Should manual J be used?

[anacdoctor]

What is the correct way to do a change out.
Do a heat load or don't do a heat load of the house?

[beenthere]

I vote for A.

[anacdoctor]

It seems to me alot of people ask about the correct tonnage that should be used for there change out.
Now they have a 3 ton system, one company tells them to put in a 4 ton the next company tells them to put in a 5 ton.
Yet the companies are not doing a heat load.
And then if the customer does go from a 3 ton to a 5 ton does the company tell the customer the duct works needs to changed out to be sized for a 5 ton system.
Why do companies operate like this?
All of the companies I have worked for tell me they use rule of thumb.
And most of the change outs have major cooling problems.
Is not doing a manual J on the house just to keep the cost to the customer down?
It costs companies alot of money in call backs.
It just something I don't understand.

[gplant]

What is the correct way to do a change out.
Do a heat load or don't do a heat load of the house?

Consider this:

1) Are you absolutely certain that the original unit was sized correctly using a heat load calc?

2) Are you absolutely certain that in that time there have been no changes to the insulation, roofing, windows, etc?

If you can say “yes” with confidence about both above conditions, then no, you probably don’t need to do a heat load calculation when doing a 1 to 1 change out. But it would not hurt.

If you can not say “yes” to both of the above conditions, then yes, you absolutely need to do a heat load calc.

Glenn

P.S. I have my doubts about anyone being able to meet both conditions. Even if you "think" the answer is yes to both, do a heat load anyway.

[beenthere]

Why do companies operate like this?

They don't care, or else they don't know ay better.

All of the companies I have worked for tell me they use rule of thumb.

Still not the right way.


And most of the change outs have major cooling problems.

Oversized units on ductwork that was already undesized for the old unit always do have trouble.

Is not doing a manual J on the house just to keep the cost to the customer down?

Depends what you mean just to keep the cost down for the customer.
If you mean operating cost, yes.
If you mean install cost, no. You add the cost of doing a Manual J into the job price.
A Manual J is to size the unit so it cools and dehumidifies right, and so you know how much air each room needs.

It costs companies a lot of money in call backs.

Thats what can happen when you guess.

It just something I don't understand.

As you learn will will understand.

[adrianf]

Or in this day and age of the ever popular higher SEER one 4 ton unit may be 44,000 while the other 4 ton unit is 48,500 btus with one having an SHR .82 and the other has a .69 Shr. Without a heat load your pi$$ing the closet after being out all night.

[dan sw fl]

All of the companies I have worked for tell me they use rule of thumb.
And most of the change outs have major cooling problems.

Is not doing a manual J on the house just to keep the cost to the customer down?

It just something I don't understand.

How do you apply a rule-of-thumb for:
1) lots of windows
2) Dew Point regularly > 74'F
3) people who insist that 72'F is just barely cool enough.
4) Air Flow to each room


Manuals J, D and S address these issues in a rather systematic manner.


Having said that, simple situations may be addressed with experience.
Just have the one using the with experience approach provide
the +/- 1'F between rooms Guarantee work for free and
on-call for the weekend.