New Boiler causing chimney condensation

[Bc02eagle]

Hi - I am new to the forum but I am hoping someone maybe be able to assist. Approximately, four months ago I replaced an old Hydra Therm NG boiler with a new buderus ga144 and an indirect hw heater. The old boiler was running fine but it was well past its prime (about 50 yrs old). For the first three months everything with the new boiler was great but over the last 4 weeks I have been getting a lot of condensation dripping back down the flue. This boiler is in my basement and is vented up through an exterior brick chimney. At the time of the install we put in a new stainless steel liner. The old boiler never had this issue. I am not sure what I can do here.

I do notice that the Beckett aquasmart has an economy feature that will lower the high water temp automatically. This winter has been mild so far in new England so many times the high water temp will only be at 160 or so. Any chance that turning this off and going back to 180 degree high water temp would help?

 

[Bc02eagle]

Sorry meant gc144

 

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[Dana]

Any 85% efficiency gas-burner will experience some flue condensation when it's actually cold out (especially with chimneys on exterior walls) which is why the chimney needs a liner, as well as a means of collecting & disposing of the condensate. If the stainless liner was oversized for ~103,000 BTU/hr input rating it would have higher than normal flue condensation. Unless you have another large gas-burner sharing that same liner or very low stack height the liner should be 5", no larger. The smaller diameter means higher stack velocity (less time in the liner for condensation) and lower surface area (for less liner to warm up.)

If there is space between the steel liner and the old terra-cotta, it's legitimate to fill that space with rock wool insulation, which will help some.

Right sizing the boiler for the actual load helps too, since it delivers longer burn cycles, resulting in a warmer liner. Very few houses in MA actually NEED the 87,000 BTU/hr output of that boiler!

If there is anything obstructing air flow to the draft hood it would result in less-dilute stack gases, with a higher dew point, and more condensation.

Bumping up the high limit to 180F won't have a huge effect on the dew point of the exhaust gases, but it will knock a percent or three off the as-used average efficiency.