Outside fresh air intake pre-heater idea. Thoughts?

[MaineWoods&Pond]

I have been watching and using the forum for years but this is the first time I needed to post:

Necessary backstory:
I finished building a new off-grid workshop/home in central Maine for my family last year. The building is 3,800 sqft with tall ceilings, very well insulated and very tightly sealed. We heat exclusively with wood, no backup sources. We have a old pre-epa non-catalytic woodstove in the downstairs workshop (40' clay lined masonry chimney) and a Jotul 602 woodstove in the upstairs living quarters (20' clay lined masonry chimney). The downstairs stove is the primary stove, the upstairs is a secondary.

I heated the building exclusively with the downstairs stove last winter burning about 7 cords, our lowest outdoor temp was -27F. Both stoves have very good drafts - when they have air to draw from. The problem is that I have to crack a window open so that the stoves have a source of air to draw from since the building is so well sealed. I know this is a problem for both the stoves and for good/healthy indoor air quality. I don't want to do another season this way. Whatever I do to solve the problem cannot use much if any electricity - we are 100% solar powered, the harsh winters here can sometimes cover my solar arrays for days at a time in snow, requiring us to conserve power.


Possible solution:
I am thinking of installing a 6" insulated duct to the windward (North) side of the building, the duct would run to the downstairs woodstove, stopping short by about 5' and transitioning to a metal non-insulated duct with a manual dampener installed at the transition. The duct would join to a 8" tee with 3' long duct section above it that would be centered/wraped around my 6" single wall chimney pipe about 18" after the woodstove and in a vertical section. This would in effect create a heat exchanger that uses both the draft of the woodstove and the convection of the 1" gap of heated air between the 6" chimney and 8" duct to draw outside air into the building while simultaneously pre-heating it with waste heat.

I verified that my chimney temps in this section range between 400-500 degrees with my non-catalytic stove, and with the 40' well drafting chimney I am not too concerned about exhaust temps dropping too low to draft well or create excess creosote. If the exhaust temps drop too low I can always close the outside air dampener a bit. I am thinking that if I do this I should change my lite gauge chimney pipe in that section to a heavy gauge since I will not be able to monitor it as closely and since it will operate under harsher conditions.

I am interested in hearing thoughts on the idea?

 

[Zack R]

Seems like you have thought this through, the idea is to try and bring enough outside air in for the stove but not to have the incoming air be cold. I really don't know much about outside air systems but it seems like a better long term solution would involve replacement of the older wood stove with a newer one that uses less wood and less air. At that time you could install a factory outside air system if needed.

On another note how would you be sure the outside air system wouldn't operate in reverse? If you are heating the pipe with your chimney it seems like you might run into a reverse flow situation depending on the pressures involved.

It'll be interesting to see what folks suggest.

 

[bholler]

I am not really sure why you want to preheat the air for your stove anyway. But is that 40 foot chimney internal or external?

 

[electrathon]

4" insulated duct bringing in outside air. Build a inlet connection so the air goes directly to the stove, not the room.

I think you are trying to build a heat exchanger instead of trying to bring outside air to the stove. Change directions.

 

[MaineWoods&Pond]

I am not really sure why you want to preheat the air for your stove anyway. But is that 40 foot chimney internal or external?

Thank for your reply bholler

The chimney is internal (almost at the center of the building). I am trying to preheat the air entering the building from outside to avoid very cold drafts. I cannot duct the outside air directly into the stove, it has two downdraft dampeners entering via the stovetop which would be difficult to duct into.

 

[MaineWoods&Pond]

4" insulated duct bringing in outside air. Build a inlet connection so the air goes directly to the stove, not the room.

I think you are trying to build a heat exchanger instead of trying to bring outside air to the stove. Change directions.

Thanks for your reply electrathon

I would if I could. Sorry, I should have elaborated that my woodstove has two downdraft dampeners entering via the stovetop which would be difficult to duct into. So, it seems best to preheat the outside air as it enters the building. Although, you are right - I could just bring the insulated duct to the wood stove and allow it to blow cold air onto the stovepipe/top near the dampeners. That would be a simpler/cheaper solution, I will think about that.

 

[MaineWoods&Pond]

Seems like you have thought this through, the idea is to try and bring enough outside air in for the stove but not to have the incoming air be cold. I really don't know much about outside air systems but it seems like a better long term solution would involve replacement of the older wood stove with a newer one that uses less wood and less air. At that time you could install a factory outside air system if needed.

On another note how would you be sure the outside air system wouldn't operate in reverse? If you are heating the pipe with your chimney it seems like you might run into a reverse flow situation depending on the pressures involved.

It'll be interesting to see what folks suggest.

Thanks for your reply Zack R

I am trying to make do with this stove for a few seasons. I have plans for a outdoor wood boiler system and thermal storage tank but it will be a few years until I have time/money to do it. A modern wood stove would certainly solve the problem though and is a great suggestion. I don't think I have to worry about it working in reverse, I have never observed positive pressure on the lower level of the building, only negative. I am sure this is in part due to the architecture - high ceilings on both levels plus a over-sized copula with windows = heat rising.

 

[MaineWoods&Pond]

I thought I would post an update in case anyone had an interest in this project. I did end up installing this cold air intake/heat exchanger on November 15th 2017. It was installed mostly as my first post describes - the only exception being that I installed the dampener at the outside wall and ran a control rope to a location near the woodstove. I open the dampener when I start a fire and close it when the fire dies down. I also used mineral wool to fill the gap between the stovepipe and heat exchanger duct (at the base of the duct) to prevent the system from working in reverse and sending heat outside, I have yet to observe warm air reversing direction in the duct.

It is hard to definitively say that it increases overall heating efficiency - I could study it, but it seems like a waste of time since very few people are in the same situation and not much would be gained. I will say that I immediately noticed the air in the building was fresher (likely much healthier), that we no longer have any noticeable drafts and that both our stoves are drafting better/starting easier now that we don't have a low level vacuum in the building.

Exhaust temps are still at a safe level of 230-350F when the stove is in its proper burn range and the heated air is entering the building interior at 90-140F - with outdoor temps around 0F. Our lowest outdoor temp so far this season was -29F and the heated air from the exchanger was 85F. A check of the stovepipe in the heat exchanger section with a inspection mirror and flashlight does not show excessive soot accumulation but I will keep monitoring regularly to play it safe.

I am very happy with the results, it met all of my goals. I am not going to suggest/promote that other people do this, if it is not done right it could be a major safety and/or fire hazard. It is also unlikely to pass any building/chimney inspections and may void your homeowners insurance.