Smoke question

[mtaccone]

I was noticing that almost every house I see that has a wood fire has alot of smoke coming from the chimney. It seems to me when I was a kid my grandparents had a hot air wood addon furnace that was their primary heating the first was a johnson and the last was an oneida royal. The only time that alot of smoke could be seen was when wood was added and then it turned to black smoke then to almost none. Has something changed? Although Gramp cleaned the chimney monthly there wasnt much creosote either. Their wood burning days are gone now so no observations can really be made only the sad fact that this year oil cost $500-$800 every 5 weeks in their home.

 

[Nofossil]

I dunno - here's a theory:

Back in your grandparent's day, folks were perhaps more content with indoor temperature swings and frequent wood tending. In general, a wood boiler or furnace running wide open will burn much more cleanly than one that's damped down. The price that you pay is that the output of the unit may not match the heat load of the house at any point in time.

The trend since then has been towards thermostatically controlled units that spend a lot of time smoldering. You load them up, they burn a long time, and maintain a more constant output better matched to the heat demand.

 

[Eric Johnson]

I'm guessing the big difference is that the old woodburners (stoves, furnaces, boilers) were not airtight, which means that they burned inefficiently, but relatively clean compared to the more modern ('70s) airtight stoves, etc. My fireplace, for example, goes through wood like you wouldn't believe, but once it's up to temp, no smoke. You can control the burn rate (and thus relative efficiency) pretty well by choking off the air supply, but the result, unfortunately, is smoke and creosote. Modern stoves and wood gasification boilers get around both problems by restricting the air supply, but cleverly burning off the resulting smoke and converting it into heat. So we're basically at Stage 3 in the evolution of residential woodburning.

 

[WRVERMONT]

Just an added note. The clean burn of your grandparents furnace was probably accompanied by relatively high exhaust temps. You see this with many wood burning maple sugaring rigs up here in VT. After loading wood, the chimney smoke cleans up to clear "heat waves" relatively quickly. Exhaust temps are often higher than needing to extract the exhaust gases. Overall efficiency is sacrificed for maximum heat under the pans. Matching heat exchange surfaces to available heat is part of the key.