# Thread: Heat Gain Calculation Question

1. Regular Guest
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Jul 2005
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Using a HVAC software program I calculated the heat gain for my house to be approximately 32000 BTUs. However, the software for some reason won't let me change the outdoor summer design condition. It defaults to 105 degrees and I think for my area the recommendation is 91 degrees. I'd like to add a little buffer to the program of 7 degrees. My question is, approximately how much would the heat gain change from 32000 BTUs at 105 degrees to 98 degrees. Would it be enough for me to want to go with a 2.5 Ton versus a 3 Ton?
Thanks!

2. Originally posted by bdschet
Using a HVAC software program I calculated the heat gain for my house to be approximately 32000 BTUs. However, the software for some reason won't let me change the outdoor summer design condition. It defaults to 105 degrees and I think for my area the recommendation is 91 degrees. I'd like to add a little buffer to the program of 7 degrees. My question is, approximately how much would the heat gain change from 32000 BTUs at 105 degrees to 98 degrees. Would it be enough for me to want to go with a 2.5 Ton versus a 3 Ton?
Thanks!
Where are you located? Maybe the summer design temp. IS 105°F.
If I knew your approx. location I can look in The Manual J. As to how much more gain at 105°, that depends on the total square footage of the house that is exposed to outside temps.

3. ## GEEZ, it was in your profile

I guess I should have looked in your profile. It says Baltimore, and yes my Manual J says Baltimore summer design temperature (2.5%) is 89°F. -city office and 91°F. - airport.

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pay for the program and that will allow you to change the design temps.

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## Equipment Selection

Originally posted by bdschet
Using a HVAC software program I calculated the heat gain for my house to be approximately 32,000 BTUs.

It defaults to 105 degrees and I think for my area the recommendation is 91 degrees. I'd like to add a little buffer to the program of 7 degrees.

My question is, approximately how much would the heat gain change from 32,000 BTUs at 105 degrees to 98 degrees. Would it be enough for me to want to go with a 2.5 Ton versus a 3 Ton?
A 3-ton may not provide enough ( 32,000) Sensible Cooling.

S.H.R. = 0.74 for some equipment mfgs, so 3-ton unit generally provides 36,000 BTUh Total /26,640 BTUh Sensible.

7'F less for Outside Design (98'F versus 105'F)
may lower the calculated load by ~4,000 + BTUh
for the walls and ceilings.

__ Seems like__ you may need ~3 Ton to

S.H.R. = Sensible Heat Ratio

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There is no need to add a buffer. Manual J is already 25% oversized. The best thing for you to do is check your ducts for leakage (if they are outside the thermal envelope) and seal all holes between the attic and rooms below. This way you are saving winter and summer.

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