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New oil boiler venting

DJD775 Member Posts: 211
@Bob Harper
The ‘Chimney and Flue’ discussion has me thinking about an upcoming install that I plan on doing early this summer. I’ll be replacing an oversized, 210 MBH input, pin style oil boiler with an 84 MBH input 3-pass oil boiler. Since I have a ~35 ft tall exterior masonry chimney with an 8 x 8 clay liner I was originally planning on lining the chimney with an appropriately sized SS chimney liner prior to the install. Now that I’ve read through the ‘Chimney and Flue’ thread, I’m thinking the additional lining might not be needed. Based the on the outcome of a chimney inspection I have a few questions:

1. If the inspection reveals that the clay liner is in good shape, I can potentially use the current chimney as is. In the previous discussion Jim alluded to the importance of proper venting of the equipment to mitigate flue gas condensation. What is considered proper venting for an oil boiler beyond what’s in the instruction manual and codes?

2. If the inspection reveals that the current clay liner is compromised, then I will have re-line with a SS liner. The new boiler has a 5” flue pipe, what is the appropriate size for the new liner? I’m thinking between 5.5” and 6” based on information in previous discussions.


  • captainco
    captainco Member Posts: 755
    If you had an oil water heater in the flue with the boiler, no one would say you need a liner, even if the water heater had a smaller flue. They operate independently and do not rely on each other for venting. Firing the equipment with proper temperatures is all that is necessary. Your boiler should operate with a flue temperature minimum of the outlet water temperature plus 245 degrees, So if the boiler had 120 degree outlet water temperature the minimum flue temperature should be 365 degrees which is plenty to vent without condensation. The biggest issue is to make sure there is a good rain cap. Wet flue cause serious venting problems.
    Having the technician check the flue draft above the barometric after the burner shuts down will verify there are no negative pressure problems.
  • DJD775
    DJD775 Member Posts: 211
    I never even thought about the impact of the rain cap. I currently do not have one on the chimney so I will definitely add one.
  • SuperTech
    SuperTech Member Posts: 1,919
    My company normally recommends installing a stainless steel liner when upgrading to a three pass or Energy Kinetics boiler, but the advice the captain gave you is everything you need to know. See what the boiler manufacturer recommends and check combustion after it's installed. Pay attention to the stack temperature during the burner cycles. Hopefully the smaller boiler will run long enough cycles to keep the stack temperature up. That oversized boiler might have damaged the terracotta with short cycles and condensation. 
  • DJD775
    DJD775 Member Posts: 211
    I'm hoping the significant decrease in boiler size will improve the current short cycling. Even on the absolute coldest day of the year the current boiler easily heats the house and short cycles. I'll have to see what the outcome of the chimney inspection is before I go with the current chimney as is.