Help interpreting Manual J Calculation - General HVAC Discussion - HVACSite

Help interpreting Manual J Calculation

 
Thread Tools Display Modes
Old 06-10-2010, 08:23 AM   #1
Junior Member
 
clos's Avatar
 
Trade: Homeowner
Join Date: Jun 2010
Posts: 1
Default

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.

clos is offline  
Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. HVACSite.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

Old 06-24-2010, 11:40 AM   #2
Junior Member
 
cool_air's Avatar
 
Trade: Condition air non-restricted
Join Date: Apr 2010
Location: Jesup Ga. 31545
Posts: 7
Default

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.
cool_air is offline  


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Join Now... It's Fast and FREE!

I am a professional contractor
I am a DIY Homeowner
Drywall Talk is for
PROFESSIONAL CONTRACTORS ONLY!

At DrywallTalk.com we cater exlusivly to professional contractors who make their living as a contractor. Knowing that many homeowners and DIYers are looking for a community to call home, we've created www.DIYChatroom.com DIY Chatroom is full of helpful advices and perfect for DIY homeowners.

Redirecing in 10 seconds
No Thanks
terms of service

Already Have an Account?

 

SEO by vBSEO 3.6.1