Do I need a manual J load calculation?

16 years ago

I am having a large house built - about 5700 sf single level with 10' ceilings. The HVAC rough-in is pretty much complete. My mechanical contractor sized it at 2 5 ton and 1 4 ton Trane XL14i condensers. I asked if he "sized" the system and accounted for the ICF R27 walls and the 4" Icynene foam on the roof deck. He said yes, but I did not know to ask about a "manual J load calculation". Is this something I should make sure is done? Is it too late at this point? If he can't/won't do it can I have another contractor do it? Any idea what it would cost? Does the current sizing seem appropriate for this size of house?

I'm really sorry for all the questions. I tried to search for answers in the forum, but the search function seems to not work and I could only find so much by manually reading through messages. Thanks for the help.

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Comments (6)

airscapes
16 years ago
yes get it done! See this link for an inexpensive program you can use. I was qouted $100 to do this calculation by a guy in another forum. With the money you are spending to build I would pay a pro to do it but if you want to take the time to validate what your guy want to sell you and not go to another pro this will help
http://ths.gardenweb.com/forums/load/hvac/msg1023232112899.html?9
energy_rater_la
16 years ago
do you need to be comfortable in your new home?

the going rate I hear for a competent manual J is
about .10 per sq ft.

these calculations arewell worth every penny when inputs are correct.

as you are not cutting corners on the construction of your
home, why cut corners on the comfort & health of your new
home??
an improperly sized system, just like a poorly installed system will effect the comfort & health of your family every day for as long as you are in your home.

best of luck with your project.
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Also, the units in the report are what was proposed and in the bid but we are planning to upgrade the SEER. Just trying to work out the size of the unit so that we can get the price on the various SEER levels. Another question: when we decided to foam the rafters, the hvac company said that would allow them to downsize the second floor unit and they gave us a $1000 credit which we're planning to use to upgrade the SEER. Would the foam not also affect the size of the downstairs unit? Our contract only specified the type of unit and SEER rating but didn't give a budget. We were told the foaming would reduce the cost of the hvac equipment but not by how much. I guess I've lost some trust and my concern is that a 5 ton system was budgeted but only a 4 ton system is needed and we won't get credit back for the reduced size needed for the first floor. Is it fair for me to ask what amount was budgeted for the hvac equipment? It is not a cost plus contract but rather a fixed price. Our builder is good about changing things and if it's cheaper, he adjusts the amount in our favor. Of course some things have been more but he's been very reasonable and I think he's just basically charging us the actual amount.
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I have to agree with the two posts before this. With the numbers that your are providing you would typically need the 2 1/2 ton to size for both the latent and sensible sides of the load. To find your sensible capacity you must ask the manufacture for their expanded ratings of their condensing unit. Then you must pay close attention using your wet bulb and indoor dry bulb temperatures. This information will give you your sensible ratio. Both loads will have their percent requirements that need to meet or exceed the loads required from the original Manual J report that you conducted. For Manual J, Manual D, Manual S, heat load calculations, and energy code compliance reports please visit our websites. THIS IS NOT AN ADVERTISEMENT!!! We only want to offer additional sources for assistance. Our goal is to help our industry toward more energy efficient building practices. Good luck with your building project! http://www.larsentech.org http://www.myspace.com/manualj Here is a link that might be useful: http://www.larsentech.org
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I can imagine that being in any jobber biz you're eventually going to get shopped after quoting a job. In my business, we take that risk by submitting our best ideas on multimillion dollar public works projects. They may like your idea but not your price, and you spend tens, maybe hundreds of thousands in the pursuit over multiple years. Think that's just a lack of ethics that pops up across the business community. Energyrater- I've been participating in a discussion on FH's Breaktime forum. Someone putting in vinyl windows where the mfr instructions said not to foam them in. I was shocked by the number of folks who agreed that you should follow the directions and use fiberglass. I can't imagine putting windows in and not using the window/door poly foam to seal them in. dallasbill- the only thing that you'd need to wing it on is the ACH. If the house remodel is in progress, you won't really know how leaky it's going to end up. We did a blower door test before construction and then based the sizing on an estimate of how leaky it'd be after doing lots of improvements. Still have work on weatherstripping old windows/doors to do. I'd intended to get it tested again afterward, but we'll see if that actually happens. yoyoma- ideally you want to test the new system with a "duct blaster" test to see how well sealed up the ductwork is. Not as big a deal if it's all w/in the conditioned space, but most of the time it isn't. A well-installed new system will leak .05 cfm/sq ft of conditioned space. Here is a link that might be useful: Southface fact sheet on blower door & duct blaster testing
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Manual j is garbage even most software has missing information even with a basic block load. You can take 10 different contractors on same house and you will have 10 different answers. Only the original person who built an existing house is only going to have the correct answers . Yes only a mechanical contractor has no buiness doing calculation on a structure that's license engineer only. Any time you answer question on a work sheet your guessing what you think it is. Equipment Installation is rule of thumb and years of experience. You don't see to much over sizing Equipment anymore. You got a lot of GC crap out there like 2 story house with damper systems. Putting a 3 ton on 4000 Sq ft house with spray foamed walls. It should be up to the discretion of a licensed contractor to make a tonnage or size call . You can argue 10 million points on the manual j never seen and thing good come out of this garbage. Let's say to have 4000 square foot house 1 door no windows walls 8ft thick and 20 Rooms in it well you dam good will fucking 2 ton not going push 800 cfm to all those rooms. If any thing the manual j will put you in a liability problem undersized the equipment.
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bus_driver
16 years ago
One fellow permitted me to look over his shoulder while he did the Manual J and D calculations for my house on his computer. He had never seen the house, but I brought him a set of plans. The plans and my answers to a few questions formed the inputs for the computer. Now I can say without hesitation that part of every proper installation is those calculations. Experience may enable a contractor to get many installations to be close to the ideal without the calculations. But by all means do the calculations. And install accordingly.
baymee
16 years ago
There are several posts recently about a house being too hot, or too humid, or too dry. This is why you do the manual J
baldwin_hobo
16 years ago
Sounds like 14 tons. Wow. I'm doing ICF house with urethane spray at ceiling. 5600 sq ft plus 2400 sq ft basement - in Maryland - 4 tons. Worksgreat - geothermal heat pump, radiant floor, 3 chilled water air handlers.
udarrell_2007
16 years ago
Quote: My mechanical contractor sized it at 2 5 ton and 1 4 ton Trane XL14i condensers.
-----------------------------------
That should probably read 2.5-Ton & 1.4-Ton.

Every home should have a record of the materials insulation amounts & areas insulated, etc.

Always have a manuals J, S & D done. udarrell

Here is a link that might be useful: udarrell.com

Do I need a manual J load calculation?

Comments (6)

airscapes

energy_rater_la

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