stove in greenhouse context

Kiko Denzer and I were hired to build a heater in a greenhouse for some very avid gardeners. They had experimented with using a wood stove but there simply wasn’t enough heat retained for the stove to heat the greenhouse from one day to the next throughout the winter. Our task was to incorporate the old wood stove, mostly taking advantage of the glass door and firebox it offered, and plug it into a system with channels and enough mass to absorb sufficient warmth to heat the greenhouse with one firing a day. This article chronicles that build and the results.

flue liners in bench

Here you can see the long path that the smoke makes through the bench after it has exited the original stove.  All of the heat retained by the bench would have otherwise gone straight up the chimney.  There is a bi-pass damper which acts as a valve and allows for a straight shot to the chimney when you are making cold start.

stove with bench

Visiting in the Winter Time

The following photos were taken on December 3rd in the Willamette Valley of Oregon.  You can see tomato and pepper plants maturing inside the greenhouse while the ground outside is covered in snow.





Step-by-Step Photos of the Construction Process

IMG_0001Dry lay-out.  I had carefully drawn out the heater and we had poured the pad to planned dimensions… but it is always still important to check that everything fits together as planned.

IMG_0006Kiko instals wooden guides, often called “leads”, that will help us to keep our corners plumb and allow us to run lines to level our courses.  This is more helpful on larger projects.

IMG_0007The original wood stove that inspired the project.  I used a 4 1/2″ grinder with a metal cutting disk to cut out the baffle shelf it had inside.

IMG_0020We laid up as many courses as we could behind the wood stove before moving it into place because we knew we wouldn’t be able to get there to work once it was there.

IMG_0022Now we have placed the wood stove which will act as our fire box.  We are examining how the linteled course will go.

IMG_0025You can see in this picture and the subsequent one that we have wrapped the stove in 1″ ceramic wool blanket.  This is both to ensure that the metal stove has adequate room to expand and contract within its new brick housing and also to raise temperatures in the firebox which will contribute to a cleaner burn.  In this photo, we are beginning to fully enclose the stove with brick and beginning to define the down and up-draft channels that will enter into and leave the bench.

IMG_0034Detail of the creamic wool and brick work surrounding the stove.

IMG_0030A cleanout is being installed at the bottom of the downdraft.  We are using a soot door by Pisla and a masonry frame that surrounds it.


IMG_0037The down and up-draft channels getting further defined.  Lintels to span the openings.

IMG_0042Before the mortar has set up, we knock the lintels back and forth at least a quarter inch to ensure that they will have room to expand and contract when heated.

IMG_0047Project coming along.  We have gotten above the height of the bench so….

IMG_0052We switch gears and focus on the bench.  A rare shot of Kiko using cement mortar! 🙂  The area the greenhouse is in experiences occasional flooding so we built the first couple of feet with cement based mortar and made some weep holes in the brick work.

IMG_0056We chose to insulate the bottom of the bench channels (with a perlite-lime mix) so that less of the heat would soak into the pad and ground below it and more would go into the bench top.

IMG_0067Detail of first flue liner plugging into brick work.  We used ceramic wool to help seal the union while giving the two different materials room to move when heated.

IMG_0068The flue tiles are mortared to each other but very minimally mortared to the bench surround.

IMG_0070I’m not sure why we hadn’t set up the brick saw.  Cutting block with a diamond blade on a skilsaw can be a dusty and noisy affair so put on your safety gear.

IMG_0072The next couple of photos show us marking a flue liner for an angle cut using water in a wheel barrow to give us a mark all around the shape.

IMG_0074Each bringing the flue liner down to our marks.


IMG_0077Working out the corners and also inserting the copper heat exchanger that Ed had made to warm water up for watering starts and seedlings.

IMG_0080Flue liner assembly is completed.  Cleanouts at end of bench to access both horizontal runs.

IMG_0092Framing up and reenforcing the concrete pad which will act as the bench top.

IMG_0100Mixing concrete, shoveling, and screeding.

IMG_0106Bench completed, back to brick work.

IMG_0108This is an important photo which shows the basic design.  You can see the firebox and the bell that will be formed above it, and the down and up-draft channels.  The heater has been designed around the size of the brick units to minimize cuts and complexity.

IMG_0118The bi-pass damper is put into place.  When open, it gives the flue gases a direct shot up the chimney.

IMG_0119Another angle on the bi-pass damper and the shaft which leads to its operation on the front face.

IMG_0122You can see the lintel above the firebox as well as the bi-pass damper handle.

IMG_0130We have used slabs made of castable refractory to cap off the “bell” above the firebox and the entrance into the downdraft.  An anchor plate with a shut-off damper has been installed for the chimney to plug into.

IMG_0137A plaque made of left-over mortar to record the year in which it was built.

IMG_0138The top of the heater is insulated with a perlite-lime mix in order reduce the amount of heat accumulated at the top of the heater and encourage it to absorb it into the rest of the heater.

IMG_0141The heater is near completion.

Analysis and Suggestions for Improvement

When we went back to visit, Ed was thrilled about how the heater was performing.  He expressed that it is a huge commitment to keep the heater fired once a day through the winter but that the results were compelling, both for extending the season as well as getting a head start on the next one.  He said the seedlings literally jumped out of their flats when started on the heated bench.

We went through a firing cycle with him and my main reflection on the heater was that the original steel stove we had used to power it was probably it’s biggest limitation.  The firebox is small for a batch-style heater where you want to be able to put all your fuel for a 24-hour period in at once (or, at worst, in two batches).  The stove’s firebox is not at all optimized for combustion…. it is basically a box with a hole at the top which is inferior to a higher firebox and throat where the flame and gases have more opportunity to mix.

A clear improvement both in terms of optimizing combustion and longevity would be to build a firebrick firebox and mount a door.  A good precedent for this modification would be the basic Gymse heater pattern developed by Lars Helbro.  The main question for someone considering this project would be whether this additional cost and expertise would be a deal-breaker for the project happening.

A Call for Support

If you have read through this article and reached this point, it’s likely that you have interest in this subject and can appreciate what it takes to conceive of, realize, and document a project like this one.  If you feel moved, you might….

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Additional Resources

We don’t know of many other precedents for mass heaters in greenhouses so please let us know of other experiences and online resources.

  • Gymse-
  • Rocket Mass Heaters in Greenhouses – I don’t think that the original iteration of the rocket mass heater is right one for “powering” greenhouses because its small, albeit efficient, firebox would require more attention than likely possible for a greenhouse and because of potential for smoke backing up and filling the greenhouse.  The new batch-style rocket mass heater, which is similar to masonry heater design, featured in the case studies of the 3rd Edition of Rocket Mass Heaters does have some promise for this use.




  1. Hello from Scotland. Thank you for your excellent article. I’m always on the hunt for ways of solving our heating problems. I’m not two big young men, rather a smallish not so young woman. I’ve seen mass stoves before but not quite understood the principle. I think I can adapt your designs to build something in my veranda. We’re living in a static caravan at the moment, and every winter I threaten to get the stove sorted. It’s a confidence thing and also a resources thing, ( never have the cash). Now I know what to collect to make a thermal bench, the new old stove we’ve been lent, has a flue rather than a chimney, and will go through the side wall, and I can build the bench on the outside and heat the veranda, a warm seat and a drying room too, sorted, Angela.

  2. Dear Sirs,

    I was wondering, given you’ve utilised a bell chamber (as per the free-gas-movement type school of thought), why then change to a channel based system in the bench? Is there an advantage to this that is not obvious to the layman like myself. Intuitively it would seem easier to construct another low bell using piers, much like a Roman hypocaust. What am I missing?

    Your sincerely,


    Ben Scriven
  3. Pingback: To Greenhouse or Hoop-house, that is my question

  4. Great idea guys…

    I don’t have such inclement weather that would require constant heat provided to a greenhouse (although I would love to do it), but could, as Angela said, use it to heat up a varanda in the winter or make it a heated bench for the varanda (love the idea).

    Question: what is the surface temperature on the bench area?

    Thanks. great job!


    Rom Linhares
  5. I have always been concerned that rocket mass heaters would have poor chimney draw giving a possible smoky room – at least before the stove heats up. Your damper seems to address the issue wonderfully: do you have a supplier of same?? Thanks in advance.

    Brian O'Rourke
  6. I am wondering if there is any problem with pouring the concrete directly on top of the clay flue liners? Seems like you would need to break the bond so they could expand and contract at different rates? But I really like the idea.

    Do you think there would be any problem with forming the sides and pouring concrete for the sides and top at the same time? I think this would be a great way to make a bench.

  7. Very cool, Thanks for sharing.
    I wonder how an auto feeding pellet stove could be used for one of these. That would eliminate the daily firing up the stove. the thermostat would be the trick because of the delay in heat transfer. It might over heat waiting for the thermostat to register ambient temps.
    But a ton of pellets is pretty cheap and easy, cleaner than handing wood. no hauling wood every day.

    Thanks again for posting this! you guys are awesome!

    Annandale Va, USA
    1. Pellets are made for producing smoke, not heat. They will produce heat, but much lower than wood. If you don’t want to bother with wood, want easy to use and cheap maybe a propane heater is for you.

  8. Hey there,

    I’m curious to see if your solid mass heater/ fireplace is still working. Or, if not, how long did it last? Was it as efficient as expected? Were there glitches? Would you change anything if you had the chance to build a new one?

    Let me kbow.

    Thx. =]- Meeschko

    Michel Levesque
    1. Hi, Michel, last I heard (a year or two ago?), the owner was gardening so much less that the greenhouse didn’t warrant heating. Aside from that, I think the heater did a good job — ripe tomatoes for Christmas (see here). Changes, hmmm. Hard to say. Depends on so many variables. The client had an old stove and wanted to use it. It made some parts of the build easier, some harder. Did it save the client any money? Short-term, yes. Long term, maybe not. There’s no such thing as perfect. (Or, if you look up the etymology of the word, you’ll find that it just means “thoroughly made” — which is what it’ll be when you’re done.)

  9. So grateful for this article. I was looking specifically for a way to heat a greenhouse and water through winter. Your article really answered all those questions. Here’s another: If I have an attached Walipini/greenhouse can a rocket mass be shared for heat and hot water? Or two separate heating systems?

    Jacqueline A Marcott
    1. Thanks, Jacqueline, glad you found your way here. Have you discovered the rocket stoves experimenter’s corner at There’s a lot of knowledge, info, and advice there. Heating water is definitely do-able, but can be tricky — and dangerous, if it’s a closed/sealed system. Shared or separate depends on your needs and heater design, etc. Good luck! I’d be interested to hear more about what you decide to do…

  10. Hello from Seattle! Quick question: how does the smoke (heat) from the wood stove travel through the channels? It’s a little mysterious. When it exits the fire box, the smoke directly enters another brick box, then goes left and down, around the bench, back up and then through the chimney?

    1. Hi, Samir, it will be less mysterious if you think of it not as “smoke” (which “always rises,” yes?), but as simply something that flows, like a liquid. Imagine turning the stove/heater upside down; think of the firebox as a lake — which fills with fire/rain until it spills over. When you make channels for overflow, you can control and direct it — thru more channels or into ponds. Turbulent flow mixes and maintains regular temperature. In smooth (laminar) flow, cold and hot material stratifies: the cool stuff is heavy, and sinks to the bottom; the warm stuff is lighter, and floats up where it can flow out into the next channel. A stove works exactly the same way, just upside down. Hot gases rise; cool gases sink. If you trap the hot gases up high, they’ll transfer their heat to the surrounding masonry, and then sink to the bottom, where they can flow out. They will be replaced by new incoming hot gases. In a pond, incoming water and gravity provide the pressure to generate flow. In a stove, the flow is generated by draft in the chimney at the end — which is why most stoves have a bypass valve that heats up the chimney; once initial draft is established, it will pull the gases thru the system. Gravity also pulls the cooler, heavier gases down so they can be sucked thru the openings, which are always at the bottom of each stage of the system. I hope this helps. If not, read it again a couple of times until you can picture the upside down pond effect…

  11. I was wondering if your design could be used to hear an entire home as well? I am looking g to build and if I could make that work for a home I will do it. This would be cheaper than electric where I am.

    Dawn Maurer
    1. it all depends. If you live in a 5,000 sq. ft. 12 bedroom room mansion, you probably can’t heat it with any kind of single stove, unless you have a large central heat source and a complex distribution system to deliver the heat everywhere you want it. If you live in a one-room or open plan house that measures hundreds rather than thousands of sq. ft., yes, you probably can. Either way, you’ll need to educate yourself, do some research, ask lots of questions, talk to people. If you’re looking for professional masons to do the work, or even if you just want more info about heaters, I’d recommend looking up the Masonry Heater Association. If you want more DIY info, there’s a scary amount of info on youtube and elsewhere on the web, so start w/some of the more reputable books to get a grasp on the basics as well as an overview (Rocket Mass Heaters, by Evans and Jackson, is just one of many; there are others on masonry stoves). I’d be happy to talk to you if you have more questions or want more guidance. If you want to go that route, I’ll need to ask something for my time; you can write me direct via the contact form.

      1. Thank you for your help. When the time comes I will most likely get with you. I am all about being frugal in what I’m doing and if I can do it DIY I will, however, this might be a project for a professional mason. I want to make sure it’s right and not a danger to my home when I build

        Dawn Maurer

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