A slightly disturbing trend

I’m concerned about some of the posts I’ve been seeing over on LE in the past few weeks. I’ve been hoping that “someone” would step in and correct some of the mis-information that is being spread, but evidently he doesn’t care enough to bother.

First of all are the CFM units per square foot to use. I’ve been seeing a lot of “under and over” the recommended fan sizing. For the record, the recommendation is this: use 125 CFM per square foot of hood area.

Next, I see a disturbing trend towards using a fan that has as much as 50% more CFMs than the design calls for. I understand that some folks are trying to get in on a good deal (and heck, who wants to pass up a discount, right?), but even 25% over design spec is moving far too much air. There’s one person who has a 5 square foot hood area (design spec is 625 CFM) and is using an 800 CFM fan (22% more air than needed).

The design specs do have some “flexibility” built into them, and I it is good to go slightly higher, maybe by 10% (so the 625 CFM spec could use a fan that draws 690 CFM), but usually not much more than that. I also understand that there is a limited choice in available fans. HOWEVER, the higher the CFM you use, the more fresh air you will need. Many people are already complaining about the cold air in winter and hot air in the summer, so why even go there?

If you follow the proper method of designing your ventilation system: figure out what size hood you want, then find the fan based on the hood size. Estimate your ducting requirements and do the calculations and see if the fan you selected can handle the static pressure requirements of your system.

Don’t buy a fan just because someone else is using the same fan! Their ventilation system most likely isn’t anything like yours, so why buy the same fan? And along those same lines, don’t buy a fan if the manufacturer cannot provide static pressure numbers for the fan. This is the fastest way to burn out a fan I’ve ever seen.

Just because a fan is being used as a hydroponic ventilation system does not mean that it will work for glassworking ventilation. The two are not equivalent systems.

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3 thoughts on “A slightly disturbing trend

  1. your voice at LE is missed, and hopefully you will post there again. i no longer give advise as all i got for my effort was grief and arguments.

  2. Well, I’m officially persona non grata over there…I’m on moderation due to my ‘continued lack of respect’ whatever THAT means…so, I will continue to pick up juicy morsels from there and present my answers here. Stay tuned.

  3. Gentlemen, do keep in mind that there is ONE forum out there where your advice and opinions are welcomed (as long as things don’t turn into a major pissing match…which the moderators there are very, very good at preventing IMLTHO…)

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