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    draft (5 Posts)

  • drhvac drhvac @ 8:52 AM
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    draft

    In at least alf of the gas units I service the draft is always higher than it should be, this obviously cuts down on the efficiency. Is there a quick fix to this? If it was oil, you could adjust the baraometric damper to lower the draft, can you do this with gas? The only time I've seen barometric dampers on 80% induced draft furnaces is when they have a side vent kit connected to it.
    How about with 90 plus units? If you follow the manufacturers combustion piping directions, and the draft is to high, how do you lower it?
    excessive draft is ok when it comes to safety because you know the stuff is going out, but it effects the efficiency because those gases are flying through the heat exchanger and all of the heat isn't being extracted.
    I know some will say the flue piping is probably wrong, and they are probably right, but what if the flue piping checks out, then what?
  • Tim McElwain Tim McElwain @ 6:02 PM
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    Not sure what you are measuring for draft.

    Normal draft on an atmospheric gas system boiler or furnace should be a -.01, -.02 to -.03 is standard and it is measured at the breach before the draft hood. I however check draft on both sides of the draft hood because of possible excessive draft after the draft hood called "curtain effect". Curtain effect can be corrected by removing the draft hood or in the case of a built in draft hood blocking it off and replacing it with a double swing barometric.

    As far as 90 plus units those typically have positive draft and and it can be affected by the length of the vent. It should in most cases however be pretty stable as the design of the equipment determines the combustion air blower RPM due to lets say ODR. The temperatures should be somewhat low as these units have very little flue loss so most of the time flue gas temps are at or below 200 degrees F.

    There have been some problems with Category I Fan Assisted boilers and furnaces and it is usually when they are flued to together with a natural draft appliance such as a water heater. Separating the two appliances before they enter the flue or connecting them with a "wye" will sometimes solve the problem.
  • drhvac drhvac @ 7:25 AM
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    im measuring

    readings of .05 and above. sometimes as high a .10. Im talking mostly about your common 80% gas furnaces which don't have a drafhood. Are you saying a barametric damper can be installed on these units to lower the draft?
  • Tim McElwain Tim McElwain @ 10:46 AM
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    If the unit is using a power venter

    and it is on the outside of the building like a Field SWG then they show a barometric being used before the venter.

    If it is fan assisted 80% no draft hood I would use a flue restrictor or better yet contact the equipment manufacturer for their suggested solution. Some furnace manufacturers include a CPVC restrictor in their vent kit.
  • Jim Davis Jim Davis @ 10:30 AM
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    Draft

    The draft pressure of -.01" to -.02" is the draft applied above drafthoods during factory certification testing.  No residential gas appliance has been certified with flue draft higher than that. 
    Draft in the field will normally be much higher assuming a good chimney and functional combustion air.  Yes this can cause a loss of efficiency and can be dangerous on drafthood appliances.
    Gas barometrics have been available for gas appliance since the 1920's.  Drafthoods came later to prevent pilot outages and allow appliances to operate under most dangerous conditions.
    A barometric can be installed on an induced draft 80% furnace to help control combustion and efficiency.  Gas barometrics are set using combustion analyzers, not draft guages. 
    They can be installed above drafthoods with spill switches to add additional safety but do not do much for combustion.  Drafthoods should have been made obsolete as soon as safety pilots were invented and definitely since spark ignition.
    If one checks carbon monoxide poisoning, equipment safety recalls etc., drafthood appliances are responsible for over 90% of them.
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