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draft in 80% furnace

drhvac Member Posts: 190
I know on 80% furnace the draft in the flue is negative. But how is that? The combustible gases are pulled through the heat exchanger by the inducer motor, once they go through the inducer and out the flue, you would think the other side of the inducer would create a positve pressure? What am I missing?


  • icesailor
    icesailor Member Posts: 7,265
    80% Furnace Draft:

    If the 80% power vented furnace is vented into a chimney, the draft in the chimney may be greater than what the draft fan produces to scavenge the gas from the flueways. It will be negative on the inlet side of the fan where it is providing negative draft through the flueways. Once it leaves the fan, it doesn't matter what the draft is as long as a positive draft doesn't overcome the fan.

    If this case is true, an analog draft instrument like a big Bacharach in the flue pipe AFTER the draft fan will show a change in pressure when the burner/fan starts and stops. Sometimes, you need both instruments, digital and analog. Digital will react slowly but with more accuracy. Analog is just as accurate but will react instantly. A digital Amp Clamp may not show you a spike on a motor start up. But an analog one will show it instantly.  

  • drhvac
    drhvac Member Posts: 190

    I still don't get it. I'm thinking like how a circulator pump works. In order to create flow, it creates a pressure difference. the water flows from the lower pressure on the intake, to the higher pressure on the discharge. This how I am thinking a inducer fan would work too. Lower pressure on the intake side ( heat exchanger ) higher pressure on the disccharge side ( flue or chimney )
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Relative pressures

    The inducer fan does create a pressure difference, but if the outlet is already at negative pressure, the inlet will be even more negative. The inducer just subtracts from the already negative pressure found at the chimney flue.
  • JStar
    JStar Member Posts: 2,752

    An induced-draft 80% appliance is still natural draft. The inducer is needed to overcome the heat exchanger restriction. Inducer motors weren't needed on older furnaces because the HX's were larger, less restrictive, and exhausted hotter. If you compare an induced-draft and a natural-draft appliance, they both act the same way after the exhaust leaves the appliance.  They just arrive at that point differently. The exhaust will be hot enough to draft on its own. That's why 80% flues are larger than 90% flues. A truly natural draft appliance (draft hood) requires a larger flue to ensure less restriction on the HX. When it's induced, the restriction is already overcome by the motor. 90% appliances need to create a positive draft because the exhaust is too cool to draft on its own.
  • drhvac
    drhvac Member Posts: 190
    starting to get it

    Ex. When I pump down an air conditioning system, sometimes you pump it down below atmospheric pressure. When you do that and take your hose off, air gets sucked in to the line because the pressure inside the line is less than outside ( negative pressure ) If I take an elbow off of a 80% furnace flue briefly, the air coming out is positive to me. Its pushing out not pulling air in. So that is what confuses me. When air is pushing out instead of pulling in, to me that is positve pressure.
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604

    If you disconnect the flue pipe at the inducer outlet you will get a positive pressure there because of the fan, Once you connect it to the chimney and draft is established the negative pressure of the draft will overcome the positive fan pressure and the net pressure will be negative.

    In other words, the suction of the chimney is greater than the slight positive pressure of the of the fan and during operation there still should be a slight negative pressure at the outlet.

    You will not be able to see this when disconnected. Only if you drill a hole in the flue pipe and insert a draft gauge sampling tube will you see the net negative pressure while the fan is operating.
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    You are talking about

    an 80 % Fan Assisted furnace I assume. The purpose of the inducer is to simply overcome the internal resistance of the furnace. These furnaces are still atmospheric furnaces so they are not a sealed combustion chamber. So picture it this way the fan is simply sucking the products of combustion out of the heat exchanger and then using the Delta T and height of the vent as the system to develop negative pressure. The furnace would be classified as Category I for venting so it has to be non pressurized, negative pressure with a flue gas temp greater than 275 degrees.
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