ducted combustion air for indoor gasifier

[powerspec]

Someone out there must have some numbers on this subject. When an indoor gasifier burns it draws cold combustion air into the room where it is located. None of the boilers I have looked at closely (Econoburn, Tarm, Eko) are made to accept ducted outside combustion air. Why not? There is an efficiency gain to be realized by bringing O2 rich outside air to the boiler without first drawing it into the building the boiler is intended to heat. My experience with high efficiency fireplace inserts tells me that an indoor gasifier without ducted combustion air has a serious compromise in efficency from the same boiler located in an outside shed.
Sure would like to hear others opinions/experience re this.
powerspec

 

[twitch]

My Tarm is in a boiler room in my garage, and I used a 4" dryer vent to bring in combustion air. It isn't directly connected to the boiler, but it does get the fresh air into the room. I might be wrong, but I don't think you would want really cold air ducted directly to the boiler. The boilers do heat the air before it enters the combustion and secondary chambers, but I think colder intake air could have an affect on gassification??

 

[pybyr]

I asked Econoburn about this a while back, and they recommended against feeding cold outdoor air direct to the boiler, and said that room temperature air was more likely to maintain an optimum burn. As the last poster mentioned, there's no reason that you couldn't feed make-up air to the area near the boiler.

 

[DaveBP]

Sealed combustion air to a solid-fuel boiler would cause insurance underwriters to faint at their desks and liability lawyers to shout with joy. If that duct was ever obstucted it could be a mess and a disaster. And on the Tarm the flow of cool air through the grills around the top and into the blower helps keep the control panel and instruments cool.

I wondered the same thing for a while but this is my take on it.

I'm going to bring an insulated duct in near the flue end of mine to bring in combustion air and to feed make-up air to the barometric damper without having it cool off the rest of the basement/workshop. That's the best compromise I've come up with yet. Any others solutions?

 

[Eric Johnson]

It's always been my understanding that the warmer your combustion air, the more efficient your burn. Therefore, I would think that preheating the combustion air in some fashion (say by running it past the exhaust pipe) would result in greater combustion efficiency. Same deal with the wood.

 

[Piker]

I believe instructions for barometric dampers state that they have to be in the same air space as the appliance being vented. If you seal the duct to the damper that would not be good. If you just bring it in "near" it, that should be acceptable.

Without having any hard data to compare indoor air infiltration heat losses to outdoor installation radiant heat losses, it's hard to say which is best. My inlcination is to think that the indoor is more efficient... if you have a well insulated shed... then that might be as good.

warm air is definitely better for combustion since. Since the secondary air is heated through the refractory... it only stands to reason it would heat up faster and hotter if the air going in is warmer.

cheers

 

[Medman]

I am bringing a 4 inch duct in, running it under the bottom of the EKO. this way, the air can warm up before it is pulled into the boiler. I intend to put a U-bend in the duct, similar to the fresh-air ducts on a furnace so that the cold outside air cannot simply spill into the room.

 

[DaveBP]

I wouldn't connect any outdoor air duct to anything. Just open it into the room at the ceiling near the exhaust flue end of the boiler and the barometric damper, so that cold air can go more directly to the exhausts of those two. The outside air is going to cool the air in the room. One way or the other. I just don't want it cooling ME on the way up the chimney. It's already warmer in that corner anyway.

The difference between 50F or 65F is more important to me than the difference between 50F or 65F and the 2100F in the boiler.

 

[twitch]

Medman,
That's a good way to do it. My setup is a 4" vent through the exterior wall about 2' above the floor. From there I ran a 4" flexible duct up to about the ceiling. Because warm air rises, the warm inside air would not tend to go down and out the vent and that the cool outside air wouldn't just blow in.