cresote question.

[Just Me]

I have a wood furnace in the basement tied to a gas furnace, the kind they sell at farm stores. Works well, probably a bit big but I bought it new/used for $600 so its what I have. I run it into a masonry chimney with a 8x8 flue. No problems there, but the metal pipe from the wood furnace to the masonry chimney gets build up. It comes out of the back of the unit, ells up at about a 45 degree angle to another ell and a 6" pipe into the masonry. This pipe gets plugged up, not the masonry.

Suggestions> Insulate the pipe? with what? I wish I had the two switched around to eliminate the 45 degree part but changeing all the ductwork would be a pain I don't want to go through.

Anyway, love to hear your thoughts. I think the unit is too big as I said so I can't run it too hot although I do every morning while I am drinking coffee.

Thanks Larry

[gspren]

   Get drier wood.

[sawguy21]

If you get creosote buildup you are not running hot enough. Either get drier wood or let 'er roar once in a while to burn it off.

[beenthere]

Have a similar design here.
As mentioned, dry wood... meaning at least two years drying (3 for oak).

Then once a week, let the fire roar through that pipe and tap it with a stick to shake the creosote loose and get it to burn. When you do this, watch and listen for a chimney fire and shut the damper down to choke it off. This keeps the short pipe cleaned out so it will not support a chimney fire when you are not there to control it.
If not burned out, then once a week take it apart and brush out the creosote buildup.
If only small fires are needed, then don't fill for long burn times which end up dampered down and smoldering while waiting for a call for heat. Control the heat output with amount of wood in the charge.

It is, to me, all part of burning wood.

[Kbeitz]

creosote is created when you have a temperature change.
Keep you pipe hot and you wont get creosote.

[John Mc]

One of the downsides of having a wood furnace that is too big is that wood combustion devices don;t have a very good "turn down ratio", especially as compared to gas or fuel oil.  A gas or fuel oil furnace will generally run fine at as low as 10% of full fire. The efficiency will not be as good, and it will pollute a bit more, but otherwise it runs OK.  A wood boiler with very good controls might go as low as 50% of full fire without a problem. below that (or often even above that on a furnace without a well designed combustion and control system), wood combustion systems really suffer. Trying to run one at low fire is almost guaranteed to produce creosote.

If you can, when sizing a wood boiler (at least in my climate) for best utilization and efficiency, you typically want to size it for maybe 70% of the required peak load, and supplement with some other form of heat on the coldest days. If your supplement is gas or oil, you generally end up burning LESS oil doing it this way, rather than sizing for peak load and running your oil furnace in the shoulder seasons where that big wood burner has trouble turning down to match the load. The alternative is to try to turn your wood boiler way down, which results in a cold, smoldery-type fire that produces a lot of creosote (and a lot of pollution). Another thing some people are doing to get around this is to add a lot of hot water storage - a large tank (often buried) that lets you run your boiler flat-out to heat that big storage tank way up, then just shut your boiler down completely (often for a few days, perhaps increasing to every day in a real cold snap) until you need to charge that storage back up.

[Timberwolf0530]

Causes of Creosote:
     - Wood not seasoned enough
     - Not enough air to the combustion chamber, aka burning your fire too slow
     - Oversized flue
     - Stove design. The older smoke dragon stoves are more likely to cause creosote.

     Creosote is formed by moisture, carbon, and unburned wood particles.  This will condense on the coolest surface it contacts.  Keeping your fire hot keeps your flue hot and reduces it.  With a proper burn, you should just have fly ash build up, which is harmless and easily cleaned up.

     Here is what doesn't make sense.  You are getting creosote on the 6" pipe, but not in the 8X8 flue.  Ideally, you should have the pipe going all the way up the flue, and have it insulated.  This helps to keep it hot, and reduce the build up.  A 6" pipe is the most common for modern stoves, and has an area of about 28 sq in.  Your 8X8 flue is 64.  The 6" pipe should be much hotter that the masonry flue because it is smaller and closer to the fire, so any creosote should be forming in the 8X8 part first.  It may be on both.  If it cools very quickly, it can cause a glazing, so you may have the chunky stuff in the pipe which is easier to see, and the glazing in the flue.  I would have a sweep come look at your set up.  You may be on the road to a big chimney fire.

[Just Me]

 Thanks for the replys gentlemen.

I have burned wood pretty much all my life so I am not new at it just for reference. Just first time I have had this particular problem.

I have a couple of things I am going to try. One is tying the return air to the furnace return air, and insulating the 6" pipe so creosote will not be so likely to condense. The masonry chimney is clear, which is what led me to think I need to quit pulling air across the metal pipe on its way to the fan.

I knew the furnace was too big when I bought it but it will eventually end up servicing the woodshop and a smaller unit will be in the house. Still, it did a fine job last winter, seven face cord to keep the house warm with about $35 dollar gas bills for backup. It was a deal that fit into my budget at the time as I am building the house for cash as I go.

Thanks, Larry

[Ianab]

Masonry is not a good conductor of heat, while steel is pretty good.

So the inside surface of the metal flue is actually cooler than the larger brick flue, once things get up to temp.

That means insulating the metal flue might be useful.

[4x4American]

I guess everyone missed it...to fix your problem, you gotta build an addition so you can give it to her!!

[Just Me]

More background......

When I built my current house I used every trick I have learned in 30 years as a builder to make it energy efficient other than I have too many windows. Walls are 2x6, wet cellulose with 1/2" foam on both sides. Ceiling is a double ceiling that I developed over the years, first layer of 1/2" foam sealed, 3/4" spacing for a thermal disconnect, and then 5/8" drywall, 2' of cellulose above with a small amount of binder to keep the loft, and all ventilation airways constructed out of 1" Thermax, sealed. No wires were allowed to go through on eve sides to compromise the insulation in that critical area. All windows and doors are sealed with foam.  No icicles on my house!

That is part of the reason I am having trouble with my wood furnace, house does not call for enough heat. I heated it at 65 degrees when I was working on it and my worst bill on NG was $115. Not sure why I am messing with wood other than I like to do it and for a sense of self reliance.

Larry