- Visiting in the Winter Time
- Step-by-Step Photos of the Construction Process
- Analysis and Suggestions for Improvement
- A Call for Support
- Additional Resources
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.
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.
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
Now we have placed the wood stove which will act as our fire box. We are examining how the linteled course will go.
You 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.
A 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.
We 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.
This 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.
We 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.
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
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We don’t know of many other precedents for mass heaters in greenhouses so please let us know of other experiences and online resources.
- Lars Helbro’s website – lots of inspiration, hopefully more details to come.
- An MHA build that shows the basic pattern of a Gymse, but is confusing because it also tests drafting smoke downhill.
- 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.