Hello all, I hope this is in the right sections.

I live in the eastern part of Canada and I am looking a. Building a hoop house in the spring 45L x 25W. Due to certain circumstances I can't build a permanent one, or 100% the way I would like. So I was going to just put up the ribs and throw some poly over it. Main purpose is to thaw the ground out quicker and be able to direct sow my plants earlier. Then I will pull the poly off come summer, until the fall and put it over to extend the season a bit. I wont be using it in the winter.

To create some thermal mass to help heat the hoop house I was going to build a rocket stove and circulate water, through a copper coil into some 55 gallon drums.

I would really prefer not to build a wooden end wall to vent the rocket stove out of. I would much rather just vent through the poly somehow without it melting and/or catching on fire. Any insight would be greatly appreciated. Thanks

I would get some aluminum flashing, cover a section of the hoop house with it,  and run my vent through there.
Flashing should be easy to cut with scissors,  and easy to affix to the hoops with self tapping screws.
Between the chimney and the aluminum ,maybe cut uosome dollar store silicon cookware for a fireproof gasket.
Given that a properly functioning RMH will have exhaust no hotter than a dryer,  these precautions  will probably be sufficient.

That's kinda what I was thinking I just wasn't sure how hot it would get. I like the idea of the plate gasket. Cheap and effect.

I will not be doing a RMH this spring, I want to make sure that my design will work first. So this year I'm just going to to a simple 4 block rocket stove and motor in some chimney pipe, prob about 6' - 8' of pipe before I reach the flashing. I imagine that will be hotter than a dryer vent. But still not hot enough to melt the plastic. I will also be running water through a coil around the chimney so hopefully that will aid in it not being hot enough.

Thanks for the help!

There are some challenges with the "water in a coil around the chimney" part.  Lots of people have tried that to preheat maple sap when boiling syrup and it's really tricky to get the flow right.  Too fast and you hardly heat the water at all, too slow and it boils to steam, spits hot water and the bubbles won't let the water through.  Then, on top of that, it's dependent upon the heat of the chimney.  So the flow rate ends up never being right.

But if you're pumping water through it and not trying to warm the water appreciably, then it's doable.  Just keep the flow high enough that it never gets close to boiling and you'd be fine.

Good luck!

I understand the need to keep it simple, starting with a basic rocket makes sense.
Be careful of using any fuel burning heater in an enclosed space.
A  carbon dioxide  detector is cheap life insurance in such a circumstance.

I have been chewing on the idea of heating water for thermal mass,  as a way to have an outside rocket stove heat my house.
I like idea of evaporating water and condensing it again, because the phase change embodies a huge amount of energy, 8000 BTU's per gallon of water boilers into steam.
By contrast it only takes 1500 BTUs to bring a gallon of water from freezing to boiling.
For my own hoop house,  I want to try boiling water into steam and blowing that hot vapor though perforated pipe,  buried beneath the floor.
Another approach would be to boil water by the batch and let it drain into a old water heater, or even into the ground of the greenhouse.