Modifying old Commercial Greenhouses

Hi! I recently moved to upstate NY to a property that was run as a commercial flower greenhouse operation. There are 4 giant greenhouses and I'm starting with one of them to try to make it less of an oil guzzling monster. Right now they each have a 250gal oil tank and a hot air furnace (which to me seems ridiculously inefficient)
The one I'm starting on is smaller - maybe 20' x 50' double polycarbonate and is attached to the chicken/duck coop. I'm trying to work with what I got as much as possible, so please don't suggest I tear things down and start over. (even though I would love to since the greenhouses are oriented the wrong way and two of them are in the shade in the winter... SMH).
So far I've got a couple of 55 gallon barrels full of water in there that don't seem to heat up super well during the day so I got a super cheap inflatable hot tub off the internet and am planning on trying to pump the warm water into the barrels at night, or maybe use that water to pump through planting beds to keep the soil warm.
But right now this is my dilemma: I moved an old Fisher woodburning fireplace insert into the greenhouse that the old owners left sitting on the porch here. I want to try to hook it up out in the greenhouse, pile as much thermal mass next to it/on top of it as possible and see how that goes. I can grab a bunch of galvanized (maybe?) stovepipe off the boilers in the other greenhouses, but I'm wondering if it's worth spending money to get new stovepipe for the 15 feet, give or take, that need to run up inside to the roofline. I know that single walled pipe will give off more heat inside the greenhouse, but I'm not sure if I should even use the old pipe if I go that route - I don't know if galvanized is even usable on wood stoves? I'm also worried that maybe with the single wall pipe there will be too quick of cooling one it gets away from the stove and not enough draft?
Any thoughts/suggestions welcomed.

thanks!

The very first thing I would do if I were going to re-use a commercial greenhouse would be to fully insulate all the walls except the south wall.  I don't understand having the entire greenhouse made of glass.  No appreciable sun comes in through the north walls in the winter, and arguably, the East and West lose more heat than you gain from them.  If I didn't have enough insulation or wanted to really maximize sun, I may leave the East wall, or part of it open.  I would also insulate the roof on the north.  I wouldn't try to use any that are in the shade in the winter.

congratulations on the greenhouses and, welcome to the realm of 100F summers and -20F winters 😋.

I would get some new stove pipe, that is not all that far to go (compared to doing the larger greenhouses) and it's not that fun playing with half rotted pipe full of soot. Galvanized is not safe to use on a wood stove flue pipe. (well it is but only near the end where it is cool... zinc fumes are bad news)

you could consider a solar hot water heater set up in one of the other greenhouses and then pipe the heat to a heat exchanger(big radiator with built in fan) in the greenhouses that you plan to use.

it may be that the greenhouses are oriented to limit the amount of incoming light in the summer, even with a shade cloth and a tree blocking 1/4 of the glazing my neighbor's greenhouse hits 105F in spring

as far as your current season's heating issues the best thing might be several box fans pointing at the stove/pipe. Perhaps you could have a fan pointing from the stove into a heat exchanger (even a car radiator will work if you are mechanically inclined) and then pump the hot water(with antifreeze) into barrels to store the heat

also if you are only trying to keep some cold hardy greens alive some row covers over the plants inside the greenhouse might be sufficient.

Finally some resources that are specific to our area, First is the Cornell Cooperative Extension has a bunch of online resources as well as offices in most counties, second is Northeast Sustainable Agriculture Research and Education who have all of their research articles freely available online and who might even help you with some crazy ideas.

May you have success in all of your endeavors,
William