Manipulating Stack Temps

[Eric Johnson]

This question pertains to the EKO, which is what I have to work with, but I suspect the approach is similar with an Econoburn or Tarm or Wood Gun.

I'm about to the point where I want to start playing around with my stack temps, and I'd like some advice on where to start. Can someone come up with a systematic approach to adjusting the air intake on the blower(s) and the secondary air controls?

My temps generally settle in the 300-400 degree range, but I've seen them over 600 at times.

 

[slowzuki]

When you say playing around what do you mean? - Edit oops missed a line!

Ok if you want to start playing with your nozzle settings you are gonna need some thermocouple before the stack. The stack is affected by the hx too much. You'll need, I think its the yellow type tc for somewhere in the output chamber of the nozzle and one for the primary air area.

The problem is without a feedback adjustment it will be different for each load of wood and even during the burn, you may drive yourself crazy!

 

[Nofossil]

It may come as a surprise to you, but I've been thinking about this as well. The European tests on the EKO that yielded 90% efficiency had a really low stack temp. Part of that is probably because they ran it with a very cool outlet temp (34gpm pump and 130 degree inlet water will do that). Part of it is that they ran a virtually perfect air to wood ratio. They were burning 42 lbs/hr of birch chips at 15% moisture - a LOT of surface area compared to normal splits.

The cool water would certainly help drop the flue temp, but I'm struggling with the effect of too much air. Too much secondary air would have the effect of increasing your heat loss, but I don't think it would increase your stack temps.

My suspicion is that the fans blow way more air than you need for normal fuel. I've started experimenting with dialing back the fan speeds and I'm seeing modest and perhaps not significant decreases in flue temps and increases in efficiency. I'd start in that direction. More ponderings when I have more time.

 

[slowzuki]

Every thing I've read from the manufacturers side is the loss for say 50% excess air in the secondary is acceptable so you never are in a fuel rich situation from finely split material. It will lower stack temps a bit "artificially" due to heating the extra air via dilution.

My Jetstream with a proper fuel config operates at about 100% excess air so if someone loads it full of kindling it can still maintain some kind of burn without clogging the hx.

 

[solarguy]

If you lower your stack temperature too much it will cause condensation
in the flue & chimney & that's not a good thing.

I'm not sure if this would even work because wood doesn't fire at a consistent rate but it'd be a cheap experiment. Try installing a barometric draft damper on the flue. This would pull ambient air from the boiler room just like a oil burner & as long as you can maintain your draft on the boiler side of the damper you should be able to lower your stack temp.

 

[slowzuki]

Yup but you wouldn't want to do it via a barometric damper, you are not doing anything to the actual combustion process or heat exchange process.

 

[sled_mack]

Eric,

I think I just learned that taking more heat away from the boiler also reduces the stack temp. And I can watch the stack temp rise with my storage tank temp. If you don't have enough pump taking away heat fast enough, your stack temp will rise and your boiler will idle.

Ironically, I understand the theory of more air diluting the gasses going up the stack and cooling it. But when I open the fan up, the fire gets hotter and the stack temp goes up. Maybe I'm just not at a saturation point with the air yet?

 

[Reggie Dunlap]

I've though about wiring my draft fan to a reostat. When the boiler is up to temp you could dial it back a little. The only problem I see is that you need to be there to do it.