This started as a long response to an email question nobody here has seen.
Despite the pedantic style, I'd welcome input from anyone in the building, HVAC, or fire-safety industries who has counter-arguments, or points to add. This is basically a way to get a common question/rant off my chest, and make my opinion available in full to those interested.
Thanks for your kind words!
I agree with most of your points ...
The possible exception would be outside air for combustion.
I think this is one of those things that sounds good in theory, but for an efficient stove that doesn't draw much air in the first place, it can be a waste of time and effort, and introduces unnecessary complexity and possible hazards into certain projects. Houses must exhaust/exchange about 1/3 of their volume per hour, minimum, to keep the occupants healthy and the structure sound. If your stove is pulling significantly less air than that, for a small part of any given 24-hour period, it may simply be considered as providing the additional benefit of a little extra fresh air. To retain stale air in the home, while feeding fresh air to the fire, can reduce indoor air quality, and can lead to safety problems such as those below.
Negatives that crop up with outside air:
- Almost impossible to do "correctly" for any wood-burning device installed in a basement or below grade. The need to drop the outside air intake down to the fire increases the risk of the fire reversing and using the air "intake" as a shortcut chimney. Where outside air is made mandatory, basement emergency stoves are effectively illegal to install. Not that I like basements as a
rocket mass heater location particularly, but the option of make-up air is a much safer alternative and this is one reason not to categorically support outside air requirements.
- All air intakes are susceptible to this kind of fire reversal, especially if they are intended to address an existing, known problem involving house pressure/negative pressure/wind pressure. Fixing the negative pressure problem seems like a much more important way to deal with the situation, perhaps with an air-to-air heat exchanger that ensures an adequate supply of clean, fresh, warm air to the house despite anything the stove, vent fans, or natural house convection can demand.
- The outside air
feed makes it possible to run the fire with a closed door between it and the room, limiting smoke. This appears to be a benefit, but then, what happens when you open the door to feed the fire? Almost all outside air designs that I've seen will increase the likelihood of trapped smoke being released into the room, simply by providing additional air currents and lessening the draft's ability to pull room air into the room through the large door opening. Especially if the outside air feed is compensating for an existing pressure imbalance, that pressure imbalance combined with an outside air source near the fire can allow the house to serve as a second chimney, effectively drawing smoke into the house as well as the fire's chimney. Trapped smoke in the presence of flame is a risk for flashback; so smoke coming into the room when you open the feed door is both a respiratory and a burn hazard. The best way in my
experience to ensure that this doesn't happen is to design a stove that consistently draws
enough air with the doors open to overcome any tendency for smoke to be released into the room, and in parallel, to build the operators' skill at understanding and preventing the pressure conditions that could lead to smoke-back. Outside air relieves the operator of responsibility when the door is closed, and makes his job more difficult with the door open.
- For J-type rocket mass
heaters especially, the process of installing one successfully is complex. Air intakes into the lower part of the firebox, a simplistic first guess, allow the J-style feed to act as a second chimney and smoke copiously into the room. Outside air must be provided consistent with the firebox design, in the intended air-feed area, in a very limited range of horizontal elevations so that the outside air neither draws heat upward, nor fails to reach the
wood evenly. Most designs that could plausibly work will also make fire-tending and firebox-cleaning more awkward. They can also make it more difficult to close off the stove after operation. In short, the outside air feed prototype I have in mind arguably makes the room colder than it would be without it.
- I have also heard about outside air feeds being a source for overheating or dangerous hot spots on previously successful fireplace designs; the example I have in mind was described by Jim Buckley somewhere on his excellent website www.rumford.com.
Outside air sounds good in theory, mostly to people accustomed to thinking of air as the thing that holds heat in the house.
In a mass-heating situation, air's low heat capacity makes it a minor consideration compared to the heat stored in the mass. The warmth in the air that you conserve by all this complicated building will not last 3 hours; it will be exchanged with outside air in the natural ventilation of the building. The warmth in the mass will last for up to 3 days.
Making the mass heater easier to build and operate seems like a much more important priority than any small increase in air-heating efficiency that could be gained by the complex process of properly supplying outside air in all building-site situations.
The distilled question-to-answer-this question from several
rocket mass heater builders at a Pyromania! event in 2010 was:
"Why would you want to live in a Ziploc bag, breathing stale air and farts, and feed the fresh air to the fire?"
Peter van den Berg has had good results with pre-heated secondary combustion air drawn from the room, and his designs are worth looking at for any secondary pre-heated air feed.
The amount of air and secondary air needed for these heaters is small enough to be a miniscule consideration in any reasonable building-ventilation scheme. I do not think drawing it from outside makes enough difference to be worth the trouble.
Yours,
Erica W
That was the initial response.
In fairness,
should we add the pro's of outside air, especially pre-heated secondary air to the firebox, as my original correspondent suggested?
Pro:
- It makes the stove independent of the house air pressure situation, especially if the outside air feed is designed as a fully heat-proof Class A chimney and the stove can run backwards safely. You can shut the fire door and have a gymkhana or a baby or whatever, and the worst that's going to happen is your house gets cold or a creosote fire in the supposed in-pipe of your outside air feed.
- It "saves" a certain amount of the heated air in your house, and may reduce cold drafts or negative pressure problems that could plague other appliances. Let's look at this in two parts:
a) - It "saves" heated air: - I don't think so. If you are pre-heating cold outside air to heat the air supplied to the firebox, that heat has to come from somewhere. If you get it up to hotter than room-air temperature before it reaches the firebox, then you are chilling whatever part of the heater you ran it past, and that means less heat is reaching the room from that part of the heater. I don't think you're saving heat, just segregating your air supply according to where it most recently came from. If you are used to a woodstove that makes things too hot near the stove, and too cold in other parts of the house, the idea of locating your cold drafts close to the stove itself could have some appeal. But again, using make-up air (supplying fresh air to the room) seems just as efficient as outside air, and immensely easier.
b) - If you have a well-drafting stove, it could be drawing enough air from an over-sealed house to impact performance of other appliances. In this case 'well-drafting' means 'drawing a lot of air,' and a
rocket mass heater is more likely to be operating efficiently when it is nearly-balanced draft, drawing a very limited amount of air just sufficient for clean combustion. But if you have a big one, it would certainly need to draw some amount of air, granted. And the near-balanced draft ones are susceptible to negative pressure if they're operating in near-balance conditions, such as warmer weather. Whether the stove draft would impact other appliances is addressed below.
- Secondary Air Yes: If carefully designed, a secondary air feed (not necessarily outside air as such) has proven to be a very good tool for complete combustion, making many batch-box or simple stove designs burn cleaner and more efficient. Since simple stove designs are much easier to build, operate, maintain, and repair/replace, anything that makes a simple stove design work better is a wonderful thing. But a design with secondary air is necessarily a little more complicated than one without. It's a choice.
One requirement to successfully run secondary air of any kind into a
rocket mass heater, and especially a J-type firebox, is as Peter puts it, "the whole system operates at under-pressure." That means it's drawing from the exit chimney end, not pushing from the heat-riser. If the secondary air relieves the draw at the feed on a J-type, you can get smoke coming up out of the feed. So in order to use secondary air successfully, you may need a little extra draft in the system as a whole, and you may need to limit the primary air, and you need to know just how much secondary air your system can pre-heat and suck into itself before it starts having too much air to deal with. To know how far to go on this, you may need a combustion analyzer or a proven set of plans. Peter's made his results permies, so that's a good option for getting the dimensions for both air feeds / total air intake. Our plans don't include secondary air, and are arguably easier to build for a novice builder.
All that leads to more cons:
- If your house has enough negative pressure problems that an air-sipping
rocket stove is an issue (as compared with, say, a wide-open woodstove or fireplace), your other appliances probably have problems already. Appliances burning natural gas may not exhaust or burn properly in negative pressure, and could be dumping CO into the house through leaks in their exhaust or backdraft from their combustion areas. You may actually need to increase the fresh air (make-up air) to the house itself, and using the heater to move more air volume through the house at intervals could be one way to do that.
-
Any negative house pressure problems that are "solved" by outside air will likely re-appear when the room/fire door is opened to refuel the fire. The outside air supply may in fact increase the risk of smoke entering the room, and the need for room air flushing / added ventilation. You might as well just increase the ventilation with a sensible air-to-air heat exchanger, or resolve the negative pressure with better attention to upper-story weatherization, which is what the negative house pressure indicated was needed in the first place.
-Outside air feeds are not usually made operator-accessible to control the rate of air flow; if oversized, they would also contribute to the fire running inefficiently, effectively exhausting more heat even if the air itself wasn't heated room air to begin with. (
All that air has to be pre-heated by something, so isn't the same effective amount of heat leaving the home?) Peter's secondary air feeds are nicely calibrated to the fire and come in from above, so they can't be blocked by falling
ash or collect sparks. But this is not the way that code allows for outside air, it must be from below (due to backflow danger from running hot, see above).
- Making the stove independent of the house may release more of its heat outside the house, reducing efficiency. You also need to ensure that the exit chimney always draws better than the outside air feed (it should anyway), so the system will automatically siphon/draw no matter whether the fire is burning. If the stove is less efficient, then you to need to burn more fuel for longer hours to make up for the inefficiency. Ignoring the stove could lead to not shutting down the outside air when the fire goes out, and then your warm bench and exhaust chimney work as a thermosiphon chiller, drawing cold outside air through the firebox and thermal mass to un-do all that your fueling hours have done.
While it's also possible for your stove to draw after the fire goes out with a room-air feed, it will not chill itself as quickly due to drawing room-temperature air, and it will likely slow down sooner due to the built-in U-locks in the J-type thermosiphon design. And the operator will more likely still be present to shut down the air, as they are not thinking of the stove as "running on automatic". And the air controls to fully shut down the stove are easier because there's less infrastructure in the way - just slide a couple bricks over the fuel feed, or close the purpose-built lid all the way.
- An outside air feed needs to be fully close-able in order to avoid the runaway chiller effect above. To design an outside air feed so that you close off the outside air and the stove with one motion, yet when you go to re-light the stove you cannot inadvertently run the stove without access to any air at all, takes planning. Making it foolproof takes a level of telepathic understanding of the human foolery that even God does not evidently possess.
- More complexity means more points to fail. If you automate the outside air to smartly close itself when the fire goes out, how would you do that? Thermal probes for fire temps are costly, and not 100% fail-safe; would you know when they failed? would all operators know how to operate the system in their absence? A simple pressure-sensor like a balanced one-way flap valve would mean you'd need to make the heater draw far more forcefully than current models, which would likely lead to (or be caused by) making it draw harder and exhaust more heat, making it less efficient. You'd be putting more heat out of the building in order to conserve heat.... and at greater risk of running the stove as a chiller if the flap failed to close properly after the fire went out. These are fail points for a workable system; there are also the afore-mentioned very dangerous fail points for a poorly-executed system, such as the fire running backwards or dropping sparks and embers into a poorly-installed outside air feed, or the possibility of dumping large amounts of smoke and flame into the room when opening the door to refuel the fire. Even the less dangerous points like the poorly-executed outside air feed causing more cold air leaks than it solves are a very likely possibility.
I'm not saying it can't be done. It has been done, and done well, by skilled stove builders and designers.
But I question whether it's always a good idea.
Outside air should definitely not be made mandatory, especially if it hasn't been a popular and well-tested option up until that point in the
local area. (I resent our collaborators adding a last-minute outside air requirement to the Portland
rocket mass heater guidelines, for example; at that point, I do not believe there was a single working prototype known to anyone on the committee that had successfully used outside air, let alone proved it would always fix the concerns they had in mind.)
And I wonder if outside air might sometimes be the last
straw that pushes a particular
project into the realm of experts-only, or causes a carefully-considered project to operate less successfully than it otherwise might.
Another opinion novella from yours truly.
-Erica W
Edited because, well, I do that. If words are good, more words are better, yes?
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