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drhvac Member Posts: 190
If there were no blockages in a flue, and the pipe was sized correctly, what would would be the reason for a poor draft? The pipe goes up about 25 - 30 ft, so it has enough lift. I did a service on a 25 yr old furnace yesterday, old heil 80% furnace. took the thing apart from top to bottom, including the plate behind inducer so that I could clean the heat exchanger and check from the top down. When I put it back together I did a combustion test and it was fine, around 7% with the o2 and co2, and the co was 10ppm. the draft however was a little on the positve side. Any idea on what else to look for? This was a 125,000 btu furnace, which they had 3" flue ran about 6' to the y. I even increased the size of that to 5" right off of the furnace, and that didnt help that much. any help would be appreciated.


  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    What was the location

    you checked the draft?

    Is this single wall pipe?

    Is this into a chimney or a "B" vent?

    With those readings you are at about 50% excess air which is high. What was the net stack temperature? Does this unit get its air for combustion from within the room (atmospheric)? If it does then this would be classified a Category I Fan Assisted Furnace. Have you checked NFPA venting tables for sizing for a fan assisted appliance?
  • drhvac
    drhvac Member Posts: 190

    The exact readings were o2 7.5% co2 7.51% excesss air 49.7% co 3 stack temp 387. No I didn't check the venting tables, but the unit is in the basement so obviously thats where it is getting its air from. The vent was a 6" b vent going up about 30'. It has the furnace and a 40 gallon water htr. going into it. The flue pipes from the water htr. and furnace are single wall smoke. 5" from the furnace and 3" from the water htr. thx
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    Try changing the vent connectors

    both furnace and water heater to "B" vent, the fact that you have poor draft says that there may not be enough combustion air. What is the size of the combustion zone and is there dedicated air into the zone?

    Were did you place the combustion analyzer probe to get your readings?

    What do you have for connector rise?

    I checked NFPA 54 and the vent sizes are okay. The thing that helps is to have some rise of the appliances before going into the common vent and double wall is better than single wall. It matters also were the readings were taken from.
  • drhvac
    drhvac Member Posts: 190
    The only

    two appliances are the furnace and water htr., and they are in a big basement about 2000 sq. ft. so I don't think lack of combustion air is a problem. Wouldn't the excess air reading be low if that was the case? Connector rise is about 4' from the out let of the furnace to where it connects to the smoke y. The furnace has a 3 to 5 increaser right off of the outlet of the flue, then about a 3'ft piece of smoke pipe before it elbows to the y. I put my probe about 6" above the 3x5 increaser. here's a stupid question. IF you have a backdraft on an atmospheric burner with a vent hood, the back draft would come out the hood. since there is no hood on these type furnaces, where does the back draft go? it was a very slight positive pressure like .004.
  • icesailor
    icesailor Member Posts: 7,265
    In spite of the fact:

    In spite of the fact that this is a huge cellar, try opening a window to the outdoors and see if the draft improves. That large cellar may be tighter than you realize.

    Remember, draft is the inside pressure trying to equalize with the outside pressure through the flue.
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578
    draft reading?

    How high up in the flue did you take your draft reading?  Could you be too close to the inducer? 
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