Heat load help: err on smaller furnace size?

KiwigemJuly 22, 2014

Hi, everyone.
We are building a new home in Ohio- potential for very cold winters and blistering summers, but you never know what you are going to get from year to year. Has felt like Colorado this summer!

Anyway, new house will be 3400 square feet with good cellulose insulation (can get more detailed if you want), a Tamarack ghost whole house fan, lots of glass facing west and south, some north and east. We are window openers and prefer house fan to AC whenever possible. One fireplace. 7 people. Average appliances. Unfinished but insulated basement.

We were suggested a 100K BTU furnace by the GC's recommended (not required) HVAC guy. He made no mention of a heat load calculation. So I got online and attempted to do one myself. I tried two different sites, and unless I am really messing up (possible) the 100K BTU seems oversized. Heat load calc indicates 80K would be adequate.

Reading I have done indicates that undersized is actually better; yielding more "highway miles"-type usage, vs "city miles" cycling.

Would someone knowledgable chime in, please? I'll be happy to give more info as necessary. Thanks!

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What outside design temperatures were used for both heating and cooling and inside thermostat setting?

What are average lows and highs for your location? You can always add a fudge factor.

Are you more concerned about heating or cooling? If heating then go 100 K on one of the nice modulating model furnaces. Problem solved.


    Bookmark   July 22, 2014 at 11:12AM
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Thank you for your response, tigerdunes!

I figured -10 winter, 70 indside. 20 degree cooling for summer (gets 100 degrees, but I set it for 80 then) Heat is definitely more important.

Does this quote seem reasonable/appropriate to you?

Carrier Infinity 96.7% 2 stage gas furnace 100k BTU (starts at 60K)
Carrier 5 ton 16 SEER AC
All ductwork for new 3400 sqft home (25 supply, 10 return) cooktop hood ducting, 5 bathroom exhaust fans, 2 dryer vents, 5" filter on furnace.

Would welcome any thought on the above units as well.
Thank you!

This post was edited by Kiwigem on Tue, Jul 22, 14 at 14:14

    Bookmark   July 22, 2014 at 11:26AM
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Doubt that U would need a 5-Ton A/C; can't U make a 4-Ton A/C work?

    Bookmark   July 22, 2014 at 12:49PM
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Udarrell: 5 is what was suggested. Your kind of response is precisely why I was asking! Please excuse my ignorance, but could you explain why you think 4 ton is enough? My DIY heat load calc had our house at around 60K. I'm not sure how that translates to AC, but I read somewhere to figure 12K per ton, which would make the 5 ton make sense. I'd welcome your input. Thanks!

This post was edited by Kiwigem on Tue, Jul 22, 14 at 14:12

    Bookmark   July 22, 2014 at 1:33PM
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Well, most homes & duct systems can be energy retro-fitted so a ton will cool 850-sf; 3400 / 4-Ton is 850-sf per/ton...

Even if on the very highest heat-gain days for a few hours the temp went as high a 80ðF if the humidity was below 50% & U had adequate air movement U would be very comfortable.

Then for all the major other times when it's operating at partial load conditions it would do much better controlling humidity & keeping U comfortable with minimum costs.

It is extremely difficult to get enough airflow for a 5-Ton system; rarely happens, so many times they operate at 4-Ton or less.
The decision is all yours...

Here is a link that might be useful: Free online whole houseload-calc

    Bookmark   July 22, 2014 at 3:13PM
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That's really helpful, thank you! I know markets are different, but do you think that the price I was quoted seems reasonable?

    Bookmark   July 22, 2014 at 7:33PM
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I would want to see the load calc in writing both cooling and heating and understand and totally agree with the design temperatures used. On the letterhead. Home layout/design is 2 story or single story?

Certainly you don't want to be oversized or borderline undersized either cooling and heating. You did not list the AC condenser quoted. I would like to know what was quoted. I assume the ubiquitous Infinity controller was quoted. It should be.

Nat gas is fuel choice for heating? Please verify.

I assume new home construction has superior insulation/building qualities.

Post back.


    Bookmark   July 22, 2014 at 7:49PM
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Thanks so much, tigerdunes. I don't know if this contractor does load calc- he scoffed just a little when I mentioned it. I could try to get someone else to do it. Not sure if it has caught on in our neck of the woods. I tried to do one on my own, but I honestly don't trust my work. I was guessing a little with some of the terminology.

Home layout it kind of a hybrid: about 2400 square feet down and 1000 square feet up. If you can envision a barn with a one story wing tacked on, it's like that.

The AC is infinity.

Natural gas is fuel.

General idea of insulation/heat factors: Full poured basement, insulated R8 from outside to grade, and inside above grade r19 insulation to exterior band, blown cellulose to r19 in walls, r30 in ceiling. Integrity wood-fiberglass windows (lots of glass facing west and south, moderate north and east)). Wood siding exterior. one fireplace. raised seam metal roof in light color. will be using our whole house fan whenever possible.

    Bookmark   July 22, 2014 at 8:32PM
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For a home with a layout like yours, I would want zoning controls at the minimum. If not that, then two separate systems. One or the other. If not you will be very disappointed in home's comfort. Insist dealer perform load calc broken out by upstairs/downstairs. Remember you are paying for this and are the boss.


    Bookmark   July 23, 2014 at 7:46AM
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As a general rule, general contractors know very little when it comes to HVAC. They like to hire the lowest cost HVAC contractor in the area since the budget for HVAC is a low priority when building a house. You are doing the right thing by questioning what the HVAC contractor is "suggesting".

New houses have no heating and cooling history so in my opinion a heating and cooling load calculation must be done. There is no excuse for guessing the required sized. This often leads to an over sized system which will never operate correctly or efficiently. The online calculators are OK for a ballpark estimate, but it is not a substitute for a load calculation.

Based on the information you have given so far, I think for new construction a 80K BTU furnace and a 4 ton AC would be the right sizes assuming the equipment will be installed in the basement and all duct work is in conditioned space. This could change if you have a lot of glass.

At 3400 sq. feet I would suggest either zoning the first or second floors, or having a furnace and AC for each floor. The two systems are initially more expensive, but you will save money on operating costs if you use temperature set backs. It is the best way to get even temperature distribution in the house.

I question the wisdom of a whole house fan in a house the has an Infinity AC. If you live in a humid area, then you will allow humid air to enter the house every time you use the fan. In my opinion you are better off setting the thermostat at 78-79 and keeping the humidity at 40%.

    Bookmark   July 23, 2014 at 8:41AM
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Thank you all for your input. I will hunt down someone who will do a real heat load calculation for me!

mike_home, you are right that humidity can be a factor (sometimes) in our area, but we just really like the open window breeze, humidity be darned- can't help it :-)

    Bookmark   July 23, 2014 at 11:43AM
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