The Pros and Cons of Direct Air for a Wood Stove (when there’s no mandatory requirement)

[openspaceman]

4 hours ago, Muddy42 said:

Sorry I'm a bit following the last two posts here?.

Can you elucidate, I don't understand the question?

 

Also how do you define efficient? Is it how close to complete combustion of the fuel different heaters get or how much of the heat ends up in the living space?

[Muddy42]

13 hours ago, openspaceman said:

Can you elucidate, I don't understand the question?

 

Also how do you define efficient? Is it how close to complete combustion of the fuel different heaters get or how much of the heat ends up in the living space?

 

sorry my typo.

 

What i was trying to say was I got a bit lost with those two posts.  I dont really buy the concept of an air supply cooling the (post burn) exhaust and flue, condensation and needing fans to improve the draft. What i was trying to say was that biomass boilers, which are the peak of design and efficiency (by both of your measures) and they have an extreme external air supply and dont have these issues.

 

hope that makes sense.

 

 

[Rob_the_Sparky]

Heating the intake air from the flue is going to improve efficiency (which is why balanced flue gas boilers do this)  However, it also cools the exhaust gasses that contain a large amount of water along with a cocktail of other compounds.  Balanced flue gas boilers have to drain the water that condenses but burning gas does not result in the same flue chemicals.  With a complete burn that may not be a problem but condensing water running back down into the fire would be bad.

 

All this has nothing to do with external air as such, it is about running the external air down the flue pipe...

[Rob_the_Sparky]

It is all in theory as well, not sure anyone has ever tried this.

[Muddy42]

12 minutes ago, Rob_the_Sparky said:

It is all in theory as well, not sure anyone has ever tried this.

super interesting thanks. I guess it wouldn't be that hard to experiment by mixing heated intake air with cooler air (to give varying temperatures) to see what happened. Rather like mixing valves do, between the flow and return water of a biomass boiler. You get problems if the return water is too cold.

[openspaceman]

32 minutes ago, Rob_the_Sparky said:

Heating the intake air from the flue is going to improve efficiency (which is why balanced flue gas boilers do this)  However, it also cools the exhaust gasses that contain a large amount of water along with a cocktail of other compounds.  Balanced flue gas boilers have to drain the water that condenses but burning gas does not result in the same flue chemicals.  With a complete burn that may not be a problem but condensing water running back down into the fire would be bad.

 

All this has nothing to do with external air as such, it is about running the external air down the flue pipe...

Perzactly

[openspaceman]

1 hour ago, Muddy42 said:

 

sorry my typo.

 

What i was trying to say was I got a bit lost with those two posts.  I dont really buy the concept of an air supply cooling the (post burn) exhaust and flue, condensation and needing fans to improve the draft. What i was trying to say was that biomass boilers, which are the peak of design and efficiency (by both of your measures) and they have an extreme external air supply and dont have these issues.

 

hope that makes sense.

 

 

@Rob_the_Sparky has explained why it is best not to rob heat from a wood fired boiler and I never saw it done on the commercial chip stoked boilers, or the smaller batch fed gasifying boilers I dealt with.  Also they were never room sealed as they sat in dedicated boiler rooms, so the air was drawn in from the highly vented room and the exhaust was drawn out by fans after the heat exchanger had cooled it to around 115°C.

 

I am not saying it couldn't be done but balanced flues run concentric to the air intake in ss pipes and only for a short distance. The OP says he could take air from the loft into the chimney then down to the boiler, all I am saying is it should not take heat from the flue and a fan would be needed to maintain a depression in the boiler (so any leaks could not allow combustion gases out and to eject the exhaust at some speed to get it away from the building as it would have less buoyancy than a normal wood stove exhaust ( as they tend to be hotter than boilers' exhausts. There may be regulation and practical difficulties as well.

 

What I should have explained is that with a gas boiler the aim is to condense water vapour in the exhaust in order to extract latent heat from it. Gas burns more cleanly and only gives off water, CO2, Nitrogen, some O2 and a little SO2, the condensate is mildly acidic and gets drained via a plastic pipe. Wood is more difficult to burn and gives off a whole rake of acidic compounds, if these condense in the flue they will eat through a stainless steel liner.

Edited 11 hours ago by openspaceman
added last paragraph

[carbs for arbs]

10 minutes ago, openspaceman said:

The OP says he could take air from the loft into the chimney then down to the boiler, all I am saying is it should not take heat from the flue and a fan would be needed to maintain a depression in the boiler

 

I'm the OP.  

 

The other OP is the one who derailed the thread   

 

Ha ha - my thought for not having replied to all the people who responded a few weeks back.  A few "life events" got int he way.  But I do hope to respond to those later today, and hope I don't confuse the detour in doing so...

 

Cheers

[openspaceman]

42 minutes ago, carbs for arbs said:

I'm the OP.  

Sorry my mistake I should have looked back a bit further and seen it was @JMA46 that was wanting to supply direct air to a 21kW boiler.