Help me on HVAC proposal and calculations

signet77May 25, 2013

Hello, first post here; long time reader.

So we are renovating our new home in East TN and are working on HVAC proposals. We have very few qualified contractors in this area from what I hear. I need some opinions on what I was presented.

The house currently has two A/C units and two system for 1500sqft downstairs/1000sqft basement and one system for 1500 sqft upstairs.

Dehumidification is important to us especially in the basement so I definitely want a dual stage compressor unit for that area. He recommended single stage for the upstairs.

The contractor in question had apparently worked on this house for years before we bought it so rather than do a Manual J calculation, he just sized it based on what was there because he "knows this house". Here is what he proposed:

All York equipment
2 ton 16 SEER single stage condenser
95% AFUE 60K BTU/hr single stage furnace
Honeywell t-stat

3 ton 18 SEER two stage condenser
98% AFUE 80K BTU/hr modulating furnace
evaporator coil
Honeywell t-stat

I was concerned that we might be oversized so I did my own calculation with HVAC-Calc and got these numbers:

Basement Heat Gain 7,175
Basement Heat Loss 13,152

First Floor Heat Gain 27,164
First Floor Heat Loss 46,434

Second Floor Heat Gain 14,373
Second Floor Heat Loss 24,498

Total House Heat Gain 48,713
Total House Heat Loss 84,086

The contractor has spec'd 3 tons main floor/basement of cooling with the main floor/basement dual stage and 2 tons for upstairs. This seems pretty reasonable, yes?

Now the furnace is where I see a problem...he has spec'd 80K BTH/hr for main floor/basement and 60K for upstairs. Looks like it should be 40K BTU/hr for upstairs. But on the main floor/basement, total heat loss from these calcs is right about 60K BTU/hr. Should I go with a 60K in this case or is 80K modulating OK? Seems like 60K might be pushing it on really cold days. Plus I assumed R-13 for the walls but it may be R-11...hard to say.

Thanks SO much for your input.

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Looks to me that both furnaces are oversized.

Even the second floor condenser is oversized. They do make 1 1/2 ton AC condensers.

I would take a chance on a 60 K mod furnace for downstairs system.

You may want to review your calcs again paying close attention for design temps for your climate/location both inside and outside, cooling and heating.

BTW, where will furnaces be located? You normally do not place high eff condensing furnaces in an attic except for location with mild winter.


    Bookmark   May 25, 2013 at 3:31PM
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"Knowing a house" is not a substitute for doing a Manual J calculation. This is another way of saying "I don't want to be bothered".

I think your first floor furnace is over sized. A 60K BTU furnace will be more than adequate. I don't understand why your second floor heat loss is so much less than the first floor. What insulation did you enter for the attic?

I also question how the second floor heat gain can be less than the first floor. The second floor gets a lot a heat from the attic.

Why is the contractor recommending a single stage units for the second floor? Is it to save money or he thinks it will work better than a 2-stage?

How what did you use for your average high and low outdoor temperatures? What are your indoor design temperatures? Are you getting a 2-stage thermostat for the first floor?

    Bookmark   May 25, 2013 at 3:42PM
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To answer the first reply, the upstairs furnace is to be installed in the attic. Our winters are typically mild with average highs of 45 and lows of 26. But we definitely have lows down to the mid teens from time to time.

The design conditions I entered in HVAC Calc were:

1) Outside temps summer 89, Winter 14, summer grains of moisture 92, daily range medium (taken from their list of default temps from a nearby city to me)

2) Inside design temps summer:75, winter 72 degrees, 50% RH

Attic insulation is R-38

The single stage upstairs was just to save money and because I was more concerned about dehumidification in the main floor/basement hence the two stage A/C.

I intend to get a 2 stage thermostat for the main floor/basement.

Thanks again for your feedback.

    Bookmark   May 25, 2013 at 7:01PM
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With some brands, you will find it difficult to get sized correctly for your upstairs furnace assuming your heat loss calc is accurate.

What is size of living area for upstairs?

Post back.


    Bookmark   May 26, 2013 at 7:45AM
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I'd do a 2 ton single stage for each floor. If you install a 3 ton downstairs it will never hit 2nd stage, which means you have spent double the money for nothing. How much of the 27,000BTU 1st floor heat gain is from appliances or unshaded windows? Simple solution is to use common sense and close blinds/reduce appliance use on the days when it's 90+ outside. I also have to question the 7,000 heat gain from the basement, basements generally are cool even when no AC is used.

FWIW I have a 2 ton on my own 1600 sqft single story house it it will keep it 75 inside on a 100 degree day. Typical construction for a 14yr old house, but the AC is installed RIGHT.

    Bookmark   May 27, 2013 at 9:31PM
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Instead of getting a 2-stage compressor for humidity removal, get a single stage with a variable speed blower. Should cost less and still be effective.

    Bookmark   May 29, 2013 at 12:59AM
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