rocket mass heater - air from outside?

So I was recently asked about the rocket mass heater having an air intake from outside. 

There is conventional wisdom - and I think it is even legally required sometimes, to make your wood burning appliance get the air from outside.  The reason being is why heat the air and then just suck the warm air out of the room to burn the fire.  The replacement air would just have to come from the cold outside.  So you burn your fire and suck cold air in from the outside. 

There was a design on the internet where a fella routed a dryer vent tube to the wood feed before burying it in cob (mud).  And then he had a BBQ or smoker lid that he could put over the wood feed.  That way, all of the air would come from the outside - via the dryer tube. 

So I saw this picture before going to the workshop, and I thought this would be an awesome question.  Now, keep in mind that there are FIVE instructors at this event, because this class just happened to align with some sort of national pyro-instructor-convention .... so when you ask your question, it is actually fielded by five experts instead of just one .... 

So I ask. 

The answer took a good 15 to 20 minutes. 

And I want to emphasize that these folks struck me as rather brilliant, so I'm still trying to digest this nugget ...

The consensus was:

A)  You don't want to live in a ziplock bag.  A lot of modern home designs are just messed up that way.  I think this is a good point.

B)  There is a lovely home in (I think it was ...) Sweden where there is a three inch opening along the roof line all the way around the house.  You have a thermal mass heat system.  Everybody is warm as toast.  The focus on heat is more about radiant heat ant conductive heat.  American heat is all about convective heat.  I'm struggling to digest this one.

As an example:  where we were sitting, it frosted outside and the building we were in was far from air tight.  Visible gaps around windows, large (one inch or more) gaps around doors and a large hole for the cat to come in and out.  It frosted outside every night.  And we were warm as toast with one fire per day.

And a lot of the projects that we did were to "play with fire" and realize that we (well, maybe just me) really know very little about fire. 

So ...  adding an outdoor air intake ... the final story was, yes, you could.  And it could help a little.  But not enough (in their collective, unanimous, opinion) to be worth the hassle.  Not even in the mountains of canada would it be worth fiddling with.

Page 90 of Ianto's book shows a method to get outside air into the house sorta pre-warmed via the roof...

...radiant heat vs the US standard HVAC method does seem counter intuitive.

Regarding exterior air intakes:
- I've also heard of cobbing an air intake into the thermal mass.  I haven't seen it personally, though.  Sounded good in theory.

Since this is a "Paul question," I'm going to post an exhaustive, thoughtful, and probably inaccurate theoretical answer.  With lots of components to consider.

Firstly: Please note that the experience of being in a cosy home with good ventilation and thermal-mass heat outweighs all theory.

I've had this experience in
- homes with passive solar heat in New Zealand and Oregon,
- in Cobville's cosy rocket-heated cottages,
- in southwest desert adobe homes,
- in California Mission architecture, and
- in my own living room. 

So I'm a believer.  Thermal mass has a bigger impact on heating and cooling a space than air flow.  Thermal mass can also be used in conjunction with air flow in very clever ways, with minimal power inputs. 

While air-heated homes can be quite comfortable, mass-heated homes can be equally or more comfortable, and tend to use a lot less energy.  Appropriate design is key; badly-designed thermal mass can make a space unlivable (like most concrete boxes anywhere).

External Air Source for a rocket stove (or any masonry stove):
"Why bring fresh air in for your stove, and breath stale air yourself?"

Advantages:
-Code Compliance: Yes, they may be required by local building code.  Will local building code approve a rocket stove in any case?  Investigate before installing.  Air intake may merely be required to be in the room or general area, and your home's roof vents or windows may already meet this code.
- Mental Control / Entertainment.  Separating factors.  Playing with things to see how they work.  You could theoretically seal off the system from the inside air, adding heat to your home without altering any other factors.  If your stove smoked back, it would allow you to seal it off and let it burn out.  You could control the air separately from the fuel.
- Fresh Air: Your home could benefit from added fresh air, especially if it's all sealed up with VOC-spewing plastic goop.  It may be better to let the air in elsewhere to ventilate the house, and let the stove exhaust 'old' air. 
- Draft: Wind configuration and pressure differentials are hard to predict.  Your house might not be able to "suck in" air fast enough to allow the draft to discharge properly.  If opening a window or door improves the stove's performance, you might consider adding an external air intake near the opening.  Or taking other steps to improve draft.
- Extreme Situations: This option seems to be more popular in very cold climates, or in other situations where home insulation approaches the theoretical extreme. 

Disadvantages:
- Not needed: Your breathing uses oxygen; you would die like a bug if your house sealed completely.  So if you're not dead yet, the house is probably already ventilated. 
- Invisible = Poorly Maintained.  If your stove is smoking back, something needs to be fixed, and sealing up the problem won't make it go away.  Clean or modify the stove until you have reliable draft.   
- Diminishing Returns:  It takes a certain amount of extra work, and creates new problems.
- Design Compatibility: It may be hard to design a vent that will actually feed the stove without interfering with its proper draft.  An improperly designed vent could increase the potential for smokeback into the room. 
- [Bad] Draft: a vent could just discharge cold air directly into the room in a chilly stream, whether or not the stove is going. 
- Hole in the House: Water can leak in through the hole in the building's cladding.  Condensation can be a problem, even with good seals, because of temperature differences.  All vents are liable to be used by vermin: spiders, wasps, mice, squirrels, rats, birds, or other local wildlife. 

"Why heat the air, and then send it out of the house through the stove..."
So many odd assumptions.
Why heat the air at all?  It's a lousy heat conductor.
"Send it out of the house through the stove..."
The hottest air is near the ceiling.  Most stoves draw air in down near the floor.  So you're sending out the cooler air from your home, and hopefully warming a lot more air (and thermal mass) in the process. 

If your stove doesn't warm enough air to replace what it "sends out," there's something wrong with the setup.

It's true that a conventional stove or fireplace can draft warm air out of your home even when not in use, and this is something to be avoided. 
  Most stoves have design details, dampers, or doors that can be closed to limit passive draft.  Closing the air intake lid on a rocket stove when you're done firing it, just like with a masonry heater, helps to conserve heat.

I like the one about burning the stale air. 

Thanks Erica!

I've been asked this a few dozen times and my answer becomes more and more refined.

Suppose that the house is a zip lock bag and the air intake is from the outside. And I come to visit. And I had chili for breakfast lunch and dinner for the last three days. Between my halitosis, burping and farting, you start to get an idea of the joys of living in a ziplock bag.

The air outside is 21% oxygen. And with me in the room, breathing up your oxygen, it might now be down to 10%.

And with the stove going, I'm starting to get sweaty.

Of course, if you heat the people using radiant heat, then you could open a window and still be comfy. And if your stove got air from inside the house, it could burn funkier air and pull cleaner air inside.

But if the heating strategy leans on convective heat .... well .... I guess you better just get used to it.

Paul, you have your own unique definition of 'Refined.'

For those whose concerns lean more to safety than smell-o-rama,

I've learned about another horrifying problem with outside air intakes, that I didn't mention earlier.
Apparently, many localities are starting to remove the requirement for outside air intakes, because in some circumstances it can directly cause house fires or smoke danger.

How's that, you say? Isn't the air intake designed to protect me from smoke?

Imagine, if you will, a modern couple.
They own a 3-story house, on the grid, all the mod cons. They both have jobs to pay the mortgage, and home insurance to protect their property value. They want to install a back-up woodstove for emergencies, in the basement, where it will keep the pipes from freezing if the power goes out. They are going to do this right, so it is an investment and not a liability.
They look into local building codes and find that $150 will cover a permit to install a woodstove themselves. They shell out $500 for a pretty stove with an energy-star rating. Chimney hardware runs another $600, for UL-approved fittings including a new liner for their existing masonry chimney. At this point they are somewhat over budget, but they are educated folks and have tackled other home improvements together; it's amazing what you can find in online DIY instructions. They have a handyman friend help them install the liner from the roof. The outside air intake is a piece of cake by comparison.
This being a basement, their outside air is up at ground level, but easily accessed through the existing light-wells. They take out one cracked window pane, and replace it with a little fitting for this nifty flexible tube, like a dryer vent. It's not nearly as expensive as the chimney fittings, because it's not rated and UL-lab-tested for fire: it's just for handling cool, outside air. They insert the tube into the appropriate port on their new woodstove, and it fits just like it was designed to. Cinch down the fittings, and she's ready for inspection.

Their stove and chimney parts have all their UL stickers in the right places. The inspector goes home happy.

And later that winter, the power goes out.

The owners get take-out on the way home from work, thinking, "Weren't we smart to install that woodstove!"

Now, the house's 50-year-old chimney hasn't been used in at least a decade, and the wind storm that took out the power has been pounding at it for two days, so it's kinda cold. It has been doing what masonry chimneys often do when unused: serving as an invisible cold-air intake into the house, to balance the warm air leaking from the roof vents, upstairs bathroom fan, etc.
As the owner goes to light the fire, the air pressure in the woodstove at this point is positive: cold air coming down the chimney, hot air expanding inside the stove. A little smoke trickles out as he struggles with damp kindling; he piles in more newspaper, and finally the fire takes. He shuts the stove door as more smoke curls around the opening.
Luckily, from the fire's perspective, the unwitting owners already provided a secondary escape route: a convenient upward tube, leading to the window well! It's like a quick-heating, secondary chimney!

Unfortunately, this tube was not designed to handle heat or smoke.
If we are lucky, all that will happen is some unsightly smoke marks on the exterior wall, around the 'air intake'/stove outlet, and then the masonry chimney will warm up enough to draft and draw air in the right direction. The masonry chimney is much taller, after all, and its new metal liner may heat up fairly quickly.
But if we are unlucky, the storm conditions may draw not just smoke but flames through the 'air intake' tube. Its material might melt, or expand and break loose from its fittings; or heat up enough to char its mountings. If we are very unlucky, it could get hot enough to ignite the old oil paint on the windowsill, and those beautiful, old, dry hardwood floors. At the miserable tail of the statistical bell curve, the inexperienced owner has shut the stove door and gone upstairs to 'let the thing sort itself out', leaving all this to happen unobserved, and the next thing they know will be the sound of sirens.

That's not nearly as funny as Paul's reason.

There are a lot of ways to avoid this extreme scenario. Just knowing about the possibility is enough that most people can take steps not to re-create it. Ensure the outside air intake does not lead upward from the fire. A separate air intake can open into the room, rather than the stove itself. Or you can choose to install the appliance on the ground floor or above, where a dedicated air intake can take in the outside air down low, and come _up_ to the fire. Better the occasional puff of indoor smoke, than a house fire.

There are good reasons to add more ventilation to a house. If opening a door or window improves the draft on any combustion device, then more ventilation will probably improve indoor air quality too. Kiko's clever heat-exchanger looks good to me; a simpler option is basically screened holes in the wall. (My own lazy solution is just to neglect a few seals in the floor or windowsills; these leak into the walls, and find other leaks in the rest of the building envelope. Thus creating a heat-exchanger that is more random, but less work, than Kiko's clever overhead tubes.)

I now feel strongly that it should not be mandatory to provide an outside air supply ported directly into a combustion device. This is simply dangerous in basements, where many appliances go. Code compliance should be a means to an end: safety for the building and its occupants. Code is a good reminder, but poor substitute, for experience and common sense.

Can someone point me to the " Kiko's clever heat-exchanger" that is mentioned?

Jerry Ward wrote:Can someone point me to the " Kiko's clever heat-exchanger" that is mentioned?

I believe it's toward the back of the book Rocket Mass Heaters, in the case studies - but I can't find my copy now to check. It's basically a series of tubes or channels, that enter the wall from outside the building down low, and come up between the roof rafters to open into the house. So the air collects some of the waste heat that tends to circulate up near the roof, before entering the house.

Another version that works for rocket mass heaters is to bring in an air vent horizontally, behind the bench of the mass heater. So it gets pre-warmed by what would otherwise be wasted heat making its way through the outside wall of the house. In this configuration, the outside air can serve as part of an insulative air gap that would protect a conventional wood wall from the warmth of the heater, providing coolth where it's wanted, while helping to keep the room air warmer.

In both cases, the air vent runs from low outside, to higher inside, so that it will flow as it warms, but is less likely to contribute to taking warm air (or smoke) out of the house in winter. If it brings in warm air in summer, you could always shut it off.
I think if your area has summer heat problems, you could probably come up with a cool passive-solar vent that would help cool the house too. Maybe plumb the vent to half-way up the wall, from a shady side of the house or a cool rock-well (pile of big dry rocks), and have another vent in the roof that you can open in summer to draft air out of the house to be replaced by stone-cold air from outside.

a quick question: I'm new to this so i hope i express this ok.
is it reasonable to have the feed and the heat riser (sheltered) outside the house and run the exhaust pip though the home. I understand that you would forgo the radiant heat from the barrel but wouldnt' have the barrel in the home and you would still get the advantages of charging a thermal battery.

If your needs are such then it might be reasonable. Hopefully you'd be able to use some of that initial hot radiant heat to do something other than heat the outdoor air! If you can pipe the exhaust directly without a "barrel" through the wall and into mass inside it will be a significant amount of heat kept inside although you will need a high temperature capable wall pass through scene (like excell) and the first bit of mass will be subject to high stresses. Perhaps you might enlighten us as to why you have such needs? canyon

My RMH design does include outside placement of the feed tube... but eventually to be partially enclosed/ sheltered and hidden from street view. The barrel will be inside of existing fireplace so that it can be hidden if we feel that is necessary, as well. The vent would also be inside of this partially enclosed shelter so as to protect from wind and to keep it warm.

Lasse Holmes wrote:If your needs are such then it might be reasonable. Hopefully you'd be able to use some of that initial hot radiant heat to do something other than heat the outdoor air! If you can pipe the exhaust directly without a "barrel" through the wall and into mass inside it will be a significant amount of heat kept inside although you will need a high temperature capable wall pass through scene (like excell) and the first bit of mass will be subject to high stresses. Perhaps you might enlighten us as to why you have such needs? canyon

Peter gallo wrote:My RMH design does include outside placement of the feed tube... but eventually to be partially enclosed/ sheltered and hidden from street view. The barrel will be inside of existing fireplace so that it can be hidden if we feel that is necessary, as well. The vent would also be inside of this partially enclosed shelter so as to protect from wind and to keep it warm.

It's an attractive idea.. Most rocket stove enthusiasts think of this one and/or try it eventually.. Unfortunately, it causes more problems than it solves.
To get the burn tunnel through the wall, it needs to be extra long, to make up for it, the heat riser needs to be EXTRA tall.. Remember, the heat riser needs to be AT LEAST twice as high as the burn tunnel is long. This'll place the top of the barrel dangerously close to most ceilings. Who wants that hot barrel towering overhead? Then there's the fire-proofing of the wall around the burn tunnel, the things get REALLY hot! And so on, and so forth..
In the end, it turns out that it's not such a good idea after all.

That and who wants to go outside in the cold to tend the stove?

Dale Look wrote:That and who wants to go outside in the cold to tend the stove?

Sure, but the trade-off is that the mess stays outside..
Good theory, no?

Re: keeping the mess outside

Same theory that led to massive coal-burning electrical power plants instead of coal-fired furnaces in the home.
And smoky wood-boilers that pollute worse than woodstoves, which are bad enough; anything that makes it easy to ignore a dirty fire, makes it easy to use a dirty fire instead of a clean one.

What if you had a system that actually minimized the mess, instead of trying to hide it outside?

If you don't want to have to deal with ANY mess from heating your home, options include:
1) passive solar design
2) moving to the tropics.

-Erica

hello hello im new two your forum but just thought id let everyone in on what im up to.im breaking ground in the mourn on an underground RMH.im seperating the system from the house by running pvc piping through the thermal battery then to the house.maybe a fan and radiater set up or basebord heat?not sure yet.putting the RMH and mass outside and under ground will fix alot of the stuff that people dont like about the RMH systems im thinking.id sure like some input on this,like what should i watchout for being that im underground in minnesota,frost?thanks in advance love the site,my new fav.

Robert,
Ya know, it seems that the list of "don't likes" about RMH's really comes from people who don't HAVE them.. After you build one and use it, the "don't likes" simply disappear. The only "don't likes" that can remain are along the lines of "I wish I could see the fire better" and maybe the "barrel is a little ugly".
What I can gather from your post, in an attempt to avoid the "don't likes" you are throwing away the BEST part of RMH's, the "do likes" or the "best likes".
Circulating air is the about WORST heating method! Sitting on warm mass is just about the best. A combination of the two is what you get when the RMH is in the house, if it's outside, you only get the worst.
If you put the RMH outside, you will need circulating fans, thermostats, and other gizmos to make it all work. You'll need electricity for the gizmos and when the power fails, you'll be left in the cold while a perfectly good stove heats an un-used building outside somewhere.
My advice, BUILD IT INSIDE AND SIT ON IT!

im back with blisters need a better shovel.kirk thanks for that advice but you missunderstood me or more likely i wasnt very clear of my intentions ,it was real late.im not circulating air to the house ,its antifreeze and im trying to figure a thermal siphon for circulation if theres no electric.damn blisters making it hard to type. i kinda thought that one thing people didnt like about the rmh was having a barrel in the house.but ive also seen the permit thing thrown about alot.if you are hiding this heater, easer to do it outside . i like the idea that when the fall weather starts coming round i can have my mass preheated and ready to go and if we get an early spring i wont suffer with over heat like a infloor.making a run to my garage also off the same mass.what pitfalls am i missing,besides the one im digging.my nabour asked if i was putting sharped bamboo sticks in the bottom,hes funny.thanks for the reply kirk im going find a better shovel and gloves ill be back later dig dig dig.

Ive just recently come upon the idea of rocket mass heaters, and as I was reading about it all the first thing that popped into my head was that the air intake should supply outside air. Secondly, the intake should have a check valve to prevent reverse air flow, as well as the end of the outlet pipe (chimney), to prevent cold air from entering into the system while not being used.

To prevent having to go outside to feed the stove, an access door could be placed inside,(with glass window to see the firebox) and the wood would alrerady be preheated to room temp.

Why should this not be done?

The comments about it being healthy to bring in cold air from outside are moot - in any case, heat is being lost and efficiency sacrificed.

I agree that fresh outside air is healthy, but in modern low (or zero) energy home design, this is already addressed by a regulatory air system.

I live in Switzerland - the standards for energy efficiency here are fairly high (highest in the world), and Swiss Minergie standards (above and beyond so-called 'energy star') call for a very tight envelope of the living area. External air infiltration is avoided at all costs.

The arguments i've seen against using outside air to fire the RMH seem valid - but are moot points in overall analysis of the heating system. Most of them seem based on inexperience (with using outside air), bad system design, or personal preference.

In this country, the idea of a thermal mass heater is commonplace. Nearly all old farmhouses have what is called a 'Swedish Oven' to heat the main living area - a tile covered heat sink on the back side of an enclosed wood stove. It therefore seems potentially easy (culturally) to incorporate RMH's into modern low-energy home design, but not with using inside air.

Beyond that, most Swiss do live in shared housing/apartment buildings, many of them burning wood and or wood pellets. Incorporating a modern RMH/Swedish oven into new or retrofit apartment construction would be wonderful - warmer, more comfortable for the residents and easier on the environment.

Please, can somebody explain why it would be bad to use an outside air source? In regards to the efficiency of the system - human health and fresg air aside?

I'm envisioning applications for mainstream use, and not some backwoods unscientific DIY pursuit. Those are great, wonderful i think - but this simple technology could be much more far-reaching and beneficial for the environment (use less wood, that is) if taken from the woods to the mainstream.

Swiss Perm : Welcome to Permies, we get your question a lot ! Obviously we are getting outside air, otherwise the fire would go out ! There is no reason why YOU
can't have an outside air supply to your Rocket Mass Heater, Also you have some valid points about air handling in general ! But consider the number of recommended
air exchanges required for health and safety in modern Houses and you will see that your outside air directed exclusively to Your Rocket Mass Heater RMH, will greatly
increase the amount of air you are needing to move, and the size of the Air handling equipment ! This comes directly from not allowing your heating system to be part
of the Air Exchange Plan !

You remarked that you expect that fuel wood would be saved by bringing in outside air, this means that you have an incomplete picture of the way a RMH works. All of
the exhaust air has been heated to a point above the possibility for unburned hydrocarbons to be left in the exhaust gas stream, therefor all of the hot exhaust gases can
be channeled through the Thermal Mass, so all of the Exhaust gases Will give up their heat to the Thermal Mass! Discharge Exhaust Temperatures are 1/2 that permit
-able for all other solid fuel burners. Instead of room Temperature air being Sucked through the RMH and lost to the outdoors, that heat is re-captured within the Thermal
Mass. and re radiated back into the room !

The fuel wood is gasified and burned more efficiently, more of the heat energy is recycled into the thermal mass, a fully charged Thermal mass Will contain ~20~25~ hrs
worth of heat energy to heat our homes with Zero exhaust Gases/Heat Energy lost 'up the chimney' during that entire time period !

Two things, Re-read Kirk Mobert's remarks just above, and Strongly consider going to Rocketstoves.com to Download your PDF Copy(s) Of Ianto Evans' Great Book
Rocket Mass Heaters, In it's Brand New 3rd Edition !

Then come back here with any questions, with over 25,000 Fellow Members you should expect to come here 24 / 7 and talk to someone who Wants to talk about what You
Want to talk about. With world wide Membership they will bring widely even wildly different points of view, they will stretch your mind as you stretch theirs !

Think like fire, flow like a Gas, Don't be the Marshmallow ! As always, your comments and Questions are Solicited and Welcome Big AL !

Thanks for your response, Big Al!

I see your point, but in minergie standard houses the air handling equipment already exists and is pretty much standard. I know most 'permie' types out there are against the use of electrical switches/fans/etc. - as they only seem to overcomplicate the system, and for most applications I would have to agree. But not when trying to plan out a super tight-sealed modern energy efficient home that is viable in the mainstream. Most 'zero-energy' homes ( in cold climates, that is) are incredibly tightly sealed, and use ground source heat pumps as a thermal energy supply. In my mind, the financial costs and impacts of the complex system may potentially outweigh the benefits, when compared to a RMH. ( over 50,000 franks, $55,000 US - for the installation of a geothermal heat pump in a one-family house).

I'll check out the books you mentioned - thanks again!

Okay, so there are risks to using an outside air supply for a wood stove or a rocket mass heater.

How about this scenario? This is not theoretical, it's my family's vacation home in the Netherlands. Basically a log cabin, except made of planks not actual whole round logs. No meaningful insulation on the inside. Since we're at the same latitude as Edmonton, Canada, and located in a clearing amidst tall trees, passive solar is not exactly a big heat source either. The house has two rooms at ground level, an attic, and a basement below around 1/3rd of the ground floor.

We're about to install a wood stove in the fireplace. And I'm considering asking the supplier to install an external air supply to the stove. This pipe will not be leading to a hole in the outer wall of the house, but to the basement. (The basement in turn is liberally connected to the outside through a few ventilation holes). So when the stove is burning, it will be sucking the air out of the basement, and outside air will refill the basement. Meanwhile, the floor between our living quarters and the basement is pretty tight.

The supplier is saying "Don't bother with external air supply. That's only for super airtight modern homes where your stove would not be able to pull in enough air through various cracks in the wall." The stove will "only" consume 20 cubic meters of air per hour, or one sixth of the volume of the room in which the stove is situated. So loss of warm room air is not the big winning argument.

Except I have a special reason.

The reason I want the stove to suck air out of the basement is that the basement is extremely moist. Although the house is built on a hill of sandy soil, its walls suck in moisture like nobody's business. If we don't leave the trap door open in winter, then on our first arrival in spring we will find a white mold covering lots of surfaces (including wood) in the basement.

Of course, ventilating basement moisture into the home can barely be called a solution. So the arrival of a wood stove is a big opportunity in this regard. My hope and expectation is that once the wood stove is there, we will be coming to the house throughout winter. And by using the wood stove regularly, and having it take it air from the basement, we will be ventilating the basement sufficiently to suppress the moisture problem.

Where we live, vacation homes are rarely made of wood and situated in the middle of the woods. They're usually brick and part of a recreation park. So I'm inclined to think that my wood stove installer may not be thinking critically when repeating his stock message that external air supply is only for airtight homes.

What would you say?

Eight years later, my position on air from the outside for a rocket mass heater is a bit more solid: