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Chimney for forced hot air oil

Sidponcho Member Posts: 7
Hi All,

I'm installing a Rheem oil forced hot air furnace in my shop.  Going with an all-fuel chimney straight up through the roof (single story building).  The unit is a Rheem ROBD-067 upflow.  The manual calls for a "5 flue".  I can get 5" all-fuel chmney parts locally.  Should I do the 5" or upsize to a 6"?



  • Jack
    Jack Member Posts: 1,047
    So many times

    I have been told by a consumer that, "I wanted to make sure I had plenty of draft so Instead of the 6", I bought the 8". It is a common mistake, but a mistake none the less. Use the size specified. You are not helping yourself upsizing. Basically, you have a bunch of nicely packed heat molecules in the 5". If you look at the cross sectional area of 5"vs 6" it is 20 vs 28 sq in. When those heat molecules expand they cool, and you loose draft.
  • icesailor
    icesailor Member Posts: 7,265
    Venting Tables:

    Then, there's all those pesky vent tables you can get confused over.

    Like with Weil-McLain oil boilers. They show 8" X8" chimney and you can use a 5" exhaust from the boiler to the chimney but as you go up in size, they require a larger boiler to chimney size. And like all WGO oil boilers, the maximum breeching collar is 7" venting into a 8"x12" chimney. They have tables that get into the overall length of the chimney/vent. The reason for the 7" maximum vent breaching is because that is the maximum that can be vented out of a boiler with that input/output. The same as that the smallest WGO-3 has a 1 1/2" NPT inlet and outlet. The same as the largest boiler in that series. If you need more flow, you need a bigger series that has a 2" NPT inlet and outlet.

    The wrong question was asked. Not whether a 5" exhaust vent is OK but is a 5" exhaust vent from the breeching of the appliance and up vertically for "X" distance OK. It might be for 4" developed length into the vertical flue, but not the overall vent length. You have to look at the vent tables.