Why does P Wheaton not like the external air intake idea?

In the video below, Paul Wheaton states with no uncertainty that the external air intake is a bad idea, but doesn't explain why. Can anybody else explain why?

It makes sense to me that by intaking air from outside you're not sucking warm air out of the room and generating drafts.

This begs for Paul's response because he is kind of the only one in his brain. I know that He and Erica Wisner have talked about drafty homes not being all that much of a bad thing. Taking your semi-polluted and alergen filled air and sending it outside. Then that will be replaced with cleaner fresher air from outside.

With a mass heater warm air isn't as precious of a commodity as when you are using forced air since your heat isn't being stored in the air itself. Removed the warm air after a fire is burnt in a house and it will still be warmed back up from the heat stored in the masonry/mass.

Is there a benefit to heating cold air form outside vs. pre-warmed air? it is certainly way easier to set up a system without an outdoor intake.

Thank you, Ryan. That does make sense to me that you need some air circulation, so you might as well just allow a small draft from the RMH. Looking at a home built using the Passivhaus method; it is so air tight that it needs mechanical ventilation - so that shows how important some draft is.

The only clear advantage in my mind to intaking outside air is you eliminate the risk of smoke blow-back into the room. In the video, Paul mentions a "bubble" to eliminate blow-back but I'm struggling to visualise how that works.

I have worked on a lot of boilers as a welder, and while these were all huge (think 9 stories tall here) they all had air inlets from outside, HOWEVER...they also were preheated. Trust me, they would not include and maintain the massive preheaters for these boilers if they did not have too. It has to do with condensation inside the boiler, which of course is beyond the scope of what a rocket mass heater is.

However rocket mass heaters are prone to draft issues, and as such temperature variations, low pressure and high pressure weather systems also affect draft in every heating appliance, not just Rocket Mass Heaters, so it is a pretty complex issue. Some of that is negated by using indoor preheated air.

But only Ryan is right, this is specifically a question for Paul Wheaton himself.

Travis Johnson wrote:Trust me, they would not include and maintain the massive preheaters for these boilers if they did not have too. It has to do with condensation inside the boiler

Getting a little off topic now but - if preheating the air is beneficial to boilers, why do turbo-charged engines benefit from an inter-cooler than cools the air from the turbo before in enters the cylinder? Seems to me like these two ideas are in opposition.

Matt Coston wrote:

Travis Johnson wrote:Trust me, they would not include and maintain the massive preheaters for these boilers if they did not have too. It has to do with condensation inside the boiler

Getting a little off topic now but - if preheating the air is beneficial to boilers, why do turbo-charged engines benefit from an inter-cooler than cools the air from the turbo before in enters the cylinder? Seems to me like these two ideas are in opposition.

Cooler, denser air is great for making horsepower. However, boilers don't run very well with their burners submersed in water.

I was always told cold outside air contained more O2 than inside air, as it was denser.  

and the local fire department's book on solid fuel heating says that produces a cleaner burn, less CO


but actual measurement would tell the tale

It's not because it's outside air, but because it is cold. Feeding cold air to the fire robs it of a bit of efficiency, as it has to warm the new air that much more. Preheated outside air would be efficient, but if you are preheating it, why not deliver it to the room at large for fresh air instead of sending the fresh air straight out again?

Glenn - if pre-heated air is more efficient, why do turbo-charged internal combustion engines use an inter-cooler to remove heat from the air before it enters the cylinders?

The reason an outside intake isn't a good idea, is because you can create a competing chimney with the wood feed, and get smoke-back. Pulling the air mostly from the wood feed prevents this. Smoke-back is highly undesirable as you know.

Some designs include the P-channel, but that intake is fairly small relative to the wood feed. Interior air also needs to be refreshed in any structure you're occupying, so it's a win-win using the RMH to help with that function.

Hi Mark. Thank you for your input. I must admit I did not understand how the outside air intake actually connects to the RMH whilst also accommodating the wood feed. So I can accept there is some complexity going on there that leads to performance issues.

Matt, regarding your question on the turbo charger inter-cooler: cooler air is denser thereby increasing the volumetric efficiency of the engine. This is relevant in an internal combustion engine, but not to a wood fire.

Warm inside air burns easier and a "breathing" house helps the draft and lessens the problem of smoke-back. I don't know if these are Paul's reasons, but this is why I think a direct outside air feed is not a good idea.

Just to clarify, you can create an air intake that comes through the wall/floor and provides an air source near the wood feed, if you want to minimize any drafts in the house, my previous comment was more for having an intake that feeds right into the burn tunnel so it might not be a clear reply.

Ianto and Leslie's book Rocket Mass Heaters mentions an air intake on page 68, suggesting a 4" intake with mesh covers on both ends to keep out rodents would be plenty for providing air intake but it's not meant to be going into the RMH, just near it for that fresh air, from how I read it.

Mark TudorSd wrote:
Ianto and Leslie's book Rocket Mass Heaters mentions an air intake on page 68, suggesting a 4" intake with mesh covers on both ends to keep out rodents would be plenty for providing air intake but it's not meant to be going into the RMH, just near it for that fresh air, from how I read it.

That is how I understood it as well.  most pictures I have seen for a setup have the cold air coming in to an outlet right under the stove.  It would still cool the fire somewhat though.

more on the external air intake

Thank you, Paul. This is just what I was looking for.

I see a lot of other people have asked a similar question over the years and you've posted this video across all the threads. Thank you.