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Manual J - Correct Load size?

Posted by wakko (My Page) on
Sat, Sep 24, 11 at 18:37

Can someone please look at this load calculation and let me know if the contractors assumption of 3.91 tons is correct? I thought this looked like 3 tons? anyone know for sure?

Location= Austin, TX

Total Loss= 41871
Heating CFM= (539)
Sensible Gain= 28,639
Cooling CFM= 1,563

Heating BTUs= 41871
Heat CFM= (539)
Cooling BTUs= 30,899
Cooling CFM=1,563
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Follow-Up Postings:

RE: Manual J - Correct Load size?


A question back to you.

What size do you have now and how does it perform?

Post back.


RE: Manual J - Correct Load size?

currently there is a 4-ton system installed. however, it has been broken in the house ever since we moved in. when the freon is charged. it works fine, cycles occasionaly but still works.

RE: Manual J - Correct Load size?


What size is your home?

Who performed the Man J? Do you know the software used to provide these numbers? Design temps both inside and outside used?

I personally think the numbers are highly suspect. With that said, someone needs to run the Man J again and make a comparison. That would be my approach.

It's difficult for me to believe that a homeowner could drop from a 4 ton down to a three ton and maintain AC cooling comfort level for Austin, Tx.

Is this a HP system or AC with gas furnace?


RE: Manual J - Correct Load size?

I am not a professional, but I will share my opinion as to what this load calculation means to me.

Your load calculation has cooling air flow (CFM) for each room. I don't have this on my Manual J calculation, so I am not sure how this is calculated. The report shows the total CFM is is 1,563. If you divide this number by 400 you get the the value 3.91. The general rule is you need to provide 400CFM of air flow for every ton of cooling.

However the total cooling load is 30,889BTU. This is a little over 2.5 tons. I don't understand why the air flow has been calculated to be so high. This doesn't make sense to me.

If this is a Manual J calculation then there should be summer and winter indoor and outdoor design temperatures. If it does not, then I would be suspicious of this calculations. Have the contractor explain the physics behind this calculation.

RE: Manual J - Correct Load size?

The contractor performed the manual J. This will be a heatpump system.

The contractor has been stating 4 tons and using the general rule of thumb here in austin with tonnage 600 sq ft /ton from the begining of our meetings. House is approxiamtely 2392 sq ft. hence the 4 ton. i'm feeling suspect with the design myself, but he keeps calling for a 4 ton. and even after an hour conversation with him finally said a 4ton evolution system is the best and the system will run at a 3 ton effectively being the same and the 4 ton will call for when needed on the hot days. Finally towards the end of negotiating, he did agree that a 3.5 ton is doable.

from the page it looks like Nitek software, design temps were 100 outside 76 inside
95 outside 72 inside

I'm thinking the tightness of the house is ach .35-.45 which is what austin energy requires. House has R30 in the attic. 2 story house on slab.

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