Humidifier Bypass

Discussion in 'HVAC Heating & Cooling' started by mcnauge, Jan 9, 2010.

  1. mcnauge

    mcnauge New Member

    Messages:
    12
    Location:
    Ontario, Canada
    Hello,

    Im in the process of installing a new humidifier, however, due to the way the furnace was installed the Furnace output duct has the A-coil mounted about 14" above the furnace top and this leaves very little room above the A-coil and the ceiling.

    1) Can I mount the bypass duct below the A-coil (All the manuals say 6" above the coil, but i assume that because usually the coil it right above the furnace.

    2) Can the bypass duct be 5-6" highier or lower than the humdifier? (I have a space issue on the supply and return lines and can't mount them level to eachother).

    Thanks,
    Erik
  2. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE

    1) Do you have a heat pump? If so, bypass has to come off above the A-coil.

    2) Yes. No need for the duct to be level.
  3. mcnauge

    mcnauge New Member

    Messages:
    12
    Location:
    Ontario, Canada
    It is a natural gas furance (about 5 years old). So does that matter is the bypass is above or below the A-coil?
  4. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE
    So you don't have a heat pump?
    (I have a natural gas furnace and a heat pump.)
  5. mcnauge

    mcnauge New Member

    Messages:
    12
    Location:
    Ontario, Canada
    Its an all natural gas furance, the only add-on is the Air conditioning A-coil (its just for ac no heat pump)
    Last edited: Jan 10, 2010
  6. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    it can come off below the coil.
  7. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE

    ...as long as you are sure you DON'T have a heat pump.

    Heat pumps look just like regular AC units.

    If you DO have a heat pump, the takeoff has to come off above the coil (so you are using warm air to run over the water pad in the humidifier.)
  8. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    quote - It is a natural gas furance (about 5 years old). So does that matter is the bypass is above or below the A-coil?

    I doubt he'd have a heat pump and a natural gas furnace at the same time.
  9. mcnauge

    mcnauge New Member

    Messages:
    12
    Location:
    Ontario, Canada
    Perfect, thanks for the quick help.

    Just curious, is the reason it has to be above the coil (if it was a heat pump) is just because of the heat needs to pass throught the humidifier?

    Since mine is a gas furnace the AC coil is just used in the summer and its only important that the bypass duct doesn't get blocked by the coil?

    Thanks again,

    Erik
  10. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE

    Yes, the reason you have to take off above the coil if you have a heat pump is that you always want to send warm air to the humidifier (assuming you have a water pad system - that's the most common). Otherwise, you are just sending room temperature air over the water pad, and it won't pick up much water since room temp. air can't hold as much moisture as warmer air.

    One thing you probably already know - you want to make sure that you have a shut-off damper somewhere in the duct, or in the humidifier itself - so that in summer you aren't sending cold air over the humidifier pad. Doesn't really hurt anything, but doesn't serve any good purpose either.
  11. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE

    Actually quite common where I live (Nebraska). Many of us have existing natural gas furnaces, and then when the AC compressor finally fails, we replace the AC system with a heat pump, and tie into the existing gas furnace.
  12. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    Mc Nauge; it don't matter where you install it as long as it is in the main air stream. This Humidifier could be installed any where on the supply plenum or any where on the return plenum. the main idea of a humidifier is to provide needed moisture in the home. As long as it is installed in the main duct stream whether it is supply or return this moistened air will be sent out thru the main supply ducts.
    if installed on the supply plenum the intake air to it will be from the supplyair.
    if it is installed on the return plenum then the intake to it will also be from the supply air.
    Ps don't forget to hook(wire) up the humidistat for it so the moisture in the home can be controlled.
    Last edited: Jan 10, 2010
  13. SteveW

    SteveW DIY Senior Member

    Messages:
    1,052
    Location:
    Omaha, NE


    He is actually asking a different question - not where he can put the humidifier, but whether he can put his take-off duct below the A-coil.

    As I've said above, it does actually matter if you have a heat pump, but if not, OK to take off below the coil.
  14. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    ****************************************************
    As I said, it does'nt matter where it is as long as it in the MAIN DUCT AIR STREAM.
  15. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    No, Steve is correct. If he has a heat pump the humidifier takeoff should be AFTER the heat source, in that case it would be the A-coil providing heat from the heat pump. Otherwise there has been no heat addition for vaporizing water and the humidifier will be very ineffective. It's a process flowsheeting issue as well as a matter of thermodynamics.

    Same happens frequently with stripping towers, folks neglect to preheat the stripping gas, then can't understand why they can't strip anything with it. I've seen it several times where otherwise competent engineers cobbled something together without running it past process design/simulation.

    I don't have a heat pump, although I've considered doing what SteveW mentioned. With the high relative cost of electric to gas here, a two stage condensing furnace with ECM blower appears a better option.
  16. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    As long as it is installed in the MAIN air stream that is all that matters. Moistened air can be introduced from ANYWHERE within the Main duct stream and still provide humidification to the home.
  17. PEW

    PEW DIY Senior Member

    Messages:
    487
    I have mounted them on the A coil housing, on the side facing the flat area of the coil, and high enough not to hit the coil.
  18. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Wrong. It will not work well. With the air being 20 F or so colder it is going to have a much lower driving force for humidification. (It will be about half as great at 70 F as at 90 F.) Add to that the problem of 20 F colder air/water interface reducing the effective heat/mass transfer coefficient and less energy available for actually vaporizing water.

    From what I've seen of a properly connected humidifier's performance, connecting one improperly as you suggest would be a waste of time/resources. Might as well not even have it.
  19. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    As long as the Humidifier puts moistened air into the MAIN AIR STREAM and delivers this moistened air into the rooms of the home then it is doing what it was designed to do.
  20. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Doing less than 50% of what it is designed to do is unacceptable.
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