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Inline duct fan for zoning

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Bran_Winston
Bran_Winston Member Posts: 2
in Thermostats and Controls
I have an outdoor wood boiler with a water to air exchanger in the plenum of my forced air oil furnace. I want to add an inline fan to the duct that leads to my upstairs office so that I can draw hot air off the exchanger to that one room without having the main furnace fan kick on and overheat the rest of the house when it is otherwise unoccupied.
I haven't been able to find anything on this particular setup. Whenever people ask about inline fans on any of the forums, the topic seems to be around boosting airflow in an improperly set up system, something the experts seem to frown upon.
Does this plan seem like it would work? Anything I should be aware of? It is a 5" duct that comes right off the plenum with very few bends. I would install either a simple on off switch to turn it on when I am in that room, or perhaps a thermostat and relay to convert 110v to 24v.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,284
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    It will work. Not, perhaps, very efficiently, but it will work. The real problem is that with the main fan off the poor thing will have to draw air from wherever it can, and you may find that it pulls air preferentially from the rest of the hot air supply ducts, rather than through the coil.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 7,376
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    It also depends on how the water to the coil is controlled: if it’s shut off like it should be when the thermostat is satisfied, then the coil’s not gonna heat the air when a booster fan starts.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,523
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    here is one attached.

    Not sure if you can get a 5" but 6" is available and you can put a couple of reducers in the duct. I would suggest mounting it near the furnace to keep the noise down. I would also put a tee in the discharge duct from the fan with a damper on the branch of the tee to bleed off some air into the basement. The smallest fan will probably deliver more air than you need
  • Bran_Winston
    Bran_Winston Member Posts: 2
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    Ironman said:

    It also depends on how the water to the coil is controlled: if it’s shut off like it should be when the thermostat is satisfied, then the coil’s not gonna heat the air when a booster fan starts.

    That won't be a problem, the hot water circulates continuously, even when the thermostat isn't calling for heat. The thermostat is setup to just turn on the fan.
    Jamie Hall said:

    It will work. Not, perhaps, very efficiently, but it will work. The real problem is that with the main fan off the poor thing will have to draw air from wherever it can, and you may find that it pulls air preferentially from the rest of the hot air supply ducts, rather than through the coil.

    Thanks. I didn't think about that, but you are probably right. The plenum is probably hot enough that it would still draw heat, but I'd rather not draw heat from the rest of the house at the same time.

    It would be easy enough to install a manifold and just run pex to a convector in that room, but that is currently out of the budget. I might plan to do it that way next year.

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