Tapping Into Triple Wall Exhaust for Air Intake

I have a 42' x 60' steel shop with 1.5" of closed-cell spray foam insulation on all walls and ceiling. I'd like to build a RMH (probably 8") to heat it and am getting the general design together.

I'm planning to run triple-wall 8" stove pipe straight up near the barrel and out the roof (a little over 15' from floor to ceiling, plus another 3+ feet outside to get above the roof peak).

Could I cut into the outside of the stove pipe near the barrel, just the outside layer, and basically use the outside ring of the triple wall pipe to bring in combustion air?  Or is that a no-no or otherwise wouldn't work (it would be fighting against the up-draft due to the warmth of the air in that outer ring, so possibly it would be counter-productive and actually try to PULL air rather than bring outside air in).

The other option is a dedicated air intake, but that would mean a separate 15 foot run of pipe up the wall.  The RMH burn chamber will be 12 feet from the side of the building (see the attached image) and about 20 feet from the front of the building, so the only real way to get combustion air in (that I'm aware of) is through a vertical pipe, like the chimney.

Would the intake air pipe need to be the same diameter as the chimney, 8"?

Thanks!

You'd effectively be turning your triple-wall of insulation into a double-wall. Doing this would probably reduce draught and efficiency.

Stacking functions is great, really it is. But doing it this way means compromising your system's integrity.

-CK

Brian Allen wrote:I have a 42' x 60' steel shop with 1.5" of closed-cell spray foam insulation on all walls and ceiling. I'd like to build a RMH (probably 8") to heat it and am getting the general design together.

The other option is a dedicated air intake, but that would mean a separate 15 foot run of pipe up the wall.  The RMH burn chamber will be 12 feet from the side of the building (see the attached image) and about 20 feet from the front of the building, so the only real way to get combustion air in (that I'm aware of) is through a vertical pipe, like the chimney.

Thanks!

How tight is your steel shop?  Maybe it's tighter than the average house because it uses steel... Or maybe not so much because it's "just" a shop and not a house?

Your RMH doesn't need to pull air from a door 20' feet away, though.  It will pull the air right next to.  Which will pull the air right next to that.  And so on, until "right next to" is a door or a window or any other crack in the building.  There's your replacement air.

What's your motivation for providing a dedicated air supply for the RMH?

Hi Chris,    There has been much debate over the years about the pro/cons of adding external air to rocket mass heaters. A quick search on this forum will give you plenty of info to read. One of them is here: external air link

Yeah, I know. Not an expert,  but pretty well-versed.

If you cool the outer insulative layer of your chimney, it will cause the incoming air to warm at the cost of proper draught up the chimney.

If the exhaust cools too much before it exits the top of the chimney, it will smoke back.

If, like most RMH builds, a lot of the heat has been taken out of the exhaust stream by passing through mass, there's already less heat pushing the exhaust out the top of the chimney.

There may be work-arounds, but that doesn't change what happens in a chimney if the exhaust gets too cold to rise.

-CK

Despite the fact that it is enshrined in some building codes and its adherents are often vocally forceful, there is no scientific evidence to suggest that outdoor air supplies, either direct to the combustion chamber or indirect supplies to the living space, are reliable and effective remedial measures for combustion spillage from the appliance for which the supply is intended.

https://woodheat.org/the-outdoor-air-myth-exposed.html

Jean-Paul Calderone wrote:How tight is your steel shop?

It's pretty tight for a steel building because every joint and corner has been sealed with the 1.5" of closed-cell foam. Really the only places air is coming in or going out is around the 2 garage doors and the steel man door. There are no windows. The garage doors don't have weather seals yet, but I'm adding them.

Jean-Paul Calderone wrote:What's your motivation for providing a dedicated air supply for the RMH?

The only motivation is direct delivery of the combustion air, to keep it from pulling cold air in through the cracks around the doors and across the shop space to the fire.  I have a big laser cutter in the shop and when the exhaust fan (one of the fans used to blow up residential grade inflatable water slides, going out through 4" duct and moving quite a bit of air) is on in the winter you can noticeably feel the temperature drop, like 10 or 15 degrees in a half hour.  I was hoping to avoid the same affect (though far smaller compared to the laser cutter exhaust fan) with the combustion air draw.

Graham Chiu wrote:https://woodheat.org/the-outdoor-air-myth-exposed.html

Thanks for sharing this link.  After reading up on it I think I'm convinced to build and run the RMH without any outside combustion air (aside from what gets naturally pulled in around the doors). I can always revisit the issue if there is a problem to be solved.

However, reading that has caused me to question what will happen when I turn on the exhaust fan for my laser cutter!  It has the potential to make my RMH way less rockety, or perhaps even draft the wrong way.  It's a pretty powerful fan and is moving a lot of air directly outside. I'll have to watch that and may have to give the laser cutter area a way to bring air IN at the same approximate rate its going OUT while the exhaust fan is on.

Thanks everyone.

There is no need for a intake.

After the "hot smoke/air" leaves the building. It will create a low pressure inside the building and the outside air will find a way to make it's way inside the building to equalize the pressure.

If for some super unique reason you find that you really really really do need an intake, make another hole somewhere else in the building envelope.  

I liked Paul's (I think) explanation for avoiding an external air supply, and instead let the air leaks refill what is pulled out of the structure: you need air exchange, and with a well-sealed structure builders start adding air/heat exchanges to a building to maintain the internal temps. If you don't already have all that set up, then an external air supply leaves stinky fart air in the living space for you to breath over and over, while delicious fresh air is fed to your heater. Instead, pull some of the stinky fart air out and bring fresh air into the building instead.

Maybe your laser cutter needs an outside air intake.  Then the RMH can draw from the same inlet whenever it needs.  As others have said, a hole in the wall close to the cutter would minimize cold drafts elsewhere in the shop.

Brian. I hear what you are saying and you are close and that physics is the law and that for every cubic foot of air that goes out the chimney, a cubic foot of air is drawn into the structure.  I have two RMH and one is outside air fed and the second will be when I am done.  I built one to heat my 40 by 48 garage and one a year later for heat in my log home. I am now in my 6th and 7th season heating with them and tweaking them. Here's some of what I have learned. There are two types of triple wall. One is three layers of sheet metal with air in between and a vent for the out side gap. Back in the 70's with the birth of the air tight wood stove, it was found that this was great insulation and venting the outside gap didn't allow heat to collect. Drawing air in through here cooled the middle and outer layer enough to cause the inner layer to not draw as well. Then the second type was with the gap between the inner and middle layer with insulation. Now it was a much better combination even to the point of some stoves designed to draw the intake air through the middle and outer layer. For the RMH I do not recommend this. As we all learned cold air falls and warm air rises, wanting your cold air to com in from the ceiling where it is warm is great but it needs to be completely insulated so that the air STAYS cold and wants to fall. Mine is insulated to within the last two feet and by then it can't warm enough to fight the flow. The incoming air will expand when it burns and heats so you will not need as much incoming air.. I also would not put another hole through your roof for air inlet. I would consider an attic intake if the attic is properly vented. It wont leak. This works for me.
Brad

Thanks, Brad.  It's a steel building with no attic and the plan is to have the RMH be about 12 from the closest outside wall, so it's either bring in combustion air through the roof or through the various leaks in the building (which are few, just around the 2 garage doors and man door).

I think I'll still try it first without. I can always add it later.

Is your RMH an 8" system? How does it do heating that big of a building?

Sorry, I thought it had an attic. Could it come in under an eve or sofit to avoid leaks? The point being that if the cold outside air is drawn horizontally, it needs to be drawn by the heater. If there is fall to the cold air there will be air flow coming out of the pipe and slightly feeding the fire. Agreed trying first without is better. My garage is a 6"system because that is what I triple wall piped for my previous wood stove. Right now I do not have outside air feeding the heater but I did build a solar collector to pre-warm the air some before it draws it into the building. Like you I built a pretty air tight building so it only leaks around doors and this helps considerably. There are some real good videos on utube about solar air warmers. One last thing. I didn't think of this earlier but you have to have a way to stop the air flow when it's near out. If you don't, the warm chimney will continue to draw and start working in reverse where it is drawing the heat out of the middle of the heater all night.
Brad

Is the 6" system enough for that size of building, or do you wish it was 8"?

Yes, I plan to have a cast cover that will seal off the feed tube once the fire dies down.

Thanks!

The 6" system works fine and the end result is the same. I burn small amounts of wood and retain the extra heat in the mass. My goal is to try to keep the shop at 50 or above and the heater keeps a warmth all night. In the mornings I run a torpedo heater for a bit to drive the chill out and bring the shop up to a comfortable level depending on my needs for that day. I don't have any experience with 8" so I wouldn't know what difference to expect.
Brad