Dual fuel heat pump- right decision?

golfinatorSeptember 3, 2011

We're in Atlanta in a 45 yr old, 2650 sf tri-level brick home. The 2 of us are close to 70 and not sure how long we'll be in this house, but probably about 5-10 yrs, maybe less. Existing split system- 4 ton Lennox Mod 10AC48 (18 yr old) that's developed a leak + Singer furnace (26 yrs old) . Have quotes to replace A/C and furnace. Most of the salesmen didn't propose dual-fuel heat pump, until I mentioned it. Wondering if it's a good idea, due to extra cost ($400-600 DFHP + $800-900 var. spd furnace) and also comfort- not having hot air immediately?

My gas usage aver. 900 therms/yr @ $1.24/therm. Heating months (Nov thru Feb) aver.163 therms/mo @ $1.04/therm. (we have a gas dryer, cooktop, new water heater.) Gas provider showing $333 savings/yr for 13SEER DFHP w/80% efficiency furnace over 13SEER AC w/80% based on $0.80/therm.(not including other mo. add-on charges). My new rate is $0.67/therm.+charges). Electric aver.15,000 kw/yr @ $0.13/kw. During the cooling months (May thru Sept) aver 1800 kw/mo. Furnace & coil are on slab in crawlspace. Plastic covering the crawlspace dirt and, with a 25 pint dehumidifier running most of the time, aver 55% humidity and 70-75 degrees.

Also, should rigid ducts in crawlspace be cleaned, resealed, reinsulated vs replaced with flex ducts?

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I like dual fuel. The heatpump provides a comfy heat when mildly cold out. I would run the heatpump when above 35 out. I would get the highest efficiency single stage system.

Make sure indoor coil comes with txv valve.

    Bookmark   September 3, 2011 at 3:49PM
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Thanks neo. Does the hi eff. need a var speed motor?

    Bookmark   September 4, 2011 at 9:39AM
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They are a little more efficient and have the ability to ramp up and down in fan speed easier. Some swear by them. They do add more cost though. They are desirable. They can be used to control humidity and therefore remove more humidity.

Its not completly necessary though.
The most important thing is to have the highest efficiency outdoor unit. Demand defrost model heatpumps are better to have than models that don't have demand defrost. Not every brand has demand defrost heatpumps.

Its important that the new indoor coil has the txv valve and is the most efficient size corresponding to the outdoor unit.

In your climate the 80% furnace will be fine as backuup.

    Bookmark   September 4, 2011 at 9:41PM
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I am surprised those reading this post haven't addressed your question whether dual fuel is the way to go. While heat pumps in a dual fuel application can provide you flexibility on fuel use at moderate temps usually in the 30 deg + fah temperature, it appears you have very good nat gas rates.

However, these added charges that are incorporating into your bill each mth have to be considered as they are a cost in your true nat gas rate.

You should also contact your electric supplier to see if they offer any rate incentive on your electric rate during the winter heating season for HP use in a dual fuel system.

I would request that you review your nat gas rate and come back with a more accurate picture of what your true rate is considering all charges included in your bill each mth. And check with your electric provider about the rate incentive possibility.

Then repost what you found.

I will give you a snapshot of whether DF is a really a good system selection to consider versus straight AC with just high eff gas furnace.

Just on the preliminary info provided, I would forget the HP strictly from an operational cost viewpoint.


    Bookmark   September 5, 2011 at 1:07AM
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On your current ductwork system, what kind of material do you now have and what R rating on insulation do you now have?

Any hot/cold spots in your home?

How are you currently filtering your return air?

Post back.


    Bookmark   September 5, 2011 at 1:32AM
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