Greenhouse rocket...

[Mikimble]

Hello,
I recently built a rocket mass heater in my hoop house; and have some questions. Since a hoop house has 90 percent coverage with a sealed membrane, I am wondering how much outside air can make it in to replace the air going out. I notice that the draft by the feed tube is really touchy; just opening the mandoor to the hoop house can reverse the draft for a few seconds. If outside air is introduced, where should it come in: by the stove or by the exhaust- opposite places in my hoop house.
And I would recommend completely closing off the system at night; so cool air could not come back thru. I think you would have to close off the outside air, plus close off the exhaust as well.

Any other folks out there with hoop houses/ greenhouses noticing any strange drafting behaviors? Any ideas on the outside air questions??

Thanks,

Mike

[Mikimble]

Here's a better picture of the exhaust, covered in cob and rocks, and the stove in the distance...

Mike

PS Green house is blue from the solar pool cover I've tacked on to beat the cold temps.

[matt walker]

Hi Mike, what a cool set up! I have been hoping to get a system in my greenhouse but haven't done it yet, yours is pretty inspiring.

I think that you are on to something with your line of thinking, that the single piece sheeting doesn't allow much return air into the hoophouse, and therefore drags on the stove. As for where to put the make up air intake, I suppose you have some choices. If it's really cold where you are, I'd probably try to get it close to the stove so it wasn't pulling a cold draft through the length of the hoop house.

One other thing to consider is exhaust location, type, and whether there are significant pressure differences due to that. Do you have a vertical stack at the end of the system that reaches above the hoophouse? If not, that could be a large part of your problem. Not only does stack effect help at the end of the system, but if your exhaust is down low and straight out it could be experiencing pressure differences that are working against the system. If there is any breeze from the exhaust side of the hoophouse it will create high pressure there and low pressure on the opposite side, which often is enough to mess up the draft. If you can, stick a vertical section on the exhaust and get it above the roof line and see how that works for you. Give it a chance to get warm to the touch on the vertical run before you judge it's effectiveness.

For sure cover the feed when the stove is out, and the return air as well. Be careful covering the exhaust, if you still have hidden coals in the system you could fill the hoophouse with deadly CO. Probably better to see how well you can slow down heat loss by covering the feed before taking that risk.

[Mikimble]

Hi Matt,

I do have a five foot vertical stack, insulated with foil bubble wrap, outside the hoophouse. You can possibly see a little smoke on the left hand side of this photo; that's the pipe.

That did make a huge difference. Last night the stove really started smoking; and I ended up pulling the barrel. I saw that the insulation around my 6" round heat riser pipe and square end of burn tube had partially fallen out, and possibly smoke/fire was escaping the heat riser, and just going into the barrel. I think this was letting the stove light up ok, but smoking like crazy later. You agree?

[pa_friendly_guy]

Mikimble, I really like the design of your hoop house. It looks like an improvement to the design that Matt posted on here earlier. He used small 1" plastic pipes. It looks like you have used 2" pipe and reinforced them with 2X4s strung along each side. How did you fasten the plastic hoops to the ground? It looks like you added wood along the ground level and up the side a bit to hold the dirt in your beds. Did you have any problems bending the heavier, thicker plastic pipes? Did you have to heat them or any thing like that to get the top curve in. You have also built a wooden wall on the ends adding a lot of strength and stability. That extra strength should help the hoop house with stand the winds that caused Matt a lot of problems. Your design would add a good bit more cost to the project than Matts cheap basic design, but the added strength should be well worth the added costs. From the pictures it looks you ran the exhaust pipe under the planting beds on the right side of the unit. Is the exhaust pipe close to the large stone wall, or more in the center of the bed? Do you have problems with the heat building up too much in the bench and damaging tender plants growing along that bench? The warmth from the bench should benefit seed germination rates, but I wondered if the heater would cook them if it got too hot after they have sprouted. Do you raise your seedling trays above the ground after they have sprouted? It appears that you have planted directly in the ground that forms the bench. What all do you grow from seed in your hoop house? Do you try and have fresh salad greens all winter etc. ?

[Mikimble]

Hi PA,

Wow, that's a lot of questions...I'll try to answer them all. The 2 inch thick wall pvc pipes (20 foot pipes) are first inserted into a 3 foot piece of rebar that was driven 2 feet into the ground. It's definitely a two person job getting the pipes bent to fit on the rebar. Here's the link that I learned how to do this at: No heating of pipes required, just a lot of leverage. I also tied those into the perimeter boards on the hoop house. Those outside perimeter boards are also my bed boards as well.

The rocket exhaust is actually inside that large stone wall. It's cob and rock over a six inch pipe. No problems with overheating the bench so far. Check out the book by Eliot Coleman, Four season Harverst or the winter harvest book. He's in Maine growing about 15 different veggies year round in unheated hoop houses. I have grown lettuce, spinach, and other greens year round in WV as well.

[pa_friendly_guy]

That is a Great link Milimbli. I think we should add it to the thread about cheap hoop houses that Matt started. His was very much along the same lines using rebar in the ground to hold the hoop house down to the ground. What part if WVa are you from? I have a son living in Fairmont now. Its a very nice area, but the roads are very twisty turny with steep drop offs. The places some people have built down there amaze me, lol. The Realtor used the term " West Virginia Level " to describe the lots of many of the houses they looked at before they bought a place. There is even a 2 story trailer on the way to his home. The one trailer is on the lower level of the steep hill with the 2nd trailer above set on the hill above and then pilasters below and set at a 90 degree angle to the lower one. They put in a stairway from the bottom of the higher trailer into the side of the lower one, amazing. never saw anything like it around here.

[matt walker]

If you had gaps at the bottom of the riser, for sure that's a big problem. It is robbing you of critical draft in the burn tunnel and feed. I would imagine the stove will run a whole lot better once that is addressed. What is your riser construction?

One more thing. It's hard to tell from the photo, but what size is your barrel and how tall is the riser? If you are going to have to rebuild it I'd recommend a full size barrel and as tall a riser as you can manage. Those two things can really help a finicky stove.

[Mikimble]

Hey Matt,

Thanks for the response. My riser is a two foot 6" black stove pipe surrounded by an 8" pipe filled between with perlite. That sits on the square bricks of the burn tunnel. The barrel is not 55 gallon, probably 30.

Mike

[Mikimble]

PA,

Outside of Fairmont too.

Mike