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am i hallucinating or did there used to be a subforum for forced-air furnaces

guess this could also go in the air-con forum but i tend to think of furnaces as furnaces even when the burner is off and it is the evaporator is doin' its thang.

older building with nice main feed and return plenums but haphazard branching and branch sizing and couple large returns not well spaced.

and all the ducts are covered upon leaving the boiler room so don't have a lot of access to think about changing a lot of stuff up easily.

so it does 'fine' for heating but struggles to bring enough load and circulation to comfortably cool the same space. figure this is a typical problem although nothing with forced air is typical to me given i spend the vast majority of my time with forced water.

so it occurred to me in the quick fix department that I might be able to slightly improve circulation by increasing the fan speed.  I was busy measuring pulleys but didn't measure the squirrel cage itself which was to my recollection maybe 10 to 12" in diameter and 20" long.

my calculations on the current set up was about 900 rpms at the fan.   there was a  very little bit of adjustment left on a Maska mvl-34 (3 and 1/4" variable width pulley) so i calculate that I raised the rpms to maybe 975 with that. little bit of improvement.  so i figured maybe I'd go for an mvl-40 which takes it up to about 3 and 3/4" outside diameter.  and if i did the math right i'd come up a little short of 1080 rpms. 

But recalling the false performance notion of simply raising steam pressure to 'fix' a steam system, I thought i'd ask besides my emperical results whether noise or vibration are the worse to fear here. figure there is some logical limit to the rpms desirable for these fans.

know there are systems that operate at much higher air pressures for delivery through smaller tubing, but this is on the bubble so i'm just looking for a slight increment.

i feel like this might accentuate the difference in areas served by larger feeds and well served by returns  (which wouldn't be all bad as it would indicate the areas that most need ducting attention when they can handle me making some holes, and i'm working on how to get maybe one additional return installed for the toughest area, but the whole place needs a little more umph on cooling. so i'm pretty much convinced to give it a try. i'm goin' in there. wicked witch or no wicked witch.  i just want you to do one thing. talk me out of it ???? or not.




  • Jamie Hall
    Jamie Hall Member Posts: 17,065
    I don't remember a subforum

    for scorched air, but I could well have missed it...

    On your particular question -- raising your fan speed should increase the flow velocity and cfm.  Should.  Squirrel cage blowers can be, well, a bit squirrely.  But it shouldn't hurt anything, and is worth a try.  It will also increase the load on the motor, though, so if you try it you will want to check the running amps on the motor to make sure you're not overloading it.

    Whether it will solve the cold air distribution problems or not...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • icesailor
    icesailor Member Posts: 7,265
    Airheads revenge:

    I'm not an Airhead, but a dedicated, dyed in the wool Wethead..

    I've always considered air systems as a form of FM, Freaking Magic. Because I see old systems, balanced with a supply and return in each and every room, to some old ones where there was a supply here and there and you left the cellar door open because there was no back for the return on the furnaces. Then, there are all the modern ones I used to see with a supply in every room in a two story house, zoned, with a common return at the front hall over the furnace in the cellar. Then, there is The Octopus.

    I read once that it takes bigger ducts to run scorched air to supply heating for a building, but if you designed it for cooling only, they would be smaller. How do you figure that in?

    I also look at air systems like a hydronic system that pumps all the supply water into a big pool when it is done going through the heat emitters.  Once in the pool, another pump has to pump it back to the energy source. If the return pump is smaller than the water going into the pool, it will overflow. Unless it is a closed pool. Then the pressure goes up. Or something.

    Increasing fan/air speed doesn't usually solve a problem. But a free flow with little restriction can go a long way. In a closed air system, you're really depending on the vacuum of the fan, to "Pull" the air back. If you have a leaky air system and the ducts are oversized, you have a real problem getting balanced circulation.

    You always need a bigger pump if your boat has a bigger leak.

    Kind of like raising the pressure in a steam system. Raising the pressure just raises the temperature. Why do vacuum systems work so well when they are tight and have no restrictions?

    Just my thoughts. Worthless at best.