Barometric dampers and condensation

[laynes69]

I've been back and forth trying to find some data on barometric dampers and wood units. I know there are many users on here that use them on gasification boilers and forced air wood furnaces. Its been said many times when flue temps drop below 300* there's a chance for condensation in a chimney.

Last year I observed exhaust temps at the top of the chimney around 100*f. I was worried about condensation so I manually regulated air to keep up temps in the chimney. This year I decided to go back to a baro, and found some interesting articles.
We keep our furnace at .05"-.06" according to the manual.

The article stated that a unit with a higher efficiency requires a proper diameter liner. What caught my eye was a baro introducing a dilution of dry room air into the flue lowered the risk for condensation regardless of the flue temps. This was in oil furnaces, but one ran 300*C without a barometric damper had condensation while the other running a baro with temps of 160*C didn't have condensation. It's also stated the lower the co2 the greater chance for condensation.

Does anyone have any thoughts on this? We had condensation before lining the chimney, but after lining and seeing this I feel better about the use of a baro. Today is very windy and thinking about our baro made me look for some articles. I don't know the exact science behind this but if anyone knows please tell.

 

[Singed Eyebrows]

I don't run a baro on my Atmos, I did on my Energy Mate boiler & didn't like it. The manual plate damper holds very steady vacuume as verified with a Magnehelic gauge that is permanently hooked up. I don't run high flue gas temps so I wouldn't want the temps dropped at all, Randy

 

[laynes69]

You talking about a key damper? If so I have tried that, but with the furnace running off a thermostat I had poor results due to draft fluctuations. I thought it was interesting that room air dilution could decrease chances for condensation. Our chimney is 32' tall and we have a 5.5" rigid liner. We hit .10 with the first fire of the season in very mild temps.

 

[Singed Eyebrows]

You talking about a key damper? If so I have tried that, but with the furnace running off a thermostat I had poor results due to draft fluctuations. I thought it was interesting that room air dilution could decrease chances for condensation. Our chimney is 32' tall and we have a 5.5" rigid liner. We hit .10 with the first fire of the season in very mild temps.

Key,plate...same thing. The draft spec for my boiler is .1 & I may need to adjust once during a 7 hour burn. If a gust of wind blows across the chimney it will increase for a few seconds, no big deal, Randy

 

[benjamin]

If you're asking why room air dilution would reduce condensation, that is easy. Imagine you're burning pure hydrogen in pure oxygen yielding pure steam, you'll get condensation on any surface cooler than 100C/212F.

Imagine you dilute your exhaust with room air, the condensation point will approach the dew point of the air as the exhaust gets more diluted with more room air. The dew point of indoor air in the winter time is probably pretty close to the inside surface temperature of your windows, so 20-50F which is much cooler than any exhaust, so no condensation.

 

[laynes69]

So as long as the temperature remains above the dew point, there's no condensastion. The whole reason why I dodged a baro was simply condensation. I keep ours set on the higher end of our manual to keep it from opening too much. Also it's been said the velocity increases from the rush of room air into the flue which helps remove the gasses quicker.