Thread: help with manual j results

1. Banned
Join Date
Aug 2002
Location
Office and warehouse in both Crystal River & New Port Richey ,FL
Posts
18,836
Post Likes
Originally Posted by toireht
dash,
don't understand what you mean. can you explain please?

thanks
james
First this applies if your load calc is correct.

Your calc came up with sensible and latent btus,you need equipment that is rated to cover each of those numbers

ARI and manufacturers ratings are for what the equipment will produce at specific indoor and outdoor temp.

Charts usually show various OD temps and the capacity for each.

Indoor is at 80° F,footnotes on the chart may tell you this.

For every degree indoors below 80 the sensible capacity is to be derated ,835 sensible btus for each degree below 80 ,per 1000 cfms of air flow.

So, 835 btus X 12 degrees X 1.2(for 1200cfms)= 12,024 deuct in sensible ,and 12,024 increase in latent.

With the new latent number,half the excess latent capacity(over the latent load calc) can be added back to the reduced sensible.

Seems you calc is off since you have a 3 ton(I think) or you weren't running it at 68° F,in the past.

2. Regular Guest
Join Date
Jul 2008
Posts
12
Post Likes
Originally Posted by dash
First this applies if your load calc is correct.

Your calc came up with sensible and latent btus,you need equipment that is rated to cover each of those numbers

ARI and manufacturers ratings are for what the equipment will produce at specific indoor and outdoor temp.

Charts usually show various OD temps and the capacity for each.

Indoor is at 80° F,footnotes on the chart may tell you this.

For every degree indoors below 80 the sensible capacity is to be derated ,835 sensible btus for each degree below 80 ,per 1000 cfms of air flow.

So, 835 btus X 12 degrees X 1.2(for 1200cfms)= 12,024 deuct in sensible ,and 12,024 increase in latent.

With the new latent number,half the excess latent capacity(over the latent load calc) can be added back to the reduced sensible.

Seems you calc is off since you have a 3 ton(I think) or you weren't running it at 68° F,in the past.
thanks for the info, I guess what I am needing to do is change the 68 degrees in the software i am using. can you tell me what I should use. I am trying to determine what tonnage to size a new system. I can't get any techs in my area to do a manual j cal. I have double checked my measurements, insulation and variable that I put in the software. the software list tonnage but not sure how to read it or if it listing for ac or heat.

thanks
james

3. Use 72 for winter, and 74 for summer.
When it list gain, that is for cooling. loss is for heating.

Page 2 of 2 First 12

Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

Related Forums

The place where Electrical professionals meet.