Hoophouse heat, Winter’s here

Hi Everyone,

Winter has pretty much arrived. We’re at about 4800 feet in the mountains of central oregon and I have some urgent questions.

I’ve read a few books by Ernie and Erica, Mr. Wheaton and a few others. I think I have a fair understanding of the RMH but I’m willing to learn as much as I can and I appreciate any help.

I have a 6” system in a 45x20 insufflated double layer hoop-house . I think it is “rockety” enough for it’s size but I’m a little concerned that I may have an inferior system for the job. I built what I could afford… I managed to secure a bunch of 6” stove pipe for almost nothing. I started with a fire-box and if I remember correctly, a 15” tunnel. I have cut about 1/3 of a 55 gal drum to act as a base for the bell. This allowed me to built a fairly long riser which seems to accelerate gases upward quite nicely. The riser is an 8” triple-wall stainless pipe over a 6” steel pipe with refractory clay packed between the two. Because I am using a 6” system in a 55 gal drum, I estimated that I had a little room to spare for the added pipe size and insulation. At the base, I have 18 feet of  pipe on a slight upslope which makes four 90 deg. turns before exiting through a 14 foot chimney.

Once the top of the drum hits about 350-400 degrees, smoke is no longer seen, only steam. max temperature on the top of the drum is about 650 while the chimney gets to be about 110-120 degrees.

My intention was to build a hopper above the firebox so that I could let it burn for longer periods without having to feed it so often but the lower box edge allowed smoke to escape the firebox at lower temperatures during startup. I then rebuilt the firebox with roughly a four foot square tube down the middle… and yes, I know this is unconventional and probably not recommended but it works great to contain longer pieces of wood for longer burn times well into the night and smoke is never seen. It is difficult to get started but has contributed to the rockety-ness by providing an accelerator tube for the air. The heavy gauge tube stays cool to the touch. I have a 5” variable-speed computer fan which snaps over the top of the tube which I use to get a hot base of coals before feeding longer pieces into the stove.

I did not want to build a cob mass for the system until I could prove that works. I have built this system in an existing grow bed and filled the bed with 1”-minus rock.

Questions:
•I have fired the stove for three days during the waking hours, though not continuously. I notice the rock is warmer each day, presently 90-110 degrees. How long should I expect it to take for the stove to effect the temperature of the thermal mass? Days, hours…? Last AM temperature reading outside was 38 deg while the greenhouse was 50 deg.

•Based on the numbers provided, do you think this stove is sufficient for a greenhouse of this size? The family says I should tear it out and build a bigger one but I think 500 deg sustained should be enough to do the job and it’s efficiency may be improved by a better built mass.

Thank you again for any help. It’s greatly appreciated and you might just save a tomato’s life.

Pictures: Greenhouse in winter without heat; anatomy of the stove without the bell and mass; semi-finished system with a yard of rock and 500 lbs of steel on top of the stove (not touching the surface of the barrel).

Welcome to permies, Craig. That looks like a very serviceable RMH. At first glance at your figures, if it's keeping a marginally insulated space significantly warmer than the outdoors, you are on the path to winning the battle. You just need to keep things from freezing in the middle of winter, not grow mangoes, right? What are the coldest temps you expect to get? I've spent some time around Bend in the winter, so I imagine you're even colder than that.

First, is your riser insulated, as in perlite mixed with the fireclay? That will have a lot of influence on the burn efficiency and how much heat energy you can wring out of the fuel. Second, getting a full cob mass will hold more heat, but probably won't be a game changer when compared to the rock you've already got. So, if you are close to maxing out the heating potential of this system and winter is bearing down on you, then upsizing to 8-inch might be the only way to ensure you keep the interior space warm enough. The bigger RMH can heat a lot more mass, and I think that will be the key. But if you've got the appetite to experiment in coming weeks, you will learn from it either way.

Hi Phil,

Thanks for your response.

Well, Mangoes would be nice… We have a couple of dwarf lemon trees we’d love to keep in bloom but that may be a little ambitious for our conditions. We’re about 90 minutes from Bend and about 1200 feet higher in elevation.

Yes, perlite mixed with fireclay. In my test burn (in the pictures), I was not able to get the riser even a little warm to touch. I am confident that it is well insulated.

I have not relished the idea of starting over but if that’s what I need to do, I think I have proven to myself that this is feasible.

Do you think I can mix the existing rock into my cob? The rock seems to hold the heat nicely but it has enough air-space that it doesn’t seem to give it up. If I rake back a couple of inches of rock, it exudes heat. Is it expected that it should take a few days of heating for the mass to come up to temperature?

Thanks again,
Craig

OK, that's good that your riser is insulated. I thought it must be from your description of the burn and exhaust but it always pays to check. Definitely mix the rock into the cob. That works and gives you the best qualities of both materials (rock's density and cob's exclusion of air). If you're willing to burn the thing for long shifts during cold spells you might be all right.

By midwinter you should know whether you want to do a rebuild later on. Keep us posted!

I thought that I might chime in as I built something similar in my high tunnel here in Kenai Alaska Zone 5A. I built my heater with the thought of keeping the soil as warm as possible so I built a masonry bed stratification chamber. It works amazingly well. I didn't build it to keep the temps up inside the tunnel as I knew that it would not be able to keep up with the temp differential once the sun set. If I was to build another system, of which is my goal for my off-grid property, I would build it down the center of the tunnel and keep the plants that I wanted to attempt to over winter in that bed. That way I would be able to add frost cloth or another layer of plastic, maybe even blankets to help retain the heat overnight.

Here's a thermal image from early summer.

Thank you, Jacob for the pictures and info. Sorry for the late reply.

If I could do my greenhouse over again, I'd cut the hoophouse in half, down the middle, then insulate the north side and place a heat collector there also. I like your idea and it's occurring to me also that this is way too much volume to easily affect temperature.

Our nightly temperatures have been about 10 or 12 degrees. The greenhouse Temps have been between 40 and 50 degrees in the morning. I have a backup propane burner on a thermostat which would easily burn through a 5 gallon propane tank in just a few hours and it has lasted for a whole week. That tells me that we're getting close to our goal with the RMH. With my hopper, I'm able to stack sufficient fuel for about 3 hours of burn time. I estimate that the fuel and coals probably burn until about 3am.

I've decided that if I'm having these results with a 5" system and an inefficient mass, I should be able to bridge the gap with an 8" system and some proper cob. I'm going to go ahead with a bigger system.

Even in the 20s, with full sun, we are able to reach 100 degrees in the greenhouse, requiring ventilation. With a bigger system, I'm hoping to only need to burn at nighttime and perhaps part of the day in the dead of winter.  

'Keep ya posted.

Jacob,
Does your pipe run through the dirt or through the cinder blocks? Is there anything special about the stratification build?

Hey Craig, I agree on the insulation on the north side, I wanted to add that to my build but it just didn't happen this time around. If you get a moment check out my post on my PAHS high tunnel. Hopefully it can answer some of your questions in regards to how I built my BBRMH.

Thanks Jacob! I’ll have a look.

Jacob Klingel Would love more information about your system. I'm up in the Talkeetna area of Alaska with an off grid home as well. Just getting ready to build ours come spring and would love as much information as we can get.

Hi Vickey, if you hop over to my post there is more information on my particular build.

https://permies.com/t/231688/PAHS-High-Tunnel-Batch-Box

I don't want to hijack Craig's post anymore than I already have!

Jacob, I should be so honored to have my post "hijacked" for a great cause. And there are few causes greater than a winter- worthy greenhouse.