Complying with building codes.

In the UK we have a requirement in the regulations:

"the outlet from a flue should be above the roof of a building in a position where the products of combustion can discharge freely and will not present a fire hazard, whatever the wind conditions."

any ideas how this can be dealt with?

It's the only bit about RMHs which is difficult to accommodate.

Roy Clarke wrote:In the UK we have a requirement in the regulations:

"the outlet from a flue should be above the roof of a building in a position where the products of combustion can discharge freely and will not present a fire hazard, whatever the wind conditions."

any ideas how this can be dealt with?

It's the only bit about RMHs which is difficult to accommodate.

It's only difficult to accommodate because the through-roof chimney fittings are so expensive. We've done at least three systems recently using up-to-code through-the-roof fittings, including double-wall and triple-wall just like for a woodstove, the 'witch's hat' roof cap, and the 2-feet-above-everything-within-10-feet guideline to get it up into steady winds instead of the laminar flow down the roof. In all cases this incurred a cost double or triple the entire rest of the materials for the installation. In some cases, ten times greater, as we had scrounged everything else.

In order to exhaust vertically, the trick is not to let the exhaust drop below the dew point, so you do have to sacrifice a little more of the heat to the outdoors. We keep the heat exchange runs a little shorter (25-30 feet has worked fine), and we bring the last pass of duct back past the barrel so it picks up a little heat that would otherwise be lost to the exterior wall, and has enough rise to draft upwards and out. About 100 F has been sufficient in our experience for the actual heating season, but masonry heater guidelines state it should be a minimum of 90 (presumably c), close to 200F, which I think would give you draft in summer as well.

You can also go through whatever the UK version of the local appeals board is, and make a case that the requirement to spend an extra four figures on something whose exhaust is essentially fire-extinguisher fog, is an unreasonable hardship.

Here in the US we have propane heaters that actually exhaust into the home, as do natural gas kitchen stoves with their open flames. Do you have these in the UK also?

A RMH with a long heat-exchanger (40 feet) tends to exhaust steam and CO2 at 60-90 F (18-30 C). Our cutoff for a 'low-temperature' exhaust is 110 F or so (40 C).
So you can try to make a case that the exhaust is clean, low-temperature, and no more troublesome than what comes out of a natural-gas clothes dryer, and can be vented outside through any wall that does not discharge into a walkway or nearby air intake. Protective vent-caps or Venturi tubes can be used to ensure that wind cannot blow the exhaust back down the pipe. There are no wind conditions imaginable that could cause steam and CO2 to light something nearby on fire: these gases are the contents of type A fire extinguishers. I tell a lie; you must ensure that your neighbors are not farming metallic sodium.

As far as chimney fires: The long channel through which this extinguisher-like exhaust must pass slowly, at low temperatures, makes a spark very unlikely, and the heat of the initial burn makes creosote unlikely - even when a foolish person or pigheaded experimenter (like ourselves) burns wet and green wood, we only see a small amount of powdery soot.

I would not exhaust one into a home or greenhouse. The initial 3 minutes of a burn cycle do produce some smoke, and there is no amount of smoke that it is healthy for a person to breathe. But a horizontal discharge into a suitable place outside where wind can disperse the exhaust, should not be a problem for anyone who has experience reading smoke and steam and telling the difference.

I would not be surprised if your officials are inclined to stick with standard operating procedure, however. It takes a fair amount of money to get one of them to stick their neck out and say it's OK to do the simpler thing. The law you quote may be as old as the Great Fires that took out swaths of London, when coal-burning furnaces were putting live coals out of their smokestacks. These things tend to stick in the memories of officials charged with public safety.

Around here, the officials do enjoy being called out to see a nifty and mind-boggling thing that is much safer and cleaner than the standard options, and will go away shaking their heads and wishing it was within their authority to say Yes.

We have found that it is possible to build these heaters entirely up to Masonry Heater building code, and your country has more familiarity with the masonry heaters, and with earthen masonry, easing your path. This can be though of as a poor man's version (or an Earth-friendly and highly efficient version) of a very conservative technology. There have been several rocket-type masonry heaters built in England to our knowledge, and I believe all the ones we have heard about vent up above the roof.

Hope that helps,
-Erica

Erica Wisner wrote:
Here in the US we have propane heaters that actually exhaust into the home, as do natural gas kitchen stoves with their open flames. Do you have these in the UK also?

Yes, we have them. I used to run one until I saw how much water ran down the walls.

I tell a lie; you must ensure that your neighbors are not farming metallic sodium.

Drat! I knew there'd be something that would stop me.

I have read through the rest of your post. It certainly does help (as do all of your replies). I wish I had done more with rocket stoves years ago, but you can't do everything at once. Thank you for such a comprehensive reply.

I appreciate the questions.
If you do install a code-compliant rocket heater (especially if you use earthen masonry or recycled components instead of/in addition to the more highly-processed refractory components), would you be sure to post the results? With maybe a proud-papa photo or two?

This is one of those projects we are working on, but it's an uphill battle in the Western US where you have to start by explaining what a masonry heaters, and that it's neither a woodstove nor an open fireplace, and research the applicable codes for building officials who are unfamiliar with them. A few officially-approved installations in regions with a strong vernacular masonry tradition could encourage approvals here. So many of our homes are owned by the banks, it would be nice to get them sanctioned as a capital investment and not an insurance liability.

-Erica

I have a few questions related to installing an RMH in my basement, utilizing an existing chimney:
Local codes required the chimney be relined prior to re-use, and the standard liner material is flexible stainless flue liner). The Chimney is approximately 40' tall (basement plus 3 floors). It seems from other comments that I will need to keep the flue gas hotter (+- 200 degrees farenheit) to prevent condensation, overcome the friction caused by the rough interior surface of the flexible flue liner, and assure a proper draw. Will this keep the condensation outdoors altogether?
The way to keep the exhaust gas hotter appears to be a shorter run of heat duct in the thermal mass. I was thinking maybe to have the RMH away from the chimney and having one straight shot exhaust flue thought the thermal mass instead of the out-and-back designs that transfer more heat.
I was planning on building a fire brick heat riser in a 55 gallon drum, do I need to insulate the firebrick riser? Will the square heat riser in the round drum screw up the internal air flow because of "pinch points" at each of the four corners?
Will "rising damp" from the basement slab be a problem with the cob construction? It is an old house and has no underslab vapor barrier.
Thoughts?
Advice?
Thanks
Glen

Do your codes really require a new liner? We have people telling us that in the UK, but it is not always true. The flue can be tested for leaks, and if sound, a liner is not always needed depending on the fuel used.

Glen please read the RMH book. (library is a good place to get it) Or buy plans and read this whole forum. Every question you have just asked is here. OR take a workshop. Or Pay for us to consult with you privately. We have lots of options for you to choose from.

I was wondering about this myself. It would be very hard to get an insurance company to approve it since it's not UL listed. And there is no way to regulate or shut off the burn; no air intake regulator or damper downstream. And the barrel heat chamber would be prone to rusting out in a few years and would need a way to be changed out quickly and efficiently without having to chiesel it out. And the flue. They are going to want to see a clay tile lined flue, espeically if it is over wood. And I don't know about it running sideways without any or hardly any rise. I know from experience on a regular flue they want triple wall stainless steel or the clay liners inside the flue that funnel the flue gases into the next joint so that if the mortar cracks or falls out the flue gases will still be contained. I would think the thermal mass would develop cracks over time or you would have to at least expect it to. I don't know how long the galavanized 6" stove pipe will last encased in the thermal mass, but not permanately. From our King circulator heater to the flue we would change out the blued pipe every few years.

glen murry wrote:I have a few questions related to installing an RMH in my basement, utilizing an existing chimney:
...I was planning on building a fire brick heat riser in a 55 gallon drum, do I need to insulate the firebrick riser? Will the square heat riser in the round drum screw up the internal air flow because of "pinch points" at each of the four corners?
Will "rising damp" from the basement slab be a problem with the cob construction? It is an old house and has no underslab vapor barrier.
Thoughts?
Advice?
Thanks
Glen

The insulation of the heat riser is an "RTFM," I'm afraid. Insulation is critical to both the clean burn, and the thermosiphon properties that let these heaters work as they do.
If you don't have insulation around the heat riser it is not a rocket mass heater. You can look up "contraflow" stoves for some workable designs that are all-brick instead; these are more technical to build and seal, but time-tested. Contraflow heaters built by an experienced heater mason are usually already covered under existing building codes.

The rising damp should be handled as for any cob construction. Mike Wye in the UK has good damp-detailing advice for cob and traditional (natural) masonry in general.
We often do a layer or so of dry-stacked stone (or reclaimed concrete) above a potentially-damp surface, allowing ventilation to replace or augment vapor-barriers that can promote condensation.

Adam Smith wrote:I was wondering about this myself. It would be very hard to get an insurance company to approve it since it's not UL listed.

True. That's roughly a $60K process I'm told; we will let you know if we get any closer.
For now, they are best approached under the masonry heater section of the international residential building codes, R1002, and the ASTM standard E-1602 for masonry heaters.

And there is no way to regulate or shut off the burn; no air intake regulator or damper downstream.

Most masonry heaters use either a fire-end shutoff, or a top-of-chimney shutoff, not both. Having two shutoff dampers makes it harder to successfully operate the heater without inadvertently closing the wrong damper at some point in the cycle. Our J-style mass heaters usually use a simple, adjustable shutoff by sliding two firebricks across the feed opening, allowing any control from 100% open to 100% shut. Adequate air for a clean burn is roughly 20 to 30% or more, depending somewhat on the fuel load. The wood itself often regulates the air with a full firebox, but the firebricks (or a door configured to give the same range of open / 25% / shut) offer a greater sense of control.

And the barrel heat chamber would be prone to rusting out in a few years and would need a way to be changed out quickly and efficiently without having to chiesel it out.

The oldest rocket mass heater we know about is one of Ianto Evans' heaters in their Myrtle library-cottage; it has been operating regularly in a temperate rainforest climate for over 20 years - probably approaching 25 or more now - without showing any signs of needing the barrel replaced. You may be thinking of the poorly-designed barrel stoves featured in Mother Earth News in the 1970's, which burn a fire and re-burn combustion gases directly inside the barrel, and tend to catastrophically fail after 3 to 6 years of use. Explaining the difference between a rocket mass heater's use of unlined steel as a downdraft exhaust channel, symmetrically arranged to minimize heat shock and strain, compared with the unsafe use of thin steel as a homebuilt woodstove firebox, is a critical tidbit to bring to any conversation with an experienced backwoods building-code official. Getting a 1/8" steel cylinder fabricated and lined with high-temp (engine block or stovepipe enamel) paint may be necessary in areas that have extensive bad experiences with the other type of barrel stoves.

And the flue. They are going to want to see a clay tile lined flue, espeically if it is over wood.

A clay lined flue, or firebrick lined flue, would certainly reassure building inspectors that you are sticking with refractory materials, and contributing to the global economy, and should be considered an upstanding citizen or at least someone worth stringing along for more building permit fees. It is possible to build rocket mass heaters with clay flue liners, or for that matter clay drainage pipe, in place of the steel channel liners.
I personally consider the metal liners add a nice sort of resilience to any project where structural cracking is a concern, and might actually make the whole thing more leak-proof, especially if it's over wood.

I don't generally recommend building masonry heaters over wood, by the way. I'll talk about how it's been done, but I prefer non-combustible footings and weight support all the way down, and the codes generally insist on them.

And I don't know about it running sideways without any or hardly any rise.

It works because of the other elements of the system. Some, but not all, other masonry heater systems can also support horizontal channels - k'ang, hypocaust, and Russian masonry heaters do this routinely, and most other designs have the option of at least a small heated side bench even if the main path is an up-down contraflow.

We do often include a slight, constant rise to our pipes, mostly to even out potential hot spots and allow any condensation moisture to drain out safely while the heater is initially being fired up and dried out, before the mass is fully raised up to working temperatures. In projects where true horizontal was necessary for thicknesses or clearances or bench heights, it has worked fine.

I know from experience on a regular flue they want triple wall stainless steel or the clay liners inside the flue that funnel the flue gases into the next joint so that if the mortar cracks or falls out the flue gases will still be contained. I would think the thermal mass would develop cracks over time or you would have to at least expect it to.

Properly built earthen masonry has a strong resistance to cracking, high fiber content in the outer casing or plaster layers to further prevent cracking, yet can easily be repaired if a crack does develop due to un-anticipated thermal expansion or building settlement. You have to do test batches, and/or use a builder with a good working knowledge of local materials and their limits, or be prepared to repair cracks as you learn.

Most masonry heater builders work continuously to minimize the types of strain that cause cracking, and also advise their clients of what type of crack is "serious" and warrants a follow-up call. Anything bigger than a dollar bill, wider than a credit card, or visibly leaking smoke or smells, is a serious crack and should be repaired, and the original builder would likely appreciate knowing about this failure even if someone else does the repairs.

The simplest repair for an earthen masonry / earthen plaster surround that is cracking due to thermal expansion around the barrel is: run the system hot. Moisten the crack area with a damp rag or sponge. (Remove any loose material and re-work it to a putty-like consistency using water. If adding new material, paint the sides of the crack with a little wet clay slip. Fill the crack with new material if needed, making sure the plaster has good sand and fiber content to prevent shrinkage cracking.)
Use a small trowel, plastic paint spatula, or cut-off yogurt lid, dipped in water, to work the damp material together and fill in the gap, smoothing over the joined areas as needed. Continue running the heater until the wet material has dried and re-set, forcing the cracked area to take the expanded shape that is large enough to prevent further heat cracking.

We find this process so simple, and so rarely necessary, that we don't generally bother to use the standard masonry heater trick of adding a layer of flexible non-combustible insulation or gasket around the edges of any metal embedded in masonry. Our metal parts are round, they're not that hot at the juncture points, and the material tolerates being test-fired while still pliable so it can set its own expansion allowance. We do use insulation around the firebox and this helps prevent cracking of the cooler masonry casing around that area.

I don't know how long the galavanized 6" stove pipe will last encased in the thermal mass, but not permanately. From our King circulator heater to the flue we would change out the blued pipe every few years.

So far, the same stove is going 20+ years on the same crappy, reclaimed, part-galvanized and part-rusted-out-stovepipe that it was originally built with. The pipes essentially serve as formwork for properly-built earthen masonry, and once the masonry hardens whether the pipes corrode is almost irrelevant. I have helped to troubleshoot a number of owner-built stoves in difficult circumstances, and short of soaking the whole thing down and standing directly on the pipes (to water overhanging plants in a greenhouse, or while building with a big inexperienced crew), the pipes have simply not been the problem. Bottlenecks in the manifold, lumps of cob or piles of fly ash working their way into unanticipated places, a cracked brick where the fuel abrasion and heat stress are greatest, chimney or house-draft problems, heat riser insulation, air leaks in the lower firebox or manifold, vermin nests due to unscreened exhausts... there are lots of common problems, but pipe failure just isn't one we've seen much.

If you do use metal in the firebox or direct flame path, as I suspect was the circulator's situation, you can expect warping and degradation.

In earlier projects where we used metal forms stuffed with perlite-clay insulation to make a cast-in-place poor-mans-refractory heat riser, we understood that the inner metal would ultimately fail, and in fact our Annex 6" heat riser turned into little rusty flakes for the bottom half of its height before 2 years went by. The clay-perlite insulation fired into a rough ceramic, and is still doing duty. I no longer recommend using any metal in the firebox or heat riser interiors, with the possible exception of Peter's secondary air and some of the pellet-hopper grates, because it can potentially damage the masonry when and if it starts to seriously warp and degrade.

These concerns sound like they're coming from experience, but not with this particular type of heater.

-Erica