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1K2GO 11-20-2007 03:48 PM

only 3 Tons for 3700 Sf Ft?
I am in the process of building a new home. The home will be energy star rated and will be insulated very well with excellent windows and doors as well as radiant barrier in the attic. But still standard 2x4 construction.

I recently have been receiving quotes on the HVAC system and one guy came back recommending the Carrier Infinity line (16 seer) and said i only need 3 tons. He proposes to use only one unit and split it into 4 zones. The house is 2 levels plus a basement. He is saying the basement wont need any cooling, only heating and dehumidification since it is mostly underground and the walls are R-22.

The guy seemed really smart with 30 years experience and he did take the plans and calculate the load. It comes out to 36,000 BTU.

I am confused because everyone else is suggesting two separate units totaling 4.5-5 tons. Granted some of the others are not calculating the loads, only guessing based on sq. footage, but some of them are doing the math and they all recommend something different.

So my question is this: Is it feasible to heat and cool with only 3 tons 3700 sq ft? The climate here in western NC is mild in the winters and on the slightly hot side in the summers.

Thanks for any suggestions.

flyby 11-21-2007 09:06 PM

I wish I knew enough to help you. But this really confuses me. I'm building basically the same house, except with 4800 ft and no basement and everyone in the mid south tells me I need 11-12 tons!

Chris Johnson 11-21-2007 09:22 PM

Depending on how well your house is built, meaning insulation, thermal mass, air leakage, window locations, etc, etc. as you can see many things factor into calculating HVAC, guys who 'guess' based on square footage are usually wrong. If this guy has done his heatloss/heatgain calcs and has good references than I would say his work is accurate at 3 tons. The zoning really helps since the AC is primarily for the top floor, secondary on the main floor and redundent in the basement, and reverse it all for the heating in the winter. Zone systems are a great way to go and give you more flexibility in heating/cooling your new house. FWIW have an independent engineer verify his work, might well be worth a couple hundred bucks.

And for Flyby, that's a alot of cooling, what are you building your house with, or should I say without!! I've built small commercial units that size with less than half that cooling, something doesn't make sense unless your version of mid-south is Death Valley, CA.

coolmen 11-22-2007 06:55 AM

With your home being energy star rated, Your getting the best insulation and the best windows. You must have a load calculation performed and given a copy of this. The rule of thumb is a guess with tons and sq ft. and I assure you will be waisting energy/fuel/wearing down your equipment. With the price of fuel and electric, lets do it the right way.

LawnGuyLandSparky 11-22-2007 07:21 AM

I agree with Cris & Coolman. The Sq. Ft. guy is guessing, and oversizing to cover his ass. An oversized unit will cool too fast, and won't dehumidify as effectively.

Trust the guy who did the math and made the calculations! My home was built in '75, I have a 125,000 btu gas boiler, and over the years I have replaced every window with good, thermopane wood/vinyl windows, and all the doors. All exterior walls were reinsulated, the attic insulation doubled. My boiler rarely kicks on now.

It's probably a profit motivation, but contractors still use the same "rules of thumb" when calculating "ballpark" BTU sizes for a/c & heat systems that they used in the 40's, 50's 60's & 70's, even though homes today are built much more efficiently.

Never trust contractors who "ballpark" it.

flyby 11-22-2007 04:15 PM

I really need to find some help. Most HVAC guys here like to use the old 500 ft. per ton method. My last bid on my new construction was for 400 sq ft per ton, because "that's what I usually use and it works for me". I'm more worried about cooling than heating, as 9.5 months out of the year are pretty hot in the Memphis area.

LawnGuyLandSparky 11-23-2007 08:58 AM

You're going to have to call contractors and ask for an estimate that requires visiting the site or the plans, and making the proper calculations.

Chris Johnson 11-23-2007 06:13 PM

Contact Richard Rue at Energywise in Texas, FWIW he will review your plans and design the HVAC system based on all information you give him and when it is all said and done and you recieve his report all contractors are bidding apples to apples. Also he will guarantee your heating/cooling costs for the first year (consumption only - not fuel cost fluctuations). His company is probably the most recognized and respected in the industry.

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