Help with htg/ac system...

mcnealMay 1, 2012

I need to replace both systems in my Atlanta home. I talked to two contractors last year - with very different ideas.

I would like to have an idea about what would make up a good home system for the next 15 years - not the brand, but what do you consider to be system requirements for a SE US home.

Any help will be appreciated.

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Nat gas service to home is available?

Relatively mild winters with brutal summers and high humidity

15 SEER AC or HP

80% or 95% eff furnace

Home is two story?

What size type systems do you have now?

Post back.


    Bookmark   May 2, 2012 at 8:11AM
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Thanks for your reply...

Home is a two story, approx 3200 sqft. One unit is in basement, Tempstar gas furnace & ac. Unit two is in attic also a Tempstar gas furnace/ac system. The basic builder grade that has yielded 19 years of almost trouble free service.
I would like to have ideas of what to tell contractors I want.I did have two contractors look at things last year. One, said to go with basic 80% furnace and 15 seer ac. Two, proposed duel fuel hp/gas furnace 15 seer.

Neither did any sort of load calculation - just took the btu and tonnage of the current units and quoted similar.

Thanks for your help.

    Bookmark   May 2, 2012 at 2:25PM
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For first floor

95% eff two stage var speed gas furnace with 15 SEER AC


80% eff two stg var speed furnace with 15 SEER heat pump

For upstairs

80% eff two stage var speed furnace with 15 SEER AC or heat pump.


15 SEER HP with air handler and backup heat strips if you have capacity in your breaker box

Make certain all ductwork is inspected and any hot/cold spot issues within home are dealt with for improvement.


    Bookmark   May 2, 2012 at 3:35PM
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Thanks for your input.

Am I reading your recommendation right - 1st floor - 80% two stage var speed furnace WITH 15 seer hp -
2nd floor 80% two stg var speed furnace with 15 seer ac OR hp.

If I go dual fuel, would I not want it on both levels? I do like gas heat vs hp.

I am setting up contractor appointments in the next two weeks. Any advice on most reliable equipment brands. Opinions,I know, vary but respected advice is appreciated.


    Bookmark   May 2, 2012 at 5:16PM
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I never recommend HPs if homeowner is opposed.

Can you provide your current nat gas and electric rates and I will run a comparison for you.

Forget the HPs then.

For first floor, go 95% var speed furnace with 15 SEER AC.

For second floor 80% var speed furnace with 15 SEER AC.

Of course correctly sized.

I like Trane/AmStd, Carrier/Bryant, Rheem/Rudd.


    Bookmark   May 2, 2012 at 5:46PM
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Thanks for your reply and offer...

I am not opposed to a HP, I have one at our lake home and it is fine 80% of the time. but when it is cold - nothing beats gas heat. So, If I go with a HP, it will have a gas furnace backup. The question is cost of equipment vs cost/savings of fuel.

This may not be the official way of doing things, but...
Last month we consumed 55 therms of gas - with a total charge of $88.52 so that is $1.61 per therm - bill says 1.011 therm factor, junk fees add the balance
We burned 1028 kwh of electricity total cost $130.14 or
.127 per kwh - bill says energy charge of .076 per kwh, junk fees add the balance.
I hope this is the info you need for calculation. I do appreciate you considerable help.

    Bookmark   May 2, 2012 at 11:16PM
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Mcneal - that was actually a great way of listing your charges. The only problem is that junk fees can be fixed or not and that does make a difference. For example, I have a $10 a month NG service charge so I get $11 bills all summer long for an effective $11 per therm charge.

I have to start by asking how you could burn 55 therms last month? I live in NC with a similar winter. I didn't run the heat last month.

It is helpful to list total winter costs since I am wondering about your gas use. Obviously if you are using 1000 therms a winter, it changes the math compared to using 100 therms a winter.

As a point of comparison, I have 5000 sqft and used 80 therms this winter with a dual fuel system. My electric bills peaked at $150 in Jan down to $65 in March (ie no heat).

Dual fuel can be very cheap when you set it up. Often it is recommended to go dual fuel downstairs and HP only upstairs. I would recommend going dual fuel for both for you. I had HP only upstairs and have changed it to be dual fuel for both.

The advantage of dual fuel in our climate is that it really pays to get a high efficiency a/c unit. By using dual fuel, you are getting more efficient work out of that a/c unit. I have mine set to switch at 40 degrees since NG is so cheap right now. At 40 degrees, a heat pump is very cheap to run.

In my example, with a really mild winter, I only used about $200 this winter with the heat pump. At most that saved $100 a year vs straight NG. But a dual fuel system for you shouldn't be more than $1000 so it would payback but the big advantage would be if NG spiked in price. It also is advantageous if your heating bill is more. You have the potential to save 30-40% when your heat pump is running vs NG. Even with a 40 degree changeover (which is very high), the heat pump should be good for 2/3 your need for an effective savings of about 20%. Not bad for $1000.

    Bookmark   May 3, 2012 at 5:34AM
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If considering dual fuel, then for Hotlanta weather/climate, you really don't need more than am 80% eff furnace. Go with two stage var speed models with the correct control/thermostat.

I will make a comment on dual fuel since we all live in the Southeast. It is no longer the economic advantage that it once was. Nat gas prices have
declined although yours are definitely on high side, where regulated electric rates continue their march upward. The gap and leverage between the two fuels has sharply narrowed. 4-5 yrs ago, I was saving about 35% in operating costs. That difference has nearly disappeared.

I suggest you re-check your nat gas rates.


    Bookmark   May 3, 2012 at 7:46AM
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David, thanks for your post.

I have no idea how we burned 55 therms of NG last month for hot water and cook top, but that is the bill. We pay $27 per mo in delivery fee to Atlanta Gas Light Co, but we buy our gas from our EMC.

At your suggestion I looked over our bills for the last two year and our highest bill was 12/19/10 to 1/27/11...
Gas, 234 therms - $306.55 = $1.31 per therm
Ele, 951 kwh - $125.03 = .1314 per Kwh.

That's the data, I don't really know how to figure the relative cost for a btu of heat.

Anyway, I do apprecate your input - I do know that my 19 year old units have served me well if not most economically. I am trying to get enough info in my brain to not be at the mercy of a contractor and just buy what they want me to have.


    Bookmark   May 3, 2012 at 8:09AM
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