hvac-calc question about shgc assumption

david_caryJune 5, 2011

So HVAC_CALC doesn't have an input for SHGC. It gives you the option of low-e and then talks about tinting or reflective coatings.

I have a close call between 2 and 2.5 tons. The SHGC of the windows are .28 which I think is lower than what would be assumed. The windows represent over 50% of the heat gain so just a small change makes a big difference. If the programs assumes .4 then I will definitely be fine.

Right now I am at 17k btu sensible gain and that includes 10 people (lots of guests) and 1200 from the kitchen. I went with the tight designation but even that was tough to say. It will be new so I just assume that I should go with tight although I am not using spray foam.

The contractors Man J has 2.5 tons and I'd like to be comfortably in the 2 ton range to disagree with him.

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Knock down. Email to developer has not had reply. I sincerely hope that an HVAC contractor would have a program with better inputs but somehow I doubt it.

    Bookmark   June 8, 2011 at 5:49AM
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I don't know the answer.

two thoughts occur to me.

get the man J in writing with all details from the contractor and make a comparison. Man Js are only good as the info inputted. Design temps may be different. Could be an error on his part or your part.

the second thought is I don't want to be borderline for cooling on the NC coast.


    Bookmark   June 8, 2011 at 11:17AM
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I was wondering if the contractor would give out his man J before we went to contract. I know the last manual J I saw was filled with significant errors.

I tried to be as conservative on my inputs and I got to just over 2 tons. I used average air infiltration which it won't be and I used just "low-e" with no coverings and no tint or reflective barrier. And I raised the design temp to 95 when the program called for 91. I also put 10 people in the house.

If I change it to best case scenario, I get close to 1.5 tons. Best case being some reflective barrier, tight construction and design temp of 91 - which is what HVAC-Calc determined for the closest city.

But the really really big flaw IMO with the manual J is that east windows equal west windows. Sure they do in fact see the same sunlight but they see it before noon when the temp is never above 90. Since this house has primarily east facing windows, I really think the simplistic model of manual J grossly overstates peak heat gain.

If you try to fix this with changing the design temp to make a morning manual J peak load and an afternoon - then you get close to 1 ton in heat gain.

    Bookmark   June 8, 2011 at 3:25PM
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The software I use for heat loss/gain calculation is Wrightsoft Universal. It is the Canadian CSA version of Manual J.

This software allows the user to adjust for the city, design temperature, orientation of the building, type of window, window frame material, outer pane, inner pane, thickness, gap, fill gas, value of low-e, reflective performance, etc., all for each window.

You can also adjust for properties of roof overhang, appliances, hours of occupancy, number of people, dogs and on and on.

While all this is nice, I think we sometimes put too much faith in these calculations. If it is new construction and I can import architectural drawings directly, then the calculations can be relatively accurate. Otherwise it is the best educated guess based largely on the hearsay information provided.

In an existing home, the owner had better be prepared to modify their duct work to fit the calculations if they want optimal results - otherwise you have to work with the distribution system you got!

To agonize over a 1/2-ton of capacity today when we have 2-speed condensers and variable speed fan coils etc. is sometimes over the top - especially when clients balk at even paying for detailed calculations that may take hours to prepare using software that cost THOUSANDS of dollars to purchase!

I also know that there are people out there that are highly trained and experienced, that can accurately design a complex system after an on-site visit without the software, Manual J, CSA or whatever.

I hope you don't have a heart attack or a stroke after reading this blasphemy!



    Bookmark   June 14, 2011 at 11:24AM
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