I currently have a house under construction. The overall design is for a tight, well-insulated building with a balanced active air exchange system (an ERV: a unit with two blowers, one in and one out, with vents spread throughout the house). I live in the US northeast where firewood is plentiful and I have long wanted an efficient, safe wood stove to use as a primary heating source.

I've had vague concerns about what happens when putting a rocket mass heater into a tight structure for some time now.

Construction has now proceeded to the point where I really need to answer some questions about chimneys and airflow. My general contractor (in whom I put a lot of trust) has concerns about what the air flow from the rocket mass heater will do to the living space. The very fancy ERV being installed seems like it will do a great job of maintaining healthy air quality in the absence of an underpressure such as that created by the RMH.

It's not clear it will operate well with the RMH running (at least, it will not be able to follow the different amounts of airflow that come at different points in a burn).

The current plan is to have a vent for the out-blowing fan (a "return") in the kitchen where the RMH is planned. The venting isn't placed yet so there's still the possibility of adjusting this it the ERV can help somehow. The ERV has settings for the CFM to bring in and take out. These can be set independently but I don't understand why you would do this. The settings are via manual controls on the unit. I could imagine setting the out blower low and relying on a running RMH to take care of exhausting stale air but I don't relish the prospect of making two ERV adjustments per RMH burn.

What options exist for addressing this apparent incompatibility?

  • Please confirm or explain: active balanced air exchange means that there are two blowers, one blowing air into and one blowing air out of the room where the mass heater will be installed? Hows the control strategy?
    – mart
    Sep 21 '17 at 9:35

It's probably not the answer you want to hear, but no sensible options exist for running a RMH inside of a tightly sealed building.

At least, that's the case if you are talking about the stereotypical J-tube RMH that is all-the-rage on permies.com and other similar places.

The frequent feeding required by a RMH means that if you plumb in an external air inlet, then every time you unseal the feed tube you will end up with smoke in the room. With 'normal' wood heaters, you only feed them infrequently, and well into the coal phase, so negligible smoking occurs. With a RMH you are always burning fresh wood and producing smoke at the bottom of the feed tube. This battle can't be won without modifying the RMH.

If you are able and willing to experiment and mod the basic J-tube design, then you could run your external air inlet into the bottom or side of the feeder tube and install a butterfly valve in it. Every time you wanted to feed the RMH you would have to simultaneously constrict flow in the external air pipe using the valve, whilst unsealing the top of the feeder tube. Good luck with that. It's time consuming and never quite works. End result: Smoke in the room.

In ideal circumstances (no high or low pressure zones outside, no wind) you could eventually get it dialled-in, but we don't live on a world where those sorts of conditions exist.

The reality is that tight sealing / heat recovery ventilators is a divergent path that embraces active and artificial systems. You're basically creating a house that needs an artificial lung for the rest of its existence when you tightly seal a house. That choice inevitably makes other solutions impractical or impossible to implement, because those solutions only work in moderately sealed thermal envelopes.

When I designed my house recently I faced the same issues. I have an infinite supply of free wood, and a limited capacity to generate electricity during winter. I went with a wood heater / outdoor air combo and didn't push for tight sealing. If I was in a colder climate and/or didn't have a really good passive solar design, then I would have gone RMH / indoor air instead of the wood heater.

I like trees, I like wood heat, I don't like the constant drone of HRVs, I don't have much in the way of surplus solar in winter to run pumps continuously, and I like the idea of a house that can breathe without mechanical assistance. I didn't tightly seal the house, and I have no regrets about that choice.

If you insist on tightly sealing your house, and insist on installing a RMH, then the simplest solution is to crack open a window whilst using the RMH. Not really sensible, but it would work.

Another approach, which reduces the smoking issue, but does not eliminate it, is with something like the design shown at: http://www.naturalhomes.org/permahome/rocket-mass-heater-basics.htm. Focus on the image. See how they have added a manifold/hood over the feeder and inject external air through that? The problem with that approach is that when wind blows against the side of the house with the external air inlet, it forces air into the room and a venturi effect kicks in when you open the hatch to add fuel... pulling smoke up the feeder and pushing it into the room.

Another (functional, but not very sensible) solution is to split your RMH between two rooms. Have the feeder in a non-tightly-sealed room (e.g. a mud room) then have the burn chamber pass through a wall and have the rest of the RMH (riser and thermal mass) be located in an adjacent room. This solves all of the pressure issues caused by a tight seal, but it makes operation of the RMH more complex. Normally you place the feeder somewhere that is convenient and which you pass by frequently — so you can just glance in and top up as and when required. If you put the feeder into another room you need to make dedicated trips into a different (and usually much colder) room.

  • Why does a cracked window work when a dedicated air supply vent doesn't? Oct 5 '17 at 22:39
  • If you crack open a window, then combustion air is drawn from the room and into the RMH normally, pulling smoke with it as it goes. If you don't crack the window the room turns into a low pressure zone. If you plumb in a external air pipe, then the connection point to the J-tube has to be further down the feeder tube — which means that air hits the side of the combustion area and smoke splits two ways — up the feeder tube and into your room, and along the burn chamber. If you're not always pulling air down the feeder tube, it is inevitable that smoke will rise up it.
    – Tim
    Oct 6 '17 at 0:10
  • I added a couple of options to the bottom of the post. Neither are acceptable (in my opinion) but are presented for your consideration nonetheless.
    – Tim
    Oct 6 '17 at 0:46
  • The naturalhomes.org link doesn't seem like the right one. It's a page with general discussion about RMH and no obvious discussion of airflow issues. If it contains another idea for a solution, I can't spot it. Oct 6 '17 at 23:32
  • 1
    Don't worry about the text, look at the image. It's a J-tube that's been modified for external air. Instead of the airflow going into the side/bottom of the feeder tube, they have constructed a manifold/hood over the feeder tube. The external air is not injected directly into the burn area — it is injected at the top of the feeder tube. So it's an improvement, but it will still fail whenever you open the loading hatch and the wind is blowing against the wall with the external air vent on it (due to the venturi effect sucking smoke up the feeder).
    – Tim
    Oct 7 '17 at 23:08

I'm throwing a few ideas about to help you get started. You should get independent review of your plans (wether you go with my ideas below or others). Ventilation of rooms with furnaces is a safety issue, worst case is that underpressure (from your ventilation system) draws exhaust into the room and kills you. If not already done, consider a CO detector for the room with the RMH.

I see two ways to go forward, if they are actually doable will depend on your specific setup:

Control in-blower for pressure: Install a differential pressure sensor that measures the difference between outside and the room with the RMH. The out-blower is adjuste manually to the air change you want to achieve, the in-blower adjusts automatically to always maintain a slight overpressure (e.g. X Pa). In practice this means that the in-blower will have a higher flowrate (then the other blower) when the RMH is active. You want a warning when the pressure diff. is negative for longer than 10 or 20 seconds, because this could mean you draw smoke into your living room!
This could impact the ERV, as the flows will be mismatched.

Separate Air supply for RMH: see if the RMH can get it's own dedicated air supply from the outside. This could be as simple as a hole in the wall with a mesh and a flap, or may need a separate blower (again controlled for delta P) - I don't know enough of your RMH and the layout of your house to really say.


This is easy to do. The RMS is outside. Or rather a high efficiency wood burner is outside. You run it flat out for a couple hours at a time, and instead of heating several tons of brick and clay you heat several tons of water.

The construction at first blush is like these outdoor wood burners you see in the back of Mother Earth News. Differences: You don't use a water jacket, but rather have heating tubes in an insulated firebox. You want the firebox to be as low mass as possible, as you essentially lose all the heat in it. Having a staged firebox (like a russian masonry stove, but bigger channels) will still allow natural draft, and greater heat capture.

E.g. You have a firebox at the bottom. You have 3 chambers above it that are sized to make working on the plumbing easy. All exhaust passes through all 3 chambers. You have a stack to increase the draft.

In operation water flows down -- enters the top chamber first, and moves downward. Coldest water is warmed by the coldest gas. Basic principle of a counterflow each exchanger.

Hot water is stored in a large underground tank. This tank still needs to insulated.

Hot water from the tank is used for radiant floor heating in the house.

Ok. It's NOT that simple. Between firings the outdoor burner gets cold. So you have to circulate antifreeze in the primary loop. This loop heats the underground tank.

The temp in the tank can be as high as 90 C or as low as 30 C. This is difficult to control in a household system, so you probably want a buffer tank in the house that you keep at the optimum temp for the in-floor system. So the big tank heats the in-house tank, which circulates through the zone system.

Build the furnace out of lightweight ceramics. If you are really into DIY, perlite and clay. You will need some structural elements in it. Masonry stoves are challenging due to the temperature shifts and differential expansion. It may make sense to build it horizontally rather than vertically except for the stack at the end.This would allow you to put some form of expansion joint between successive chambers.

  • 1
    This is the beginning of an interesting answer to a different question, I think. Oct 21 '17 at 0:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.