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Post by stinwell on Sept 10, 2022 7:18:06 GMT -8
I have found some allusion to people having done this but have not been able to come up with any actual examples.
Does anyone have any results from actually ducting outside air through a mass for combustion/house make up air in a well sealed structure?
Depending on climate it seems like a well designed system could essentially function as an HRV, pulling in and heating fresh air for multiple uses.
Random thoughts to throw around:
Optimizing the relative location of air inlet to firebox.
Seems that each inlet end (opening) would have to be below the firebox. Am I visualizing the physics correctly that the pipe could travel up above fire level but the openings should be down low to avoid pressure issues in general and reduce backdraft risk? My intuition tells me a straight run end to end down low would be ideal though likely difficult in most building scenarios.
In line filter. Many simple boxes and filters available for standard duct sizes. Potentially tricky to calculate sizing of the ducting with the filter affecting flow in a non forced air setting.
Condensation point inside the mass by bringing in cold outdoor air?
Very cold air temperatures might cool the mass faster or as fast as it can effectively be heated?
What say your brains on the subject?
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Post by peterberg on Sept 10, 2022 23:48:42 GMT -8
stinwell, Before you start to build like you are presenting, a word of warning. Make-up air doesn't work well in half of the instances I've been aware of. One of those instances happened to be mine, regrettably. We live in a passive house, with only one larger(ish) mass heater in the living room. During the build, we had a provision for make-up air installed, ending just beside the heater. From there it goes under the concrete slab, out of the house's perimeter about 10 ft and up to about 3 ft above ground level with a rain cap on it. Half of the time it didn't work at all, sucking air out instead of feeding in. The other half it wasn't reliable as well, during a succesful burn just now and then the air stream direction reversed for a minute or two, belching smoke into the living room. For a time, I let air in through a small window which was at the windward side at that particular moment. This solution didn't satify me, it is a passive house, all ventilation should be taken care of by the balanced ventilation and heat exchanger unit. At long last, one of the presets of that unit was altered in such a way that there was more air fed in the house than extracted. In order to create a slight overpressure, just enough to compensate for the chimney draw. Since then, in 6 heating seasons the heater works as intended 100% of the time. There's a site which comprehensively describes the how and why of make-up air failure. Please read this first before you making more plans. Of course, it might be that in your particular case it will work as intended, but having a 50% chance of failure is a bit much, don't you think? woodheat.org/outdoor-air-supplies.html
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Post by stinwell on Sept 11, 2022 7:24:57 GMT -8
Thanks for the response peter.
I have been thinking of the outside air issue for some time and have read through your link before, but that is a nice simple and clear explanation of pressure and air flow. It did confirm some of my thinking that the placement of the air openings do have an effect, but of course simple things like a random wind gust can throw off even well designed systems.
Some context for my design thinking is a tightly sealed house, in an off grid setting where reliance on mechanical systems may not be possible 100% of the time. I’m assuming a simple open floor plan, no exhaust fans, and no vented clothes dryer or other appliances.
With fine tuned controls and a skilled operator, the heater itself could at least bring in some warmed air and reduce the need for running around trying to adjust various windows when the wind changes direction or something. This could theoretically go higher tech with an adjustable in-line fan in there, possibly powered with a thermoelectric generator connected to the heater or something of that nature, digital controls etc.
I guess in the end my goals are more towards reducing dependence on mechanical systems while gaining the benefits of tight air sealing and good insulation. The current consensus in the building world seems to be that those goals can’t effectively include wood burning.
All else aside the house is going to bring air in from somewhere, so maybe a successful system here means accepting having to fuss with the air supply more than desired.
Or maybe I can get lots of hamsters and wheels and connect them to various parts of my ventilation system, using a controlled food hopper to motivate them when I need the fans to run....
I’m currently working on a building that will eventually include balanced ventilation with through wall units, but it might be a perfect scenario to test this out since we won’t be occupying it for some time. I will probably build it regardless for experimentation. I will of course report back with results.
In my research I seem to have found several mentions of this being tried but no actual examples to speak of. There are likely too many variables for any two systems to be truly comparable, but I’m still interested to hunt down someone who has done this and what their conditions and experiences have been.
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Post by Dan (Upstate NY, USA) on Sept 11, 2022 15:30:34 GMT -8
I did it with aluminum pipes through my wall and then concrete mass, I have 7, about 1.5 inches in internal diameter, 3 feet long.
THE ONLY reason they work is because I punched a bunch of holes through my house to find out where the house wants to pull air in and where it wants to let it out. *my house is very well sealed as tested with a door fan*
After finding the correct height I installed them there.
Lets see if I can find a picture.
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Post by Dan (Upstate NY, USA) on Sept 11, 2022 15:32:00 GMT -8
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Post by Dan (Upstate NY, USA) on Sept 11, 2022 15:35:57 GMT -8
This only feeds warmed air to the people in the winter, NOT the fire directly, it works quite well as the only other makeup is from the double dog door.
This keeps the draft less in the hallway with the dog door.
No condensation issues.
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Post by Dan (Upstate NY, USA) on Sept 11, 2022 15:36:15 GMT -8
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Post by stinwell on Sept 12, 2022 6:02:44 GMT -8
Super interesting Dan.
Do your pipes just go through an exterior wall and terminate outside?
I'm guessing from your photos that you started high with your hole punching and had to move lower to find the sweet spot for correct pressure.
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Post by Dan (Upstate NY, USA) on Sept 12, 2022 14:00:46 GMT -8
"high with your hole punching and had to move lower" Yes, I started with windows and observed at different times of year and day and found a sweet spot for my house as I was sealing... and I used burning incense to determine direction.
I was also in the process of super sealing and insulating the house so it all made sense at the time. R40 walls, R60 roof.
The outside is a piece of trex plastic decking with holes in it over an aluminum screen between that and the pipes.
Works well 365 days a year, air only moves in one direction, even in storms. Until a skunk sprays outside those pipes, that's no fun...!
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Post by stinwell on Sept 13, 2022 7:01:51 GMT -8
Well thanks for the reply and posting photos and details. My thinking has so far been limited to standard ducting materials and stove pipe. Great to see things that work as intended
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