Ductbooster installation questions

Discussion in 'Electrical Forum discussion & Blog' started by MauriceS, Dec 11, 2009.

  1. MauriceS

    MauriceS New Member

    Messages:
    4
    Location:
    Denver CO
    Hello,

    I need to install one or 2 duct boosters in my HVAC system, and want to hook them up to the furnace, possibly (if needed) using a 120VAC Relay. The EAC terminals on the Furnace can supply 1A/120VAC, and I need between 1-1.5A for 2 ductboosters. The ductboosters will be further away from the furnace, so I need to run wire to it. I am planning to fuse the wires appropriately - so there will be no issues there...

    1) When using a relay, I plan to use a 5A-10A relay (so the wetting current is sufficient). Can I install an ice-cube style relay in a regular 4x4 outlet box, or do i need to find a different solution? A single ductbooster I can run off the furnace directly, but 2 I cannot obviously... And even 1 I am concerned about... I don't want to fry the furnace controller...

    2) Can I use 14/2 Romex to hook up the fans, I know that's rated for 15A, but the maximum current carried really again is 1.5A? Any code problems hard wiring using this?

    3) If I were to use a relay, do I need to add a separate branch circuit, or can I draw the current from the furnace circuit? It supplies currently the furnace and a humidifier. (Planning to add a 36W UV-C light)

    Anything else I need to be aware off?

    Thanks,

    Maurice
  2. jeffeverde

    jeffeverde New Member

    Messages:
    79
    Location:
    L.A.
    Alternatively, rather than wiring directly to your FAU you could install a pressure or temperature switch somewhere in the ducting, that triggers based on airflow in the duct. Here's an example
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,481
    Location:
    Cave Creek, Arizona
    ducts

    You would be better served wiring them into the circulation blower circuit so they ONLY operate then the main blower is running. Otherwise, they will come on too soon, and shut off too early. In other words, they would not be controlled by the fan switch, but rather by the thermostat and possibly the high limit control.
  4. Billy_Bob

    Billy_Bob In the Trades

    Messages:
    422
    A "contactor" is the same thing - relay. But these are typically designed to be mounted in a large electrical box. You can screw them to the back wall inside the electrical box.

    Then they have screw terminals to connect the wires. These will be better and easier to connect - connections to the relay (contactor).

    What is called a "relay" many times is designed to be installed on a "printed circuit board" like would be found in an electronic thing like a VCR, computer, etc. And these are designed to be soldered.

    The problem with "relays" is replacing them! Not easy to "unsolder". Also they can build up heat, so better to be in a larger box where air could circulate more.

    A "contactor"...
    (Just the gizmo on the right...)

    [​IMG]
  5. drick

    drick In the Trades

    Messages:
    392
    Agree, but if you steal power from the blower circuit the duct boosters would be controlled by the fan switch on the tstat, which seems fine to me.

    You can mount the relay in a 4x4 box. (My HVAC guy has installed a cube relay inside my furnace for my heat pump. No problems with the install, but your not really supposed to do that;)) I'd mount it right on the outside of the furnace. Be careful of what is on the other side when you drill though.

    14/2 is fine. Drawing power from the furnace circuit is fine.

    -rick
  6. MauriceS

    MauriceS New Member

    Messages:
    4
    Location:
    Denver CO
    Thank you...

    Looking at the schematics of the furnace controller the mentioned EAC terminals are energized when the blower fan is running, so the relay would be powered when the furnace blower is running to heat or cool...

    I cannot really use a sail switch, as I want to put the booster in the crawl space, just before it goes upstairs... That's the last piece of exposed duct I have access to...

    Also, I would use either a socketed ice cube relay, or one which has spade connectors. I don't want to use a high current contactor as I am concerned that possibly I wouldn't have enough wetting current through it, so it would die prematurely...

    -Maurice
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,481
    Location:
    Cave Creek, Arizona
    booster

    YOu have already said the EAC will not provide the power for two boosters, an wiring the boosters to the blower circuit will not "steal" power from it unless it is already underpowered. However if the blower is 240 v and the boosters 120v, then it would require rethinking. SUch as a 240 volt contactor switching a 120 v load. Still think it would be simpler than what you are proposing.
  8. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA
    I would investigate further to see what is actually causing your uneven heat problem before I get too far into attempting duct boosting. Just about every single residential system can be properly adjusted IF it is properly sized. Kinked & crushed duct work is another common problem.
  9. MauriceS

    MauriceS New Member

    Messages:
    4
    Location:
    Denver CO
    This is the duct the furthest from the furnace. What the builder did :-( Is to have a long supply run in the crawl space, then it goes up into the wall between the garage and master bedroom, and I suspect that there are 2 more 90degree angles before it hits the room itself. I haven't pulled the drywall off in the garage (yet), so I don't know whether it is a 6" or a 8" part of the way. I'll inspect it further in the crawlspace, and see whether I can increase the size of the run up to effectively 8"...

    In the room itself i cannot feel any air coming up through that vent at all...
    There is another vent in the room which has barely any airflow either. The problem in that room is also that there is NO return duct, which we are trying to correct by putting a vent in in the room to the next room which does have the return air duct...

    -Maurice
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    Your ducts might also be leaking like a sieve. Hard to blow air to the end when it all leaks out along the way.
  11. MauriceS

    MauriceS New Member

    Messages:
    4
    Location:
    Denver CO
    True...

    I'll get busy with some duct sealant in the crawlspace ;-) Don't want to open any walls in cold Colorado weather...

    Maurice
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    The stuff to use is the foil tape. You need to clean the dust off first, though. You have to remove the protective paper from the adhesive side. Don't use the fabric 'duct' tape. There is some caulk you can use, but it is messy.
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