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Manual J slop

Gwisejr | Posted in General Questions on

How much “slop” is there in the manual J calculations. By slop I mean, how much extra capacity is the Manual J calc already adding.. The reason for my ask is that I  was not happy with the inaccuracy of my HVAC vendors calculation, so I engaged ProCalcs to do a manual J for me. Long story short, Vendors calc showed 3 tons downstairs and 2 tons upstairs, while Procal showed 1.5 Ton down and < 1 ton upstairs.

After vendor corrected for a number of issues, It is down to 1.8 ton up and 1.3 ton down. Ordinarily I think, you would round up to the next .5 ton.. (PS this is just 1 stage equipment.. I plan on changing out to VRF’s in a couple years once I can afford it. And yes I know that it would be best to do it now but those are the realities in life keeping my wife happy with her list…) 
I had heard that the Manual J had plenty of  over capacity. Can’t recall where I read it and I’m sure that it was here or on Alison Bailes blog but I am thinking that  1.8 ton is close enough to the 1.5 ton and 1.3 to the .9 ton as calculated independently that using those numbers, 1.5 ton and 1 ton is sufficient. 

Just wanted confirmation.


PS.. Vendor is still using .4% mean instead of 1% mean.. Also, they calculated with no blinds which is definitely wrong at least for us, while Procalcs did 1% mean and light colored blinds @ 50% of window @ 45 degree tilt.. 
I understand that industry standard mean is 1% not 0.4% and i assume that most modelers would include some blinds instead of no window coverins to be more accurate in general

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  1. MattJF | | #1

    Go with the Procalc numbers, your PS points out pretty much exactly what is wrong with the vendors numbers.

    I don't think I have ever heard of a manual J being done and it later being determined the equipment was undersized. I would be interested if there have been any such cases.

    At least on the heating side, manual J does not factor in any internal load contributions or account for any thermal capacity and time dependent aspects of heat transfer.

    What are you looking at for equipment? In those sizes variable speed mini split systems are cost effective with just about anything I am aware of at this time if you are paying for installation.

    1. Gwisejr | | #2

      So, we went thru a builder so they use this vendor and they are installing Carrier's bottom tier 14 seer heat pumps. I've asked about upgrading to at least two speed units and they are crazy thinking that Im going to pay ~$6k for equipment that costs them a few hundred $ more. I'll wait and have the units removed and LG VRF's Units installed.

  2. MattJF | | #3

    At the sizes you need standard inverted driven minisplits make the most sense. Fujitsu slim duct line is the go to for cost/ heating performance in zone 5 and below.

    Depending on the exact loads, a pair of 9rlfcd, 12rlfcd, or 18rlfcd would work. There is a series that run bit higher static pressures too.

    The lg VRF units are all larger commercially oriented systems.

    Oh and the situation with builders is that they may have a negotiated contract for x number of systems with an exact set of specs for a fixed price. They may have even already purchased all the equipment. So they are pricing the change as buying a second set of equipment.

    What code is being used in your location? Maybe the carrier system is technically oversized per code and you could go down that road. Could be messy though.

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