Is Manual-J absolutely necessary?

[jacquelynn]

Sorry to rehash this point, but I've spoken to over a dozen HVAC installers (some on the phone, some actual bids). They've all been in business a long time -- some since the 1960's. Only one of them considers a Manual-J important -- and that company costs $4K more than the others. The others all say they go by square footage; that with a house built in 1948, the Manual-J doesn't apply anyway. Since the one company that did the calc came up with 3.5 ton -- what's wrong with asking for that size system from another company and getting the job done for less $?

[tinknocker service tech]

and get what you pay for. if you are looking for a rebate most ultility companies require a manual J

[mo-flo]

yeah,that figures the guy that is doing it right gets to do load calc. and then gets undercut and the other guys get the job and the load done for free for them,......just because they have been in business along time doesn't mean that they are doing things right.In my opinion it is alot harder nowadays to be going in to this business than it was 20 years ago because of all the changes that we are dealing with.............

[redhawk]

To properly size any equipment, it is necessary to do a heat load calc. From what I have seen, most older homes have over sized equipment. IMO, any contractor telling you that a heat load calc. is not necessary because he can use square footage is wrong. Room sizes, windows, doors, fireplaces, insulation, is important information to correctly size your home. Now taking the information from the one contractor that did do a heat load calc. and getting your equipment from someone that did not do one is wrong! That contractor took the time to do it right.

[trane]

There is some truth to the statement that since it was built in 1948 it does not apply. Manual J is only accurate if you use actual data in the calculations. With a home built in 1948 you are going to be guessing on some of the important data unless you have gutted the house and basically started over. Some will not agree but experience plays a large roll in selecting the right equipment in some homes.

[mo-flo]

1948 or not,a load still needs to be run on it.You can still figure windows,insulation,etc.,.....JMO.

[beenthere]

Manual J is an educated semi guess on most exsisting construction.

But it still applies to older homes, it has the r, and u values. Withj out a blower door test, infiltrtion is a guess.

If the other companies are going to guess at the size equipment needed, then they're going to guess at the size of duct work also.

So if you get one of them, and it does cool the rooms evenly, and its loud, just think of all the money you saved.

[brian in mass]

jacquelynn.....from reading your post.....it read like a school book.

It sounded funny enough that I had to reply...no offense please.

You talked to one respectable company, and a bunch of idiots.

It is very typical of alot of companies from around my neck of the woods as well. Those guys that have been in buisness since the 60's are still in the 60's. And the rest are just as bad.

The answer is simple. Yes it is important and nesasary.

The problem is lack of education. I have a good friend who owns his own HVAC company and has so for years. He took it over from his father who probably did it for 30 years or so. He says Man J is stupid, and there is no need for such things.

He is a perfect example. He has no proper training in his own field. He doesn't even realize just how un-educated he is !


Here is a perfect example........Take two houses that are exactly the same except for a couple of things. Let one house have spray foam insulation and real good windows, and let the other house have regular fiberglass bats and cheap windows.

Now send in the clown that doesn't like Man J and have him do both houses. After he gets done with his tape measure and pocket calculater, he'll come to the conclusion that both homes have the same needs.

Now what happens a year later when both house wives are out walking the dog and start talking about their climate conditions ?? I'll tell you....the contractor's phone will be ringing and he won't be happy !

[Jultzya]

Originally posted by jacquelynn
what's wrong with asking for that size system from another company and getting the job done for less $?

Besides being wrong, think about this...

If the other company is cutting this VERY IMPORTANT procedure from his job, what else is he taking short cuts on?

Doing things right takes time and material... neither of which are CHEAP or FREE!

Do it right, or do it twice... you choose, but doing it the correct way FIRST is cheaper and will cause you less headaches.

Load Calc Here

[jacquelynn]

Thanks everyone for your responses.


The calc. I received from the one company is one page only and it's titled "Right-J Short Form," Manual J: 7th Ed. Right-Suite: V4.1.10. The contractor said it took about two hours to do.

Is this the real thing, or did they use a short-cut version?

Thanks so much.

[jacquelynn]

Thanks everyone for your responses. I'm sure you're right (tho I hate to resign myself to paying the highest-priced bid of all). I hadn't thought about the ethics in using someone else's figures, and I agree with you that it's not right.

Trane: I'm not sure how thoroughly this contractor did the calc, or if he asked me all the right questions.

The calc. I received from the one company is one page only and it's titled "Right-J Short Form," Manual J: 7th Ed. Right-Suite: V4.1.10. The contractor said it took about two hours to do.

Is this the real thing, or did they use a short-cut version?

Thanks so much.

[Jultzya]

It's the real deal... and NOT a cheap one either!

[dash]

If they measured the walls,windows etc.,plus they know the r-values of the insulation,there's a good chance they did it right.

[trane]

Just out of curiosity, what sizes did everyone else come up with. Jultzya is right its real.

[Freezeking2000]

The second most importent thing is duct design and static pressure of your system. If the other contractors do not do a load calc I can almost gaurantee the seer rating of the equipment will not matter. A pooly installed and over-sized 17 seer or 90+ will cost more to operate that a properly sized and installed 10 seer (or 78%)system.

Go with someone who knows and you will be paid back every month for years to come. I know of identical homes to mine which have electric bills that double my bills every month.
We both have pools ect, my cooling system was installed properly!

[chapmanf]

There is some truth to the statement that since it was built in 1948 it does not apply. Manual J is only accurate if you use actual data in the calculations. With a home built in 1948 you are going to be guessing on some of the important data

The silver lining is that the house built in 1948 probably has a long history of actual billed gas usage, and that's really actual data: a working furnace hooked to an accurate gas meter for a few years is a darned good measuring instrument for actual heat loss. IF: 1. you have access to a year or better of gas bills (during which time you didn't make any major changes to the house or furnace), 2. your gas co. shows amt. of gas used, heat value of gas, and number of heating degree days on each bill, 3. you know the efficiency of the existing furnace, and 4. you know the design winter outside temperature for your location, you can come up with a size number that can be a very good sanity check of an estimate from Manual J. It's not hard to do if you liked drawing graphs in math, or have a spreadsheet program that can draw a graph and fit a line to it.

It's not an officially recognized alternative to Manual J as far as I know, so you might want to be cautious about relying only on this number, especially if it's smaller than the J estimate; it's probably the better number, but if you go with the J estimate no one can say you didn't go by the book. Still, if the numbers are very different it would be a good clue to look again at some of the guesses on the J form.

And the inexpensive homeowner license for HVAC-Calc (jultzya posted the link) looks like a good way you can make sure you have a Manual J estimate done, and done right, without having to count on a contractor to do it.

[bigtime]

If you have called over a dozen companies you are wasting a lot of folks time. If you call anyone else tell them they are the 15th company youve called and see how many come over for a free quote.

[jacquelynn]

Thank you again for responding.

The other companies guessed 4-5 tons, and the one with the Manual-J says 3.5 ton.

I've decided to go with a 10 seer A/C since I don't plan to use it much -- maybe a few days a year.

jultzya -- thank you for the link. And chapmanf -- do you know anyone who'd like to review my utilities bills (for pay of course). I'm definitely not the person to do it.

[dan sw fl]

Originally posted by jacquelynn
Only one of them considers a Manual-J important -- and that company costs $4K more than the others.

The others all say they go by square footage; that with a house built in 1948, the Manual-J doesn't apply anyway.

Since the one company that did the calc came up with 3.5 ton -- what's wrong with asking for that size system from another company and getting the job done for less $?

As a starter, Blower door test would be more significant for a 1948 house.

[cem-bsee]

ACCA manualJ, or ASHRAE Fundamentals (used by engineers), apply to houses built whenever! I have used both for 50+yr --
else someone is guessing | assume = ass\u\me

without it, one does not know how much conditioned air is needed for each room --
although, one can install large size pipes with dampers, then adjust these by trial & error -- ( probably by the homeowner)

another aspect is your knowing just where your biggest loads are comming from --

re: infiltration: one can make reasonable choices for individual items: eg: door fit, window fit, oversize holes around electrical boxes, ...