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Condensation in chimney w/oil boiler

skippy's picture


I have a condensation problem with Buderus G115 oil fired boiler.  In the cold weather (below 20 F) moisture builds up when fumes hit the stone chimney cap and a good amount of water ends up collecting at bottom of flue with some seepage issues. 


Using Riello burner 40F5 with .85 60B nozzle.  New installation, venting into 8” masonry flue, with total chimney run of about 20-25’.


When unit was installed gross stack temp was 350 with net 300.  I called the installer when problem first started and he removed the two flue baffles thinking this would increase stack temp enough so fumes would not cool down to dew point before exiting.  Worked a little bit but still had problem on cold days.


I really don’t want to increase the stack temp and send the heat up and away.  Ideas?  Thought about stainless liner to make the flue smaller or using some sort of power vent system to get gases up and out to atmosphere quicker?


I'm just the homeowner trying to fix a problem that my installer has nothing to suggest-"my problem now".  Thanks for any suggestions.


(post #114119, reply #1 of 8)

I bet that your new boiler is now too small/efficient to vent into the existing masonry chimney.

The installer, if he was a pro, should have consulted the venting tables to make sure it was all good. Obviously, that didn't happen.

Removing those baffles probably wasn't a good idea, either.

Proper venting procedures and materials will be in the installation manual.

I'm gonna guess that you chose price over quality- especially with the "my problem now" reference.


(post #114119, reply #4 of 8)

No, he was actually one of the most expensive heating subs and have read very good things about Buderus.

(post #114119, reply #8 of 8)

That's unfortunate because it seems like the job is not done right. If they really are a reputable company, then the problems should be fixed, not passed onto the customer.

The piece of equipment is only as good as the installation- the name on the cabinet is secondary.

(post #114119, reply #2 of 8)

Two things that I know that cause condensation (there are probably more) is either the the chimney is too cold and either needs to be smalller or insulated or number 2: the return water temp to the boiler is too cold.




(post #114119, reply #3 of 8)

I am not a chimney sweeper, but I would try a 6" diameter steel pipe within your mansonry.  Let me know if it works, because I get condensation on my cap, too,  although the tiles look very clean.

(post #114119, reply #5 of 8)

Not sure how the water temp returning to the boiler impacts the stack issue?   

(post #114119, reply #6 of 8)

"Manifold piping and boiler primary loops normally require a minimum of bypass piping for boiler protection. The use of electronic or mechanical protection will give us the best protection with changing system conditions. Bypass piping has worked for many years to control the average boiler water temperature or flow rates. Flue gas condensation is a result of return water that is too cold, or cool return water at a high flow rate."

Here's the article....


(post #114119, reply #7 of 8)

It makes a huge difference. If the return temp is too cool/cold there has to be a bypass (small) from the hotside to mix in with the returning cool/cold water to raise the temp. Boilers are designed to work on a number of factors and one of them is the return temp must NOT be below a certain temp otherwise you get poor combustion, condensation and the pipes rot.

Sure...................after I post this I read Hootowls message which says the same.



Edited 4/23/2007 11:03 pm ET by roger g