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Old 12-14-2011, 02:05 PM   #1
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Loss / Gain calcs


I am going to be getting bits on a new furance here in a couple months, I've been "trying" to do load / gain calcs myself to get an understanding for this I came out with these numbers


Sensible Gain 27,776
Latent Gain 2,675
Total Heat Gain 30,451
Total Heat Loss 82,467

my area has summer temps average at 83 F and winter at 2 F, summer grains of moisture is 90, trying to design for 55% RH

the house is 1,450 sq ft basement, 1,450 sq ft 1st floor and about 600 sq ft in the 2nd floor so about 3,500 sq ft conditioned

right now they have a 125,00 btu/h furance and a 4 ton AC... after these numbers that does seem way off, I need a furance that can keep up with the heat loss right? would dropping it down to 90k btu/h be a good idea? right now it cycles about 7 minutes of run which seems short to me, it gets really hot fast then cools fast then repeats that a lot...

I'm definatly getting a multi stage (probably 2 stage, modulating seems way too much $$$) was wondering if something like a 61.8K / 95K BTU/h would fit this design?


Last edited by BlueBSH; 12-14-2011 at 02:07 PM.
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Old 12-14-2011, 05:48 PM   #2
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Loss / Gain calcs


A 3 ton A/C will meet your sensible requirements, since you have a very low latent load. A 90,000 BTU input 95% will meet your heating load.

Is the basement above or below grade.

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Old 12-14-2011, 05:52 PM   #3
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Loss / Gain calcs


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Originally Posted by beenthere View Post
A 3 ton A/C will meet your sensible requirements, since you have a very low latent load. A 90,000 BTU input 95% will meet your heating load.

Is the basement above or below grade.
the entire basement is below grade except for 1.5', so about 7 ft is gelow grade and the top two layers of block is above, its all insulated also to about R-17 below grade, and the rim joists are R-30 insulated
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Old 12-14-2011, 05:55 PM   #4
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Loss / Gain calcs


So only around 14,000 BTUs heat needed for it then.
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Old 12-14-2011, 07:04 PM   #5
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Loss / Gain calcs


It sounds like your main problem is that your cooling down too quickly. Are all the rooms cooling down quickly are just one? A smaller two stage will still have the same off time, but a longer on time.
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Old 12-15-2011, 07:49 AM   #6
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Loss / Gain calcs


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Originally Posted by bungalow_steve View Post
It sounds like your main problem is that your cooling down too quickly. Are all the rooms cooling down quickly are just one? A smaller two stage will still have the same off time, but a longer on time.
90% of the house isn't cooling fast, it's that last 10% that is, and that last 10% is the master bedroom / WIC / Master bath... it's the only part of the house that doesnt go up 2 stories... the odd thing is its well insulated, I don't know if it gets enough air flow to start with... its 600 sq ft but only has two ducts and one 12x8 return, to get this room warm you have to overheat the rest of the house, but that hopefully will be fixed with the new ductwork and zoning (maybe if I can afford it)
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Old 12-15-2011, 10:57 AM   #7
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Loss / Gain calcs


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90% of the house isn't cooling fast, it's that last 10% that is, and that last 10% is the master bedroom / WIC / Master bath... it's the only part of the house that doesnt go up 2 stories... the odd thing is its well insulated, I don't know if it gets enough air flow to start with... its 600 sq ft but only has two ducts and one 12x8 return, to get this room warm you have to overheat the rest of the house, but that hopefully will be fixed with the new ductwork and zoning (maybe if I can afford it)
yes that way too little duct for 600 square feet

I have a friend with a 4000square foot house, only one room heats correctly, every other room has problems, some rooms are always hot, others always cold, one room heats up real hot and cools down at about 10 degrees/min it seems. He has multiple zones with automatic dampers I believe, no furnace exists that is going to fix those problems.
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Old 12-15-2011, 11:01 AM   #8
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Loss / Gain calcs


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yes that way too little duct for 600 square feet

I have a friend with a 4000square foot house, only one room heats correctly, every other room has problems, some rooms are always hot, others always cold, one room heats up real hot and cools down at about 10 degrees/min it seems. He has multiple zones with automatic dampers I believe, no furnace exists that is going to fix those problems.
yeah, well im not trying to fix those problems with a new furance, I am trying to fix a crap furnace with a new furance if those problems can be helped in the process then yay for me!

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