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05-21-2007, 05:56 PM #1Regular Guest
- Join Date
- Jun 2006
how often do professionals really do manual J tests?
I read all the time that your contractor should perform a heat/cooling load calc, and if not you should find someone else who will, but over the past few years I've had several contractors out for a couple of different projects (including installing central air for the first time), and NOONE is crunching those numbers. I even asked this last time and received the "I've done 800,000 of these.......". But when the units come in increments of 1/2 tones, what's the practicality of such a calculation anyway? Or is more than sizing the unit, like number of supplies/returns, where they are located, etc.?
05-21-2007, 06:07 PM #2Regular Guest
- Join Date
- Sep 2006
They like to do them about as much as my ten year old son likes to take a shower.
But without one they are guessing. There are just too many factors that influence the answer. That is why most residential systems are over sized. They guess and go big so that they aren't as likely to get a call back. But it ends up costing you more for the equipment and more to operate. Plus, if you are in an area that has high humidity then it will likely leave you with a more humid house and you won't be comfortable. This is your first A/C (at least in this house) don't get it wrong after waiting this long.
For $50 you can do your own. Go to the top of this web iste page and click on the HVAC-Calc link. It is money well spent.
Last edited by mchild; 05-21-2007 at 06:14 PM. Reason: Started Thinking
05-21-2007, 06:09 PM #3
A room by room manual J will tell you how many CFM each room needs.
Manual D can then be done to tell what frition rate to use for sizing the duct work.
When done right.
Helps prevent those rroms that are too cold or hot.Contractor locator map
How many times must one fix something before it is fixed?
05-21-2007, 06:49 PM #4
I agree, it's hard to find a pro who will do Manual J, at least that has been my experience for an existing house. I wanted to really find out whether I had the right size AC (not to mention we were seriously confused about what size was installed, due to illegible labels). There was one AC pro who sure seemed conscientious, and old-school hard duct metalworker kind of guy. Father and son in business together, the older guy took some room measurements and told me I needed 4 tons on one side of the house and 5 tons on the other. Naturally that required a complete redo of the duct work too.
I don't doubt the sincerity of that guy but he was *so* far off on sizing. Where he prescribed 4 tons I had at the time 3.5, and have downsized to 3.0 with good results -- I really suspect 2.5 might be the closest fit. Where he prescribed 5 tons I now have 3.5 and I expect this summer may prove that is more than needed.
My Hvac-Calc Manual J7 showed 5.0 tons approximately required, depending on what value I chose for infiltration. Since that is across two halves of the house, and the halves peak at different times, each half must be sized appropriately for its own peak demand. Manual J8 does a better job of estimating duct losses, than J7, so I found myself having to guess at that component of cooling load. The overall experience has taught me the shortcomings of an amateur like me trying to provide all the info for an important model -- but I suggest to anyone it is better than your other choices. The program is easy to work too, only took me a couple hours and most of that time was measuring walls and windows.
One caveat about Hvac-Calc, it is no substitute for using Manual D for airflow. You will get only rough ballpark air flow numbers if you omit Manual D. I predict your typical pros will fall back on "I've done 800,000 of these" when you ask them about Manual D, deja vu all over again<g>. If you find a pro who actually will use Manual D, he's a keeper no matter what he charges!
Hope this helps -- Pstu
05-21-2007, 09:55 PM #5
05-22-2007, 09:12 AM #6Regular Guest
- Join Date
- Jul 2005
05-22-2007, 09:25 AM #7
Around here, you need one if your going to pull a permit. The inspector will ask you, how do you know thats the size of a furnace or a/c you need?...and you have to be able to show them."If anybody can draw on the power, where do we put the meter?" - JP Morgan before pulling Tesla funding
05-22-2007, 09:31 AM #8
we don't need them for permits............yet.
we do need them for rebates, most of the equipment we sell is high efficiency so they all get load calcs done.
05-22-2007, 09:31 AM #9Regular Guest
- Join Date
- Jun 2006
wow. interesting point. hadn't even thought about a permit. do i need one (or want one) for installing central air and/or new heating?
05-22-2007, 09:44 AM #10
05-22-2007, 10:55 AM #11
>>I do a manual J & D on 90% of the homes I estimate.
>>The other 10% are for landlords, Asians, or Middle Eastern folks and I'm not wasting my time...
Maybe "stereotypist" would be a better word. This is not someone who goes out and lynches people, after all. Nor an Aryan supremist type, at least not evidenced so far. There are degrees of malice in ethnic stereotyping and I would not slam this man with the worst.
But if you take out the convenient cliche then it's not so easy to stop thinking about something.
ARP board, anyone?
Best wishes -- Pstu
05-22-2007, 11:21 AM #12
The use of stereotypes based on race as represented by the poster is a way to judge groups of people based on race in advance of dealing with any one individual from that group. In this case, it amounts to not doing business with certain specific racial/ethnic characteristics. Sorry - that's racism.
05-22-2007, 01:14 PM #13Professional Member
- Join Date
- Jun 2005
- The Gray Northwest