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Flue temperatures too low?

Mrubens Member Posts: 3
I have a natural gas boiler and have recently had issues with condensation in my chimney. Part of the issue is the chimney is unlined (I will be getting a liner).

However, I had a service person out and they measured a flue temp (at the exhaust port) of 280f and said this could be contributing to the condensation. They like to see 310-320F. 

My question is, is it possible to increase the exhaust/flue temperature on a boiler? 


  • Steamhead
    Steamhead Member Posts: 16,840
    I bet the gas input is too low. But don't try to fix this yourself! You need a pro with proper test equipment.

    Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Mrubens
    Mrubens Member Posts: 3
    Thanks. I'm in New York, capital region

    Wouldn't try to fix it myself :) just looking to see if this is an issue that an HVAC tech can fix by fiddling with the boiler or if it's a sign that it's on its way out. 
    EBEBRATT-Ed Member Posts: 15,526
    What @Steamhead said.

    What is your return water temp?

    Is this a converted gravity system?
  • Mrubens
    Mrubens Member Posts: 3
    What @Steamhead said. What is your return water temp? Is this a converted gravity system?
    Don't know off hand what the return water temp is but can check. Is there a threshold or band that I should expect it to be in?

    It is a gravity system I think. There is no recirculation pump. (old boiler, services 2nd floor only)
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,838
    edited March 2022
    The condensation is a result of the low stack temperature. a Liner may help increase the temperature because it will heat up faster and draft better. More draft will pull flue gas thru the boiler a bit faster. Gravity heating lends itself to lower return temperature for longer time period.

    Did this boiler heat the first and second floor in the past?
    I ask this because I remember the first time I split a boiler that did a 4 unit apartment building into 4 separate boilers. I used the existing oil boiler to heat the hardest zone to repipe. (which was not the largest zone BTW). Then disconnected and capped off all the other radiator connections.

    Then I purchased 3 small gas boilers and 3 gas water heaters and connected the individual zones to separate boilers on different gas meters. This left the old oil boiler way oversized for the apartment zone. (after 3 years the landlord purchased the final small gas boiler because he couldn't keep a tenant in the Oil Heat apartment.). Either pay now or pay later

    If your current gravity boiler did the entire building in the past, then it is way oversized. Is it possible that someone derated the boiler to a lower input in order to lower operating cost? Not such a good idea! If so, than a smaller boiler is in order.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics