Tuesday, May 28, 2019

Air, Code, and Rocket Mass Heaters

Another rocket mass heater successfully passed inspection in Washington state in 2018.  We are getting further details from the owner-builders, and I've asked for permission to share them publicly.

For now, I can say that they followed their local building permit process, and it all went quite smoothly.  They submitted plans to the county; got their permit including some detail on ASTM E1602, R1002, and state code requirements; followed those plans and the recommended clearances and thicknesses; and had no difficulties with their final inspection.  The building official brought a fire marshal along to see the thing.  Both had done their homework and were favorably impressed by the whole project.

The only help we provided was the initial drawings, and one hands-on session on "cob and natural plasters" to help them get a feel for earthen materials.

Congratulations to everyone involved. I hope we'll have permission to share some pictures and more details here soon.


One of the things that did not occur in this case, thankfully, but still sometimes comes up in other jurisdictions was an obsolete local requirement for "outside combustion air."

Most experts now agree that outside combustion air is ineffective at its intended purpose, and experienced builders often add that it can be downright dangerous.

This is the most cogent explanation of why outside  combustion air is ineffective that I've seen yet:


Here are a couple of quotes:

"An important distinction to make is spillage RATE and spillage SUSCEPTIBILITY. Tighter doors will reduce spillage rate, but not spillage susceptibility. Outside air can also be shown to not reduce spillage susceptibility, since it does not lower firebox pressure, relative to the room - this is the function of the chimney (if outside air lowered firebox pressure it would, in fact, be a chimney - not a good thing.)"
 [My emphasis]

"Like most building codes in North America, the NBC had included outdoor combustion air requirements for combustion equipment on the assumption that it was a good strategy to reduce spillage susceptibility.  Unfortunately the assumption was acted upon before any research had been done to explore how outdoor air supplies actually behave.
Although the two studies were conducted by two labs with different set-ups, different protocols and different appliance types (1. factory-built, 2. masonry), they arrived at the same conclusion: The susceptibility to combustion spillage due to room depressurization is not affected in a predictable way by the presence or absence of air supplied from outdoors, whether supplied to the combustion chamber or indirectly through a supply duct terminating near the fireplace.
In both studies the reference room depressurization at which spillage was induced was 10 Pa.  In 'Fireplace Air Requirements', none of the five tested fireplaces spilled at 5 Pa depressurization despite the fact that all were very different in their configurations and features, although all did have glass doors.  The tests at the two depressurization levels were done with and without outdoor combustion air supplies.
Once the research findings were in and analyzed, the underlying physical process became clear:  That is, air flows to a zone of lower pressure through any available opening, regardless of our wishful thinking.  In retrospect, this principle appears rather obvious, although for most of us it was not, until revealed in the lab."

From other sources and builder discussions, some of the intrinsic dangers of outside combustion air:

There is a real risk of the air inlet acting like a chimney.  This is even more likely in cases where the outside air is being brought into a basement, or atop an existing slab, and the inlet hole is above or on level with the firebox. 
  A wind gust that catches the exit chimney just wrong, or a cold plug in the chimney, can push air/smoke downward.  While this is unpleasant and dangerous at any time, in cases where outside air is installed, it more or less allows the house a two-way path to "fart fire."  If the wind situation on the exit chimney were particularly ill-fated (such as a chimney improperly installed in a "wind scoop" area below some nearby buildings or roof shapes), the outside combustion air inlet could potentially create a stable new pathway for the downdraft to continue unobserved. 
  Imagine flames, hot smoke, and embers rushing through the outside air inlet (which in many early examples was no more fire-resistant than your average dryer vent, made of flexible aluminum duct that could melt if exposed to direct flame for more than a moment).  The flames and smoke will at best emerge from the vent inlet in the crawl space; at worst, they will destroy some element of the air inlet and create a new opening to escape within the walls or home.
  There have also been problems with heat conductivity of airtight (metal) materials used to create the outside air; sometimes to the point of damaging combustible materials used for chimney-chases and exterior finishes.  Experienced fireplace builders have been surprised at the amount of heat transmitted backwards by an outside air inlet, which may reduce the customary thickness of masonry floors and walls where it is installed.

  Outside air is usually brought to the bottom of the firebox (it being an obvious problem if it is brought into the top like a chimney), which means embers and hot ash can fall into it, heating it and/or clogging it. 
We are not big fans of "ash cleanouts" generally - complicated trapdoors that are neglected more often than not, with fiddly little doors prone to failure.  A combination cleanout/air supply is almost guaranteed to not work well for either purpose over time - if it's collecting ash, it's clogging the air supply, and you definitely want that air supply to continue right through the end of the fire.
  We generally prefer a smooth, easily cleaned, easily observed firebox floor that keeps ash buildup where owners can easily notice and deal with it during routine operations. 
 Inaccessible cleanouts that fill up with ash, and then collect any dribbling creosote, can create a real mess that doesn't need to be there.  As a fire fighter, I've worked in a crawl space below a chimney fire, where over 4 feet of dead chimney space led to a crawl-space cleanout that was very difficult of access, and almost undetectable to the average person.  The door was warped, and a previous occupant had duct-taped the warped part. 
In this case, the whole mess inside the chimney was on fire, to the point where some of the chimney masonry was starting to conduct heat to framing members as secondary ignition points, and the cheesy cleanout doors made it difficult to cut off the air to the chimney.
I much prefer a little honest mess, with a tidy metal ash bucket in the room next to the fuel opening, over a hidden and dangerous mess that nobody will see or deal with until it's too late.
Likewise, I'd much rather feed my stove from the comfort of the living area, and have my stove exhaust stale room air and help bring fresh air in for me to breathe, than try to separate it from the house and go outdoors to check its inlets and tend its needs.

Finally, many owners and builders point out that outside air retrofitted to well-designed fireplaces, stoves, or heaters can substantially increase the experience of smoke in the face (and in the room). 
  Nobody likes smoke in their face.  Our ancestors did a considerable amount of tweaking of the indoor hearth to ensure it didn't happen.  One of the biggest drivers of rocket stove development is the reality that many households still have open hearths for cooking today/
  In the industrialized world, for the last several centuries, we have used chimneys to solve this problem.  A hot chimney pulls room air through the stove and out.  Most heaters, stoves, and fireplaces that have been developed since chimneys came into use have openings sized and/or shaped to allow this room air to carry smoke along with it, even when any doors are open for fueling.  Now if you swing a door open quickly, of course you can "pull" a gust of smoke into the room, but most people can learn to avoid that within a few tries.  (We even managed to live with hoop skirts and open fireplaces for a considerable amount of time, albeit with more tragic lessons in many families.) 
  In case of a Rumford fireplace, a "curtain" of room air continually sweeps smoke back into the chimney throat.  In other cases, the whole feed opening may be sized so that the chimney can pull enough air to fill it.  Glass doors on fireplaces may be double-hinged to reduce the "fan" effect, and allow users to control the opening size.  
  Now, if there's outside air coming up in the floor of the stove, how can the chimney pull room air through the opening?  The chimney is already getting plenty of air from the main firebox.  Any smoke that happens to be on the room-ward side of the air inlet is now free to come outward into the room, any time you open the door to feed the fire. 
This problem about competing air inlets and smoke escape into the room is part of why it's so complicated to install an effective, safe, outside air inlet to the stove.  The new air inlet more or less needs to be exactly where the old air inlet was, right at the room door, and it will likely be pointing in the wrong direction to prevent smoke escapement into the room.  So in many builders' experience, outside combustion air may in fact make the users experience more smoke in the room, instead of less.

All these problems are intrinsic to outside combustion air.

  The fundamental problem, as with many "solutions to imaginary problems," is that adding a new feature adds complexity.  Complexity increases the chances of failure, and guarantees more expense both initially and whenever repairs may be needed to remedy those failures. 
  Building codes are generally immune to complaints about cost (despite the huge deficit of affordable housing on the market, with homes that raised many generations in good health no longer making the cut today).  But at least the code committees do occasionally reverse a bad decision when proof is available.

Mandatory outside combustion air does not solve the safety problem it was intended to solve, and in many cases the attempt to install outside air would add other safety problems.

- Outside air is no longer required in Canada, or most US jurisdictions. 
Make-up air is allowed (air brought to the room instead of the combustion unit).  The only language still in the Canadian and US model codes is along these lines: if outside air is provided, it must be installed according to certain guidelines for safety.

- Carbon monoxide detectors are recommended, regardless of heat source. (Automated furnaces that meet code, but could leak CO under negative pressure, are arguably even more dangerous than a woodstove die-down leak, since at least the wood stove eventually runs out of fuel and stops producing CO.)  A CO detector requirement is a performance-based standard, that deals with the actual problem if and when it occurs. 
 - In cases where negative pressure problems are real, such as airtight houses with too many exhaust fans:
   Many heater installers feel that exhaust fans (such as kitchen and bathroom fans) should be made responsible for their own make-up air supply, to reduce the likelihood of indoor negative pressures over 10 Pa in the first place.  There are plenty of air-to-air heat exchangers on the market that supply incoming fresh air while capturing a little bit of the waste heat from exhausted air; both passive (convection driven) and with intake-and-outlet fans hooked to the same electrical supply.

- Another common cause for negative pressure problems is the common-sense approach to a leaky house: occupants can feel cold incoming drafts, and attack them with weather sealing on ground floors and crawl spaces - but they can't detect the outward leaks in the attic hatches, upstairs windows, and overheads, so those leaks are allowed to continue unpatched. 
Overhead hot air leaks can be detected from outside the home with an IR camera.  If you don't want to buy one yourself (a reasonable one with software can now be obtained for about $200), most home energy audits include an inspection with a professional-grade IR camera. 
  For starters, visit your attic in winter.  If it's warmer up there than outdoors, and especially if condensation is occurring on the bottom of your roof, you probably have some hot air leaks to deal with.

- Startup and Die-Down Phase Management:
  It is worth noting that the most common time for solid-fueled appliances to release indoor CO is during startup (when the chimney is cold), and during die-down.  Die-down is more insidious as there is no smell for owners to detect - but at least with a solid-fueled heater, it only lasts a short while. (Compared to automated furnaces, which might release CO undetected for hours or days in case of a negative pressure issue in the house.)
 Priming (warming) the chimney prior to starting the main fire is an old family tradition.  We used to use a wad of newspaper, light it and hold it near the chimney throat to "check the draft."  (It also allows you to detect if the damper is closed on an unfamiliar fireplace or woodstove).  Nowadays I have seen people use everything from a candle to a propane torch. 
  The startup phase usually doesn't take long to overcome; especially if the house or heater mass is warmer than outside air.  In extreme cases, we might wait until the outdoors cools down at night, or in the early morning, to prime a thermal-mass heater that was stubbornly colder than the warm outdoor air of an autumn afternoon.
  For the die-down phase, the solution is even easier. Thermal mass in the stove - even firebricks often used in conventional iron woodstoves - can hold enough heat to help the chimney draft properly through the die-down phase.  A good operator will also keep the fire going, or help it burn out cleanly, without allowing it to linger in this phase too long.
  Regardless of the type of heater or stove, a good CO detector can be a life-saver.  Combination CO and smoke detectors are now available, and commonly installed in many residences.

  Leaving a woodstove to burn unattended at night while owners sleep is intrinsically risky, not only for CO but for all the reasons the fire itself is risky.  Do you trust all the critters in your house never to drag toys or nesting materials up next to the stove before you awake?  Do you let your loved ones stack or hang combustibles near the stove to dry, that might get knocked closer in the night? 
"Damping down" the fire, a common practice to get overnight heat from a space-heater woodstove or fireplace that wasn't designed to provide it, makes it more likely to release CO into the room, and to coat the chimney with creosote and eventually cause a chimney fire.

This is one of the reasons we love masonry heaters.  After running the fire responsibly and efficiently for a few hours, the owner can put the fire safely out before going to bed, and still enjoy overnight heat.  The time window for smoke-related mischance is much smaller, and the benefits much greater, than with most other forms of wood heat.

No comments:

Post a Comment