Are rocket mass heaters and super-insulated houses compatible?

Hello,

In a recent discussion with our builder, he expressed some concerns about the heating output / heat loss combination that might come from using a rocket mass heater in a super-insulated house.  He gave examples of similar recent projects where the measured loss from the house is on the order of 15k BTU/hour around the coldest part of our winter.

If I do the math, that means a heating requirement for a 24 hour period of 360k BTU.  If we wanted to do a daily four hour burn of the RMH, we're going to be releasing 90k BTU/hour for those four hours.  If the RMH were a normal wood stove, this would be ... uncomfortable, at least.  Good thing it isn't, maybe?

Some more math tells me that if I wanted just 15k BTU of direct heating from the RMH heat riser for those four hours, I'd need a system that stores 5/6th of the heat in the mass and only radiates (from the heat riser) the remaining 1/6th.

Elsewhere, I've read that you might expect to get 40% radiated from heat riser and 60% stored.  I'm wondering if it's possible to push the balance here to something closer what I described above - does the system still work?  If so, what's the right technique for changing that balance?

Or is there something else I should be considering entirely?

(Searching the forums, I found an old, similar-ish, unanswered question - https://permies.com/t/40077/tight-house - and a comment in another technical thread about rmh - https://permies.com/t/6149/Rocket-Stove-Thermal-Mass-Heater - which only touches on air supply, not heat balance.)

Thanks!

Jean Paul, may be an all brick Bell is to consider.

Check batchrocket.eu

I asked for recommendations for back-up heat in my little solar-heated house that has plenty of thermal mass. People explained that a rocket mass heater heats up slowly and stays warm for a long time, so it's not the best choice if you are only going to need extra heat, say, quickly for a few hours, or during a cold snap in the springtime. It's the best choice if you are going to be using wood as a major source of heat all winter. I don't know if that addresses your question, though...

I will second Max's recommendation for a bell system rather than the traditional barrel over the heat riser.

If you have a house where you can get good natural draft without priming the chimney most of the time, you may be able to build a barrel-based system with the barrel largely encased in cob, to reduce the direct radiation fraction. A tall chimney completely within the house will be most likely to work this way.

Another point to consider is that any house has the same issue with regard to the fraction of heat output from the barrel, it's just a larger amount on both sides of the balance. So I tend to think that a RMH will perform similarly in a superinsulated house as in most current installations. For one thing, you would probably not do the fire in one four-hour burn, but two two-hour burns or the like.The heat would not build up so much while burning, and if the house materials have mass too (like lime plaster or stone on the interiors), that will absorb immediate radiation and act as additional thermal storage mass to mitigate temperature swings.

Also, you are not getting the heat output through the mass while you are burning, for the most part, so it's not like an additional dose of heat from the barrel, but substituting for mass output. The mass output starts to take over after the burn is done, depending on how long you burn in a session.

Cold days control how much system capacity you need, but there will be many more milder days where you will need half the total heat output. In my house with some passive solar features (but no real solar mass), a cold sunny day can warm the house until after dark with no morning fire at all.

Our home is super insulated, a real passive house in a moderate sea climate. We heat it with a high-mass batch rocket heater, no barrel at all. Direct heat is estimated as 10% through the glass door, all the rest is stored. See http://batchrocket.eu/en/applications#redbell Maximum delivered energy 3.6 kWh, or 12.3k BTU/hour. When applied to your situation the max heat output should be somewhat higher so a slightly larger heater is in order.

Rebecca Norman wrote:I asked for recommendations for back-up heat in my little solar-heated house that has plenty of thermal mass. People explained that a rocket mass heater heats up slowly and stays warm for a long time, so it's not the best choice if you are only going to need extra heat, say, quickly for a few hours, or during a cold snap in the springtime. It's the best choice if you are going to be using wood as a major source of heat all winter. I don't know if that addresses your question, though...

Rebecca, thanks for your input.  I had previously considered an RMH for intermittent heat in a workshop.  That project has been shelved now as not a very good fit (and not a high priority anyway).  Now I'm considering the primary heat source for the house we're building.  The information you shared about RMH not being a great option for quick, short-term heat matches a lot of what I've learned elsewhere.  The heat riser is good for the quick heat part but once the mass is hot, that heat is going into the living space and there's not much you can do about it (except quickly usher it out an open door or window, kind of defeats the purpose).

Satamax Antone wrote:Jean Paul, may be an all brick Bell is to consider.

Check batchrocket.eu

Glenn Herbert wrote:I will second Max's recommendation for a bell system rather than the traditional barrel over the heat riser.

If you have a house where you can get good natural draft without priming the chimney most of the time, you may be able to build a barrel-based system with the barrel largely encased in cob, to reduce the direct radiation fraction. A tall chimney completely within the house will be most likely to work this way.

Glenn, Satamax, thanks.  I've come across these bell systems before but haven't read as much about them as about RMHs yet.  One reason I've focused on the barrel-style RMH is the relative ease of construction.  I'm definitely out of my element with most of this stuff.  Cob seems just barely possible.  I really like the look of these real masonry heaters but not sure I trust myself with real masonry work.  The books and material put out by Ianto and Erica & Ernie have been a great help too - I wonder if you have any recommendations for similar kinds of material for these masonry / bell systems?

Glenn Herbert wrote:Another point to consider is that any house has the same issue with regard to the fraction of heat output from the barrel, it's just a larger amount on both sides of the balance. So I tend to think that a RMH will perform similarly in a superinsulated house as in most current installations. For one thing, you would probably not do the fire in one four-hour burn, but two two-hour burns or the like.The heat would not build up so much while burning, and if the house materials have mass too (like lime plaster or stone on the interiors), that will absorb immediate radiation and act as additional thermal storage mass to mitigate temperature swings.

That's a good point about the shorter burns.  I'm not sure why I got so fixated on the single four-hour burn.  Thanks for reminding me that's not the only way to go.

Good points about mass of the house absorbing some of the heat.  These super-insulated house systems are low mass compared to some other approaches (eg cob, straw bale) but I don't have a good concept of just how much mass, and therefore heat capacity, is there.  I guess I was expecting it to be negligible.

Glenn Herbert wrote:Also, you are not getting the heat output through the mass while you are burning, for the most part, so it's not like an additional dose of heat from the barrel, but substituting for mass output. The mass output starts to take over after the burn is done, depending on how long you burn in a session.

Yep, that makes sense and I'm counting on it.  That's basically the line of thinking that led me to wonder just how much of the RMH output I could stuff into the mass.  Then only what's left is a concern for overheating the house during firing.

Glenn Herbert wrote:Cold days control how much system capacity you need, but there will be many more milder days where you will need half the total heat output. In my house with some passive solar features (but no real solar mass), a cold sunny day can warm the house until after dark with no morning fire at all.

Another excellent point, of course.  I've been thinking about how the system will operate at peak heating season but it needs to be comfortable during the milder part of the heating season as well.  That's another good reason to think about shorter burns as you mentioned above.  And maybe skipping firings.  Thanks.

Peter van den Berg wrote:Our home is super insulated, a real passive house in a moderate sea climate. We heat it with a high-mass batch rocket heater, no barrel at all. Direct heat is estimated as 10% through the glass door, all the rest is stored. See http://batchrocket.eu/en/applications#redbell Maximum delivered energy 3.6 kWh, or 12.3k BTU/hour. When applied to your situation the max heat output should be somewhat higher so a slightly larger heater is in order.

Thanks Peter.  I've actually looked at that cast bell heater of yours before.  I forget where I came across it, probably in another permies thread.  I would love something like that.  I am totally intimidated by it, though.  I feel like I could slap cob onto a blob with some success.  Custom cast refractory components?  No way.  Also, cost is a factor...

http://batchrocket.eu/en/applications#culdesac also looks really great.  I wonder if you have more thoughts on that kind of system.  Technically it still seems like a challenge to me but I think I could probably find some local masonry experience to help out with it.

Stepping back, it's interesting to see the masonry / bell focus of the answers I've gotten.  Are these systems that much better suited to a super-insulated house than the barrel-style RMH?  Are the RMH folks all on vacation?

Thanks again everyone who replied.

We are all RMH folks, but Max, Peter and I have experience with bells as a means of heat storage from the RMH core. The bell is actually a less demanding design than the duct winding through cob; there are fewer ways you can mess it up. There is a style called the "half-barrel bell" which can be built into a cob bench and requires very similar skills as building a standard RMH.

My own RMH is essentially a brick box 26" square inside, with a layer of cob on the outside of the brick. Not counting the extra features I built in, it is about as simple as you can get, and not expensive if you can get used bricks for the inner shell.

Glenn Herbert wrote: and not expensive if you can get used bricks for the inner shell.

Or metal

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

Hi guys,
It might be a stupid question, but if the house is heavily insulated where does the new oxygen come from? The fire is burning it at a quite high rythm and the house is insulated so no big airflow comes in.

I went ahead and built a cob RMH in my super-insulated, rather tightly air-sealed house.  I find I need to crack a window a couple inches to start the stove.  Once it is roaring, the window can be closed unless someone is going to open and close an external door somewhere in the house.  With window closed and fire roaring, a door closing causes the stove to backdraft for a second or two and let a puff of smoke into the house.  As soon as the door is shut it quickly reverses again but generally doesn't manage to suck all the smoke back in.

I had plans to create a dedicated air supply (not plumbed into the firebox - just into living space) which could easily be opened and closed and didn't have the problems associated with opening a window mid-winter (condensation buildup, ice buildup, uncomfortable location to dump the cold air).  The crew doing my walls forgot to leave me a space and I haven't gotten around to digging a hole through my wall to do this.  Probably will be at least one more winter (3rd with the heater) using a window to make this work.

Andreas Delaware wrote:Hi guys,
It might be a stupid question, but if the house is heavily insulated where does the new oxygen come from? The fire is burning it at a quite high rythm and the house is insulated so no big airflow comes in.

In that case, you have no choice but to have an outside air intake, feeding the fire directly. Far more feasible with a batch rocket, than a J tube. But it has drawbacks nonetheless. The intake air isn't pre warmed, cooling the fire. And the pipe or channel leading to the stove has to be dimentioned right. Otherwise, if too small it chokes the fire.