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Post by dirtdevil on Dec 27, 2019 4:24:20 GMT -8
I built a six inch J tube heater in a ten by twenty greenhouse I put together earlier this year. It has thirty foot of horizontal duct in a cob filled bed with a three inch concrete cap on the bed. . The exit flue is ten foot tall. I filled the bed late in the summer and then we got a real early blast of winter here in Northern Michigan and I had to set the project aside a while. A couple of weeks ago we had temps near zero for a week and I believe that the probably still wet cob pretty much froze. It warmed up the last week or so and since I finally got all by fire wood supply for my house split and stacked I decided to try the heater. It would not draw so I put a shop vac hose duct taped to a reducer on top of the exist flue and it worked fine as long as the vac was running. I could shut off the vac and open the stack and it would draw for up to ten minutes and then start smoking out the fill port. I believe that the extremely cold bed sucks all the heat out of the duct run and kills the draw. Even with the vac running for an hour the surface temp of the flue has not reached 70 F. I am going to install a duct fan and run it until the bed warms up enough to work. I'll wait until outside temps drop below the temps of the surface of the bed which has not topped 36 degrees even though we hit 50F yesterday. I'm assuming that once the thing has run for a while these problems will resolve but I thought I would post this in case anyone else is planning to heat a greenhouse in this way. If I did this again I would finished the heater before I put the cob in the bed and run it while I was filling the bed to start drying out the cob earlier.
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Post by Dan (Upstate NY, USA) on Dec 27, 2019 18:14:04 GMT -8
My house stove works like this for the first few firings of the year, if you have a clean-out in the bottom of the ten foot exhaust, then light some card board in there. When the outside of the 10' pipe is hot, shut the clean-out and light the fire in the J-tube.
Also is the 10 foot exit flue at least two feet above the greenhouse roof, if not add another pipe...
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Post by dirtdevil on Dec 29, 2019 5:03:23 GMT -8
Thanks for the information. This is my first RMH and I figured there would be a learning curve. There is no clean out near the exit flue because I built it so that the bell could be raised above the heat riser so I would just clean out the manifold by raising the bell. I did it this way because I intend to later wrap the heat riser in copper tubing to heat water and warm a six hundred gallon fish tank I built into the greenhouse and I wanted to be able to access the whole heat riser. I thought about adding a tee at the base of the exit flue but wondered it it would disrupt the gas flow out the flue which it a little more than two feet higher than the top of the greenhouse. I see in the shoutbox below that someone advises using a jet pump instead of a blower fan to induce draft. I don't know what a jet pump is, but using a duct fan to get the system up to temp seems simple enough.
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Post by dirtdevil on Jan 7, 2020 4:30:16 GMT -8
I have been running this stove for three hours a day for the past four days. At first it would only run with a duct fan running. The last two days it will run without an assist but I am leaving the fan running to keep the fire as hot as possible. It seems that the cob in the bed is finally thawing and building up enough heat directly around the ducting to allow the heater to draw on it's own. It was a mistake letting it sit so long into the winter before starting it. It will still take a lot of running time to get that bed dried out and up to a useable temp.
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Post by dirtdevil on Feb 15, 2020 14:56:51 GMT -8
I thought I'd update you and what I have found about running my heater in this greenhouse. Not as good as I hoped. I built this according to the information in the Wisner book so I assume it was constructed properly. In my above post I mention some problems I had with the draw and have run it enough hours now that I assume the bed has mostly dried out. The system draws well now however I still run the duct fan on the flue to get it started just because it is so much faster and easier to get it going. With the fan running the top third of the barrel will heat up to over 500F. On it's own it will stay in the high 300rds. I guess that's fairly typical. However with a outside temp usually in the mid-to high twenties, so far this bed as only reached 50 degrees a couple of times. Not nearly enough to support plant life. I was concerned about the heat from the barrel melting the plastic in the roof so I built a wall between it and the greenhouse proper. That was totally unnecessary. the flue and barrel are inside a three foot by four foot concrete alcove with the flue exiting within two inches of the barrel and the protective wall right up against the flue. That wall has never gotten warmer than a hundred and five degrees even if the heater has run six continuous hours. I expected that a 300 plus surface temp radiating from the bell would give out a lot more heat to the surrounding areas. Here is a picture of the wall and one of the bed. Apparently the pictures are too large to post. Anyway if I had it to do over again I'd build an eight inch system to get more heat out of it. As it is I expect that when the outside temps get higher I will be able to get the bed up to at least 70F so that it will work as intended.
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Post by gadget on Feb 20, 2020 7:49:37 GMT -8
Dirtdevil I feel your pain. I have run a couple of heaters in my greenhouse and have learned a valuable lesson. Don't believe what you read on the internet without doing the math yourself. I don't know what your heat losses are on your greenhouse but I can tell you they are high. Your common style greenhouse will need a heater running continuously over night to stay warm. The BTU loss is just to high for a reasonably sized heat battery. I have a 14' x 14' greenhouse I was trying to keep warming in the winter. I had 385 gallons of water I tried with a 120F warm up and by morning they would be at 80F but the greenhouse was below freezing(20F outside). I did the math, and the BTU's in the water where good for a couple of hours max. I would need to get them to over 200F to work all night. Problem is, it takes an incredible amount of heat to get that much hot water stored. Also, the heat source needs to stay at the higher temps so radiating rate of BTUs stays high. As it cools, the output lowers compounding the problem. Problem is both capacity and surface area.
Its like trying to fill a bucket up with water that has to many holes in it. My problem is my greenhouse design, not the heat source. Now, I just heat my soil to about 50-55F and keep the air just above freezing by using well water ran through a radiator. I stopped trying to grow tropical plants in there since it was to unreasonable.
I am building a second greenhouse that is about 5 feet in the ground and about 4 feet above ground. The earth below will be insulated and the glazing will only be south facing. I'm going to focus on heating the soil primarily. It will be easier to keep warmer since this design has less heat loss. I'm also working on a blanket system to cover it with at night.
Your problem isn't the heater, its the heat loss. I read many times online how well heat batteries work in greenhouses but the math does not add up on what I had. They must have some magic dirt or water.
I ripped out my water barrel heat batteries since they where just taking up space. A J tube would also be the same thing. Just taking up space.
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Post by pigbuttons on Feb 20, 2020 15:13:02 GMT -8
Thanks to both dirtdevil and gadget for sharing the experiences. I feel your pain. Heat transfer through R-1 materials like glass and plastic is amazing and relentless.
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