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Can someone explain Manual D results?

Posted by RED-OAK (My Page) on
Sat, Oct 29, 11 at 20:32

As recommended by the HVAC world and Mr. Udarrell I performed this test and the Manual J for example,the program recommends 700 CFM and 700 FPM for supply and return, but Mr. Udarrell and others recommed @350 FPM for return which is a very big difference. So, for example if you have to pysically use three return ducts back to a 5 ton unit a 16", 14" and 12" is equivalent to @ 2,000 CFM and the FPM would be at least 700. But, if you select 350 FPM for the same three return ducts needed it recommends a 20", 18", and 16" depending on if you add additional rooms on each trunk. CFM wise it now totals 4,000 CFM which in theory makes since, but isn't the system now not balanced because you have 2,000 CFM supply out and 4,000 CFM returning? Can someone explain is that too much CFM because everything I've read, everyone says you can't have too much return, but this seems to be too much.


Follow-Up Postings:

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RE: Can someone explain Manual D results?

The filters do a better job at catching stuff if the flow rate through them is under 400 FPS.

There is no need to "balance" the flow rate of the returns to that of the supplies. You aren't going to get any more CFM in the return than the fan is blowing, even if the return is very very large. The total static drop is important, so if you can reduce the drop of the filters/returns, you have more budget for the A/C coil and supply ducts/diffusers.

It seems pretty difficult to meet the official static pressure drops, with 5T of cooling flow (which would be 1750CFM for Carrier default settings) or with a 120k+ BTU furnace.


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RE: Can someone explain Manual D results?

So, are those size returns too large to do this, because I am more than capable of making it happen and if it requires some real pros to get the supply right, so be it, but at least I'll have the returns large enough. I was also going to oversize the return boots one size with a reducer to the duct, but yet I will have larger grilles for more free space area. This is a Goodman furnace and it is designed for 2,000 CFM @ 0.5 static, which is very common, but it seems impossible to achieve since they don't make a coil with that low of a pressure drop and I have checked around other manufacturers and none are near that low except a horizontal slab coil and it was @ 0.298. Someone told me you should aim for percentages of 40, 40, 20 out of that 0.5 or coil, ductwork and filters in that order. But I said you are saying a 5 ton A-coil having a PD of 0.2 or less? Does that even exist and what is typical pressure drop or static of flexible duct work for 5 ton systems? So, is the HVAC world going to be silent on this or do what this one guy is saying that is mix and match like 4 ton condensing unit and coil or go to a variable speed DC motor furnace because they have higher PD's (0.8- to 1) etc. So, it seems to me it must be about non-existent for a standard 5 ton unit to be designed to operate within those specs. I am anxious to hear what they have been doing for years to make 5 ton residential systems using the PSC motors work under 0.5" IWC.


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