CFM and exhaust ducting
Hello folks,
Was going to install a 1500 CFM in our under construction build and suddenly found myself facing the MUA (make up air) dilemma.
I am told by my HVAC guy that for 1000 CFM, it would cost me $3200 for 2 dampers/louvers/ducting/upgrading the unit and one extra loop (geo). To upgrade to 1500 CFM, it would be close to $5200 which include all the above plus 2 extra loops.
My question is, since the exhaust duct in our house would be straight up thru the roof without any elbows, would It be wise to say, it’s efficacy could be equivalent to 1500 CFM installed in most houses, where I am sure they have 1-2 elbows?
I think, I can push my HVAC guy to let me squeeze in 1200 CFM for $3200 upgrade and be able to save some money?
Any issue with dampers on a straight thru exhaust?
Thanks
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Replies
Beenash,
I think that there is a misprint in your question. I'm guessing that in the 3rd paragraph, you intended to write "1,200 cfm," not "1,500 cfm."
If you measure the air flow rate of an installed range hood fan, the measured rate is usually significantly less than the manufacturer's specification. How much less depends, as you suspect, on the number of elbows, the length of the duct, and the diameter of the duct.
There are charts that show the result of different duct configurations, but suffice it to say that your "straight shot up through the roof" is as good as most installations will get.
Many homeowners are happy with makeup air units that have cfm specifications that are a little under the cfm specifications of the range hood fan -- but it's hard to generalize.
Martin,
Thanks for your help.
Regarding the third paragraph, what I meant was, installing a 1200 CFM in our house with straight thru the roof duct, could that be considered equivalent to most houses with 1500 CFM where the install probably isn't as ideal as ours?
I hope my HVAC guy is in good mood today and will let me get the 1200 CFM blower motor for the make-up air system that he has configured for 1000 CFM for which he is charging me $3200. If I was to get a 1500 CFM, he would charge me $5300.
Thanks again.