# Thread: HVAC-Calc help and opinions

1. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes

## HVAC-Calc help and opinions

Here is our quandary. We have been taking a crash course in HVAC as it seems that the contractors in our area go with "if the house cooled with what you had in the past, then that is what you should go with." We had one contractor (who we are going with who did measure - NOBODY ELSE WOULD DO IT>>>SAID IT WAS A WASTE OF TIME IN AN OLDER HOME).

This is a 1 &#189; story house that is going to be ONE Unit zoned upstairs/downstairs with one Infinity 19. (We need to switch to one unit.) Contractor proposed single 5T. Carrier Infinity 19 and zoned to replace 3T. down and 2T. up.

After reading, asking questions, etc. for weeks, last night we pulled out old blueprints and a measuring tape, bought the HVAC calculator for \$50 and spent the entire evening doing our Manual J. (My husband is an engineer and is very precise.) The house came out to 2362 sq. ft of "conditioned area".

Using HVAC-Calc 4.0 with conditions set to: outside = 95F and129 grains H20; inside: 75F and 50&#37; RH (based on New Orleans which is closest city listed) and using the best estimate of insulation, we came out at a recommendation of 3 1/2 tons.

In trying to understand how the contractor might have arrived at 5T, he changed to outside: 100F and inside: 70F, construction tightness poor (leaky farmhouse), dropped the low e-glass and the foam insulation in exterior walls and added 8 people; but still only got a recommendation of 4 Tons from HVAC-Calc.

Are we missing something?

Does HVAC-Calc give accurate results for New Orleans area?

Do you oversize because you are using one unit, zoning and 2 floors?

2. Professional Member
Join Date
Apr 2007
Posts
6,344
Post Likes
I have New Orleans outdoor design conditions as ....

4-10 ft above sea level depending on NO AP lakefront or NO AP
30&#186; N Latitude
34-39&#186; 99&#37; Heating dry bulb depending on if it's NO AP lakefront or NO AP
78&#186; Coincidence wetbulb
61 grains of moisture difference for 50% RH
Daily Range in the low for NO lakefront
Daily Range in the med. for NO AP

It's never a good idea to oversize especially in a climate like NO where humidity is a huge factor.

3. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
Thank you for replying. My husband just went over your comment and says that the only way he can crank this up some more is to make it really hot outside (105) and maybe 67 (inside). 5T. is just not computing...even when we make the house have the highest level of infiltration as possible (which is not the real case).

Is there something else we are missing"

4. Professional Member
Join Date
Apr 2007
Posts
6,344
Post Likes
Are you sure he used manual J to do the load calc? There are some very basic websites that don't go in depth as hvac-calc does.

If your 100&#37; on your measurements and your husband is an engineer then if you come up with 4 tons then that's what it is. Oversizing leads to humidity issues....short cycling...and loss of efficiency and possible shortening of the lifespan of the system.

Whatever you do don't fudge the readings to achieve what someone thinks it should be. It specifically states not to fudge numbers to raise equipment size in manual J.

When doing manual J the number of occupants should be 1 for each bedroom + 1 unless you know you will have more people in the house on a regular basis.

I'll quote

"Comfort system performance is only as good as the accuracy of the load calculation. Efforts to "adjust the load" to provide a "safety factor" or to produce a solution that is compatible with the "I have been doing this 30 years" syndrome only produce designs that generate customer complaints about poor temperature and humidity control, or about more serious problems, such as mold and mildew"

Manual J 8 AE.

Maybe some of the other folks that are a lot better than i am with Manual J will come in a lend a hand. I just finished this class and I am just trying to provide you with what I do know.
Last edited by BigJon3475; 10-28-2007 at 01:01 PM.

5. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
BigJon

To be very specific, we are using:

http://hvaccomputer.com/talkref.asp

HVAC-Calc Residential 4.0
Cost \$50 for homeowner use

If there is something you think is better and available to homeowner, please share. Everybody wants the job....but nobody seems to think the footwork is important. We are so frustrated...this is not about \$\$\$, it is about getting things done correctly.

6. Professional Member
Join Date
Apr 2007
Posts
6,344
Post Likes
That's a good program. I haven't used it personally but from other folks on here it's good.

7. Regular Guest
Join Date
Jan 2007
Posts
141
Post Likes
Originally Posted by Doral
Using HVAC-Calc 4.0 with conditions set to: outside = 95F and129 grains H20; inside: 75F and 50% RH (based on New Orleans which is closest city listed) and using the best estimate of insulation, we came out at a recommendation of 3 1/2 tons.
I think that you will find that most would not recommend using the actual sizing specified by HVAC Calc for cooling.

You need to select equipment based on the sensible and latent gains that HVAC Calc produces - this is part of a process called "Manual S" which uses the output of a "Manual J" calculation. Usually you will find that the sensible gain is the limiting factor. For example, a 4 ton AC unit generally will only satisfy around 3 tons of sensible gain.

8. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
That makes sense and clarifies a few things.

9. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
Okay ...we have spent hours of time doing Manual J (and \$50.....) so ....
now where do we find Manual S?

Any suggestions..... is there a program that mates with this one that provides manual s?

Phew

10. Regular Guest
Join Date
Jan 2007
Posts
141
Post Likes
Originally Posted by jkish
I think that you will find that most would not recommend using the actual sizing specified by HVAC Calc for cooling.

You need to select equipment based on the sensible and latent gains that HVAC Calc produces - this is part of a process called "Manual S" which uses the output of a "Manual J" calculation. Usually you will find that the sensible gain is the limiting factor. For example, a 4 ton AC unit generally will only satisfy around 3 tons of sensible gain.
You will also find a section on sizing ac units on page 8 of the HVAC Calc manual that talks about these issues.

11. Professional Member
Join Date
Apr 2007
Posts
6,344
Post Likes
What's the specifics of sensible and latent heat load? How many btu's of each? Might help a bit with better advice.

12. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
We are using and indoor temp of 75F and outdoor temp of 98

-Sensible: 32586
-Latent: 6444
Sq. feet 2324

13. Regular Guest
Join Date
Sep 2007
Posts
79
Post Likes
All the upstairs ducts are being replaced.....so they should be "tight"

Page 1 of 4 1234 Last

#### 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.