RMH, fresh air intake, why suck out warm air from inside?

I have been reading you site for the last few days and have a basic question that is stated in the subject line, why not feed the RMH with a controllable/variable air shaft that draws air from out side the building? It seems to me that you have gone to the effort to heat up the air in the building and you are sucking out warm air (being replaced by cold air through leaks) at a rate of ?? cfm while it is running. You might have to make a a box or something that goes over the hole for the wood, but that is very simple to do. I understand that when starting it or loading it with wood, it is done from inside where ti's comfortable, but when it is just running, it seems like drawing outside air makes more sense. Maybe this has been discussed in the past I didn't dig deep enough to find it. My only thought is you want to draw in fresh air from outside preventing CO2 issues.

Scott

There a problem related to this, which Erica addresses in the podcast, (http://www.richsoil.com/permaculture/182-rocket-mass-heaters-permaculture-podcast-019/).

The problem has to do with setting the wood-feed outside the building. If the wood-feed is on the exterior wall of the building, then the burn tunnel is going through the wall of the building, and the barrel is on the inside. Problem 1) The burn tunnel is hot. Putting it through the wall like that can be a fire-hazard or at least difficult to work around. Problem 2) the burn tunnel is best when it is short. Since the barrel needs to be away from the wall, then the burn tunnel has to go from outside, through the wall, and cross the distance over to the barrel. This means that the burn-tunnel will be fairly long. According to Ianto and Leslie's book, optimal burn-tunnel length is 1/2 the height of the heat-riser.

I guess you could bring cool air indoors through a duct and then feed that duct into the front of the wood-feed.

Here's my critique of that idea:

Disclaimer: This is all very hypothetical. --- What's your hypothesis?

One of the things that makes a rocket mass heater so awesome is how efficiently the mass-covered-exhaust-pipes reap the heat from the exhaust before it exits the building. So, if warm (indoor) air is coming in through the wood-feed, then that just means that the rocket mass heater will get warmer. In effect, most of that heat should get recycled back into the room and mass.

However, you're absolutely right: That air intake has to come from somewhere, and ultimately a standard rocket mass heater is pulling cold air through an open window, or cracks in the doors, etc. (Note: conventional fireplaces and woodstoves do this too). I guess the room can be used as a sort of buffer to assist with increasing the potential heat in the insides of the rocket mass heater. So I'm thinking that if you want the air in your room to be warmer, then make a duct that brings air in from the exterior of the building and deposits it directly into the woodfeed; However, if you want the hotter (cleaner?) burn, then let the wood-feed drag air in from the surrounding room. You could easily make a rocketmassheater which has the potential of doing both (just open or close the in-take duct).

Also, the length of that in-take duct probably needs to be included in the total length of duct (including the exhaust duct). So if your system can produce enough draft to push air through 30' of duct, and you have a 5' intake duct, then the exhaust ducts can only be 25'; if your system can produce enough draft to push air through 40' of duct, and you have a 10' intake duct, then the exhaust ducts can only be 30'; make sense?

Anybody else have ideas?

I was not suggesting that the wood feed opening be outside of the building. I even suggested that a simple box could be made (that was plumed to the outside) sit over the wood fee when you were not loading it. I agree that running a superheated pipe through a wall is not the best idea and there would be potential loss of heat in doing so.

Lets say that the RMH is moving 50 CFM, so what you are saying (or I'm asking) is that you are pulling into the room that amount of really cold air, maybe below freezing into the room. Would we put a six inch hole in the wall, to the outdoors and turn on a small fan in the middle of winter when we are trying to heat the same room? No, but that is what is happening, through all the small leaks, you just don't feel it like a small blast of cold air coming off a fan.

I think that the warm air going into the RMH, is not required (I don't remember all my physics anymore but I think that wood burning is an exothermic reaction and the amount of heat given off is not affected, but the air being heated is going to be colder). I believe you might burn an insignificant amount of extra wood to make up for that small delta (to regain that amount of heat loss), but I don't think you could measure it.

My question was digging into maybe your last comment about the total length of the entire duct system. So is the rule of thumb, that it take into consideration the entire duct length, both pre & post burn into your total length? I have not seen that restriction/dimension in the forum so far. What is that rule, or is it trial and error, base on the size of the fire box, chimney size/length, reburn chamber, exhaust duct dia..... etc? My guess would be to increase duct size to reduce the pressure that slows down air flow if you need to go longer, but I'm sure there is an upper limit that makes it to bulky or worth the effort.

Maybe it was something that you guys have not gotten around to yet or no one thought about it yet, but it was curios to me not to consider it. I have a couple of other ideas around the bench, the heat chamber and a few others....

Scott

I believe the biggest reason from EandE against doing this was the possibility of creating an alternate chimney. Essentially the alternate air supply becomes a chimney and the RMH runs in reverse. You then have fire going into ducting that was never designed to handle it. The open wood feed will not act the same way, as it is not tall enough to create the draw the ducting would.

The amount of air required to fully combust 1 pound of wood is 35 lbs, or 400 cu ft. If that air is pre-warmed there is less heat of combustion lost to heating it. What is more important is that the combustion air coming from outside will dilute the temperature going into the riser and secondary combustion chamber (drum). Reduction of temperature there would reduce the combustion efficiency, potentially increasing hazards.

While lately we have become more concerned with conserving energy by tightening our homes against drafts one of the most often encountered problems I address as a professional is inadequate make-up air. Without an air supply combustion appliances fail, malfunction, or function poorly. Another concern that has come out with air-tight homes is overall health. Cutting off the ventilation of a house leads to mold and bacteria growth. The point here is that regardless of whether you are using an oil burning furnace, an EPA wood stove, or an RMH, you can have a house that is too tight. Coincidentally, an outside air duct for combustion air would be an appropriate solution to insufficient ventilation.

Another point to consider, from what I have recently learned, an RMH uses far less fuel than traditional wood fired heaters. With far shortened burn times (down to 1/8th?) one is drawing a commensurately less amount of cold air into the house. The concern about inside or outside air may simply be moot.

- A lot of assumptions here , and I cant pick out which ones are most likely correct, E and E point out, and use the way a pocket rocket works as a guide that air entering the feed tube is warmed prior to meeting the 'flame combustion front ' (my choice of words). The alternative that we see with most wood stoves is the coldest air in the room , that at floor level, is drawn into the heaters wood box actually cooling the 'flame combustion front' ,
I have helped on the build with 2 Rocket stoves, and 2 Rocket Stove Mass Heaters, and played w/ some gasifiers and am convinced that pre warming the air (and the insulated J-bend/combustion tube) allows the Rocket stove to achieve its spectacular heat production and great efficiency,that would be the Two areas that I would be most reluctant to change. Perhaps after 20 or more minutes of initial operation, and a great bed of coals , I might try adding in some outside air, but I have the idea in my mind that the Rocket stove which needs careful tending would become more troublesome when my pre-burn Air Temp. dropped, and my bed of coals disappeared. O.K. now that I've had a chance to think about this i could see a below floor level long skinny box between the floor joists that could be used to bring in outside air, (here you could use a damper) if the whole project was a fail, it would still be a handy place for a small kindling storage box and a place to sweep up into for Daily cleaning chores ! I think a heat exchanger placed at outside eaves height would mingle cooler outside air with the over heated air at ceiling height, Yes, I can easily see how all that precious room temp. heat becoming 'make-up air' for the Rocket Stove would seem sinfully wasteful Especially with the outside wood pile frozen to the deck by the freezing rain , and no kindling split, but thats really more about survival of the fittest, than a wood stove problem ! Before i posted here i reread what has already been posted and I have only added my 2 cents w/out adding much clarity, But this forum has again changed my building plans Allen L OUTER 'Rondacs, Northern. N.Y.

I like the idea Scott gives about giving a outside air "Choice" for the feed. I never gathered from Scott that he suggested anything other than the standard intake feed location (stjohn) and I think Scott isn't suggesting to cut off all air from inside, but only to allow outside air to enter the room close to the feed location. If the Feed is pulling air from inside the house and originating from every crack and crevice anyway, then introducing a duct opening to the inside house air, would allow the feed to choose where it gets the air from, and where the air pressure equalizing occurred. IMO, this doesn't require figuring in the length of that intake duct, otherwise you may have to figure in the length from every crack, since the air has to enter the house somehow. I wonder what an air pressure meter would read in a house that doesn't offer a direct air from outside? (What meter would read that?) If you could measure air pressure changes inside a well sealed house, seems like either the air pressure would be reduced if the house was sealed really well, or air would enter through cracks to equalize it. To clarify, I am just a newby in the RMH arena, though have 54 years of thinking mechanically, and reasoning everything out to a mechanical solution, which didn't come with the means of clearly portraying it to words, lol. Sorry if it sounds muddled, I found this subject very interesting.

I think you could run a cold air intake channel with a damper in the bench so it opened into the feed tube above the flame front. This could connect to an external air source. As the system warmed it would warm the incoming air through the bench. Then you have a controlled draft in your house which has to pass through the thermal mass hopefully eliminating the idea that it is a "cold" draft. The pressure difference will always work itself out, at least this way you have some control over it. At least in my head

If we let the outside air into the barrel that is around the feed-tube, we can change whether the air comes from the duct or from the house. Which is handy, since if you can seal the fire in this system, the fire can be left alone.
If some back-draft happens no smoke, fire or burning wood can escape into the house. Also if the feed air is coming from the same side of the house where the exhaust goes, the wind pressure can not really affect the draft.

But if the fire is tended and the stove is going well, we can leave the lid open and have some air change.

I have been seriously considering this same matter, the idea of mating a RMH with a ducted outside air intake, and am glad to see that it has been discussed here already. I will have an outside air intake kit already installed to work with the conventional woodstove that my RMH will be replacing. Obviously it is tempting to me to fabricate an adapter to seat the same system over the feed tube of the RMH. The pros of the concept seem obvious, in that they should be the same pros as using an outside air kit on any conventional woodstove. Namely, the pros are:

1) More efficient heating of the home, since the heating device is not itself drawing large amounts of cold outside air through cracks into the same space it is heating.
2) Less danger of back-drafting in a well-sealed house, since the movement of air through the combustion device is not creating low pressure inside the house.
3) More efficient heating of the home, again, since the interior air that is NOT being blown up the chimney expands as it heats creating a positive pressure and pushing backwards into those same cracks in the building envelope, thus "sealing out" the outside cold air.

I heard this idea of an outside air kit on a RMH mentioned by Paul in a podcast the other day. He characterized the objections he had received to the idea as "well, you want to encourage air exchange in order to ensure fresh room air for general comfort of breathing." While this certainly makes sense, particularly in a well-sealed house, it is also irrelevant to me: I've already incorporated a mechanical air exchange system into my house plan, called an "energy-exchange ventilator" (this works just like a "heat-exchange ventilator" except that, in addition to partially equalizing temperature between the incoming and outgoing air, it also partially equalizes humidity).

So, if encouraging air exchange is not an issue for me, what are the cons associated with the idea of ducted outside air intake on an RMH? It seems like a couple have already been mentioned herein:

1) Less efficiency inside the combustion chamber itself, since the intake air feeding the fire is colder, thus sucking energy out of the combustion gases. Someone else already pointed out that this con could be mitigated by routing the air intake duct through the "mass" bench, thus pre-heating it prior to its arrival in the feed tube.
2) Assuming that the outside air intake is designed to more-or-less seal over the mouth of the feed tube, thus ensuring that maximum outside air and minimum room air ends up feeding the combustion chamber, another con is that the length of intake duct must be added to the total length of duct in the system for purpose of calculating total draw created by the heat riser vs total drag on the airflow. This might or might not prove problematic depending on the paricular RMH in question.

Are there any other cons of which I'm not thinking? Any other reasons why I should not consider trying to adapt my outside air kit to the RMH I plan to install? Anyone with first-hand experience of such an RMH/outside air adaptation?

Thank you all in advance for your feedback!

While we are on the subject of ducting in outside combustion air, I have another question: the duct that was installed for the outside air kit on my conventional woodstove (mentioned above) is simply a 4" flexible dryer exhaust vent. The soft, floppy, tin foil, corrugated-looking kind. Understand that this duct is in place, and the roof pass-through for my chimney is in place, but the woodstove itself and the rest of the stovepipe chimney is not yet installed.

It occurs to me reading through comments in this forum that this might be a totally inadequate material for ducting my outside air kit. And by this I mean 1) it could prove inadequate when and if I try to adapt it onto my RMH in the future, as discussed above; but also 2) it is probably just as inadequate for use on my conventional woodstove in the meantime, and for the same reasons. Here are my suspicions:

1. Since the intake duct will be mated with a more-or-less firm seal to the oustide air kit on my woodstove, then the total length of this duct is added to the total drag on the airflow through the system. Even though my conventional woodstove will likely produce more than enough draw to overcome the added drag - since afterall my woodstove will be blasting 400+ degree exhaust up the chimney - more drag is still more drag; it is still a drain on the efficiency of the system. So why use a "corrugated" dryer vent when I could use a much lower-drag smooth rigid metal duct instead? In the future, when and if I mate this system to my RMH, this should prove even more critical, right?

2. Along those same lines, I understand the importance in RMH design to maintain a consistent cross sectional area at every stage of the air channel through the heater, from beginning to end. As I understand the physics, this is to ensure consistent air flow. Otherwise, the air flow capacity of the entire system would be effectively reduced to the capacity of the single narrowest bottleneck anywhere along its length, right? Surely the same logic applies to air flow through my woodstove system (right?). So if that is the case, why attach a 4" intake air duct to a woodstove with a 6" exhaust pipe? And again, even if the woodstove has the draw to power through this restriction, wouldn't this become even more of an issue when attaching to the RMH in the future?

Thoughts, reactions, criticisms, suggestions...?

Thanks!

why do you want to breath the crappy air yourself?

Its not a trouble to heat the room and your house has to cycle air at a certain rate in any case to allow you to not get sick from all of the off gassing crap and your own breath. I don't think you want to breath all the stagnant crap but most folks soon figure out that a nice clean breath of air is a good thing.

Matthew Nistico wrote:

1. Since the intake duct will be mated with a more-or-less firm seal to the oustide air kit on my woodstove, then the total length of this duct is added to the total drag on the airflow through the system. Even though my conventional woodstove will likely produce more than enough draw to overcome the added drag - since afterall my woodstove will be blasting 400+ degree exhaust up the chimney - more drag is still more drag; it is still a drain on the efficiency of the system. So why use a "corrugated" dryer vent when I could use a much lower-drag smooth rigid metal duct instead? In the future, when and if I mate this system to my RMH, this should prove even more critical, right?

2. Along those same lines, I understand the importance in RMH design to maintain a consistent cross sectional area at every stage of the air channel through the heater, from beginning to end. As I understand the physics, this is to ensure consistent air flow. Otherwise, the air flow capacity of the entire system would be effectively reduced to the capacity of the single narrowest bottleneck anywhere along its lenght, right? Surely the same logic applies to air flow through my woodstove system (right?). So if that is the case, why attach a 4" intake air duct to a woodstove with a 6" exhaust pipe? And again, even if the woodstove has the draw to power through this restriction, wouldn't this become even more of an issue when attaching to the RMH in the future?

Thoughts, reactions, criticisms, suggestions...?

Thanks!

the little 4 inch intake cant provide the amount of air your fire needs to burn clean in a 6 or 8 inch RMH. it does not need to be smooth for an intake but it does need to be large enough.

Ok we will go into this a little because its important. yes the constant CSA is important in a box stove; surprise!!! your stove has more air intakes than you know many are hidden from you or inaccessible so you cannot block them off. the other thing is that you cannot get your box stove up to clean burning temp without voiding your warranty. the Temp for a good clean burn is 1200 and up. this would make your stove glow really nicely and very quickly void the warranty. So the reality is your stove is starved for air in the first place but now days stoves have more holes in them so you cannot totally shut the stove down to a smolder (Air quality). What most have these days is a catalytic converter to scrub the particles and some of the other nasties out of the exhaust.

another consideration as i said in the above post why would you want to breath stale air?

any how hope this clears a bit up.

Ernie Wisner wrote:the little 4 inch intake cant provide the amount of air your fire needs to burn clean in a 6 or 8 inch RMH. it does not need to be smooth for an intake but it does need to be large enough.

Thanks Ernie, that is excellent info! I was also very interested to learn more things about my woodstove (which is not catalytic, btw; it is a Striker 6" by Lennox, if you are interested). Fascinating. So, if there are multiple air intakes all over the thing, I am wondering if that means I got taken for a ride on the whole outside air intake kit in the first place?

Yes, I hear your concerns about breathing stale air, but since I've already installed a mechanical ventilator I am less concerned about that myself. My house is pretty tight, so I will be running the ventilator through the winter whether I'm burning the stove/RMH or not. Since I will only fire the stove from time to time, even less frequently for the RMH, I'd hate to rely on that as the only "engine" to draw fresh air into the house.

My suggestion is that the make up air be supplied by an intake that directs the air against the barrel that contains the rocket mass core. that way your fresh air gets heated and cooler and stale air feeds the fire. A damper to reduce the intake when the fire is not burning would be advisable. Some systems use a plastic underground pipe for make up air which warms it slightly in winter and cools it when exhausting summer heat.

On the subject of the 4 inch outside air intake for a wood stove. I have found it is not sufficient for my wood stove when my wife turns on the cooking exhaust fan. It draws smoke back through the smoke burning draft. The electric furnace air handler has a 5 inch insulated duct that comes down from the roof and I added a 4x6 inch duct from the crawl space but that still dose not keep up with the high speed fan.

There are two issues here.

Cold air brought from outside is denser - more oxygen for volume. Once it falls below zero it is dry. On the face of it a win win situation. This is why grain dryers work best in freezing conditions, and why race engines have larger jets fitted in cold weather.

So bringing cold air in from outside can only be good?

There is always a but……..

Cold air directed into the burner will significantly cool the flame temperature. If you have a gas oven duct some cold air directly under the burner - go and get a takeaway - see how long it takes to cook something. You might even be lucky if it does.

In my opinion a couple of air changes per hour is good in a living space. As a rough guide you should have enough ventilation for you and your rocket to breath if allow 36 sq inches for a 6 inch burner, but do your own research and find out. Yes that’s a big cold hole!

In my entire life I have seen maybe a dozen homes built as tight as Hans Quistorff's, most of them were owner built, and none of them were occupied during the build !
I Would like to wish that all homes were built this way in the future!

I wonder if Hans would share the location and date of the build of his house. I expect that it would come close to meeting todays Passive House Standards ! The Required
Air Changes per hour ( hopefully actually done as Air Exchanges per Hour thru a heat exchanger ) Is only 0.6 A.C.Hr., not even close to 2 per hr, I think that simply
changing The Cooking Exhaust fan for a 2 or 3 speed model would reduce the few problems he has now ! I live in a house with lots of green plants that can tolerate
temperatures in the 55-60* F range much better than My wife and I can, and hope to always ( when not traveling in our yurt) sustain our interior envelope on ~1~ A. C. Hr.

For the Craft, BIG AL !

Yes it is a custom manufactured home designed to have the air exchange through the heating plenum and out the bathroom vents. When the house was new the recommendation was to keep the fan set to on rather than automatic so construction chemicals would clear. Because of permitting delays it sat 6 months before delivery so that was not much of a problem. Running the circulating fan dose bring warmth to the master bedroom and bath better than natural circulation if the house has cooled down. It is all open ceiling except the two bedrooms and baths. There is vent tube connecting the ceiling from the hall to the bedrooms so heat will slowly enter and cold air return under the door when closed. The house holds about 10 degrees warmer than the outside which here in Puget sound is 58 day and 48 night. If the sun shines in the two front picture windows facing south we do not need a fire in the day time. If the fire goes out during the night the temperature will drop fro 75 to 68 in the morning.
During our occasional 80 degree days in the summer we close the blinds in front and turn on the circulator to draw air from the crawl space which is a concrete slab with cement block walls and says dry.

I hope to one day build a home with a RMH, and have been thinking about the issue of tight homes and inadequate air exchange. I was wondering about having an outside air source that is routed through the RMH via a closed pipe to heat, then is vented to a remote part of the house appropriate for proper air circulation, thereby eliminating draft and supplying constant fresh air and inside pressure pushing out rather than the stove drawing air in through cracks etc. So, the stove itself is drawing from the house atmosphere rather than feeding from the outside source. Has anyone done this?

J.W. R. : ''The Book'' Rocket Mass Heaters detailes bringing in cold air near the floor and blending into the air space within the structure at the ceiling level
this warms the air and causes a counter-flow of air to decrease temperature stratification at the ceiling ! It also helps to mix the air within the living space !

With over 100,000 Rocket mass Heaters RMHs Built, most have been built following 'The Book" 95% of all the First builds ( that worked ) followed " The Book '' !

Please Consider going to Rocketstoves.com to download the brand new 3rd edition of "Rocket Mass Heaters", You will quickly learn the simple word phrases
used by all rocketeers to describe the Sizes, Shapes, Materials and the Orientation of the parts of an RMH both to themselves, and the whole !

The little bit of math to deeply understand the RMHs Ratios and Constant Cross-sectional areas is truly 3rd/4th grade stuff and makes talking to your fellow members
a prideful thing that truly All of us Late Adult learners respect in ourselves and our fellow members !

'The Book will also save you time money and frustration as you grasp the core values, no more trying to re-invent the wheel !

Get the Book,read the book and come back herewith your questions, we all love creating fellow Rocketeers ! Big AL !

My two cents on why I like fresh air from outside for my lungs, and the stale inside air full of farts, halitosis, BO and whatever is in the compost bucket to get sucked outside: