Help interpreting Manual J Calculation - General HVAC Discussion - HVACSite

Help interpreting Manual J Calculation

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Old 06-10-2010, 08:23 AM   #1
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Posts: 1

Help interpreting Manual J Calculation

I've picked what I believe to be a good contractor to replace my gas furnace with a high efficiency furnace. The contractor recommended a 80K BTU 95% 2-stage American Standard furnace. I received a 1 page Manual J calculation and Heating Summary "Equipment load" showed 54129 BTUh at 0F design temp and 72F inside temp. The thing is, I don't remember the contractor doing any measurements of the house other than asking me what the foundation size was, so I am not sure if this manual J is accurate or not.

Since they recommended 80K BTU furnace, I asked if that was too big based on the Manual J (and since I had previously gotten another quote that recommended a 60K BTU furnace). They asked for my insulation info and said they would come by and measure the outside of the house and windows and rerun the calculation.

After measuring the outside, the new manual J showed equipment load of 35314 BTUh and they suggested that I go with the 60K BTU furnace. Again, I don't know if this is accurate. It seems odd to me, but I'm not an expert at all.

Original Manual J

Structure 36610
Ducts 14506
Central Vent (38 cfm) 3014
Humidification 0
Piping 0
Equipment Load 54129 Btuh

New Manual J after measuring house from the outside

Structure 32285
Ducts 0
Central Vent (38 cfm) 3029
Humidification 0
Piping 0
Equipment Load 35314 Btuh

What really stands out for me is that "Ducts" value going from 14506 BTUh to 0. Can this be correct? Can it change to 0 based on having real wall/window measurements? I don't want to get an undersized furnace installed. Does anybody have an opinion on these manual Js?

For reference, this is a 1920 sq ft split-entry home in MA, outside design temp of 0 F and average insulation.

Thanks for your help.

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Old 06-24-2010, 11:40 AM   #2
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Location: Jesup Ga. 31545
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Re: Help interpreting Manual J Calculation

Get another load done by another contractor and compair the more info that they get the better the closer the sizing will be Geo locating,which way the home faces, window size which they face,panes, glaszing,over hang and how close is top of window from eves, wall,ceiling, flooring contruction inside loads amount of people fire place tightness of home (infilltration) ceiling height,door contruction, attic vented/unvented ,color of roof ,attic insulation what kind and inches. And other loads inside. MANUAL J, MANUAL J!!

Last edited by cool_air; 06-24-2010 at 11:49 AM.
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