C and gas furnace or a heat pump, depending on the relative cost of buying and running each over 15-20 years. In our area, the former is usually more cost effective (as your HVAC guy has recommended). He doesn't appear comfortable with heat pumps.

This post was edited by sktn77a on Wed, Mar 26, 14 at 21:27

    Bookmark   March 26, 2014 at 9:22PM
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"The natural gas furnace is probably 60-70% AFUE."

How did you calculate the AFUE of your gas furnace? Did you take measurements? Gas furnaces sold in 2000 had a minimum rating of 78%.

You shouldn't make any decisions on heat pump versus furnaces without knowing your gas and electric rates.

My vote would be for a gas furnace and AC. I would get a quote on a 95%+ AFUE furnace and calculate the return on investment on the extra cost over a 80% AFUE furnace. The higher efficiency furnace may be eligible for local utility or state rebates which would help offset the higher cost.

    Bookmark   March 27, 2014 at 8:37AM
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You need to get another dealer involved.

As mentioned in my first post, you do need to get both electric and nat gas rates, understand them, and have an idea about your operating costs.

As to suggestion for 95%+ eff furnace for upstairs system, this might not be feasible or cost effective due to venting costs. Furnace to be located in attic?


    Bookmark   March 27, 2014 at 8:52AM
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Thank you all for your input. My 14-year HVAC systems haven't quit yet (though some in our neighborhood have done so, houses same as as mine). So, at this stage, I am simply trying to educate myself as to what is out there and what would be my best options when I need to replace my HVAC.

Here are the rates of electricity and gas (in $) in our area:
Electricity: 0.136/kWh, Natural gas: 1.013/therm.

The HVAC person who tuned up my systems told me that I can't have a high AFUE furnace in the attic because the condensate may freeze. Is it correct? Does this mean that one can't install a high AFUE furnace in the attic at all?

So, in my simple-minded way, I thought that since I can only install a 80% AFUE furnace in the attic, why not pair it with a heat pump (instead of an AC), so that I will need to use it only during the coldest days. Do I make sense?

    Bookmark   March 28, 2014 at 10:00AM
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When the HVAC guy was cleaning my outdoor compressor units, I asked him about the capacity of the electric breaker box. He said each one (I have two: one for each floor) is 120 Volt/60 Amp.

    Bookmark   March 28, 2014 at 10:06AM
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Tar Heel

You have relatively cheap nat gas compared to electric.

Even at 80% efficiency furnace, nat gas is cheaper.

Below s a fuel comparison that is to be used as a guide only. 2.75 COP for electric, 80% for nat gas.

Cost per 100,000 btu of useable heat
Electric baseboard: $3.59
Heat pump: $1.45
Natural gas: $1.24

Again this is to be used as a guide only.

While you live in a location that has relatively moderate winter weather, you normally do not place high eff condensing furnaces 95%+ in unconditioned attics because of freeze ups even however remote. If your attic does not have certain insulating properties, it simply is not advisable.

With the fuel comparison calculator, I probably would not recommend a heat pump. I would go with a properly sized 2 stage 80% var speed furnace paired with at least a 15 SEER AC condenser.

Just for info, if you went straight HP with air handler for upstairs, normally you place the condenser, air handler, and heat strip on separate circuits. Only a dealer with qualification in this area could determine if you have the capacity. If not, an upgrade would be required which probably would not be cost effective. If you went dual fuel with heat pump and 80% furnace, then your electric service would not be an issue.

Hope this helps even tho I don't care for the UNC Tarheels. Just kidding.


    Bookmark   March 28, 2014 at 10:34AM
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Thank you, Tigerdunes.

    Bookmark   March 28, 2014 at 12:05PM
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High efficiency furnaces can be installed in attics but it takes a HVAC contractor who knows how to keep the condensate from freezing. It is probably not worth and the investment in your climate.

I also recommend getting the gas furnace and the AC. At current prices the dual zone set up offers no savings.

    Bookmark   March 28, 2014 at 12:32PM
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Thank you, mike_home and all others for your kind input. So the consensus seems to be to go with straight AC + furnace in our area.

    Bookmark   March 29, 2014 at 9:34AM
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What size living area is upstairs zone?

With the fuel comparison calculator, I probably would not recommend a heat pump. I would go with a properly sized 2 stage 80% var speed furnace paired with at least a 15 SEER AC condenser. Two stage furnace will require two stage thermostat that controls staging, not the timer on furnace control board that will leave you in high stage whether needed or not.


    Bookmark   March 29, 2014 at 10:48AM
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Your HVAC guy may know how to spray water on a condenser but he doesn't know squat about electricity.

TD was asking about your main breaker panel, not the cutoff boxes for each condenser.

    Bookmark   March 30, 2014 at 11:48PM
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