In my experience, higher temps (assuming you are not roasting the plants) make for more vigorous growth. This sounds good, unless the plants in question are seedlings. Then, the higher heat makes them very leggy very quickly.

Lower temps have the opposite effect. It slows the growth of seedlings producing thicker, stronger stalks. This, however, may not be a good thing if you have an established plant.

Ultimately, it depends on what it is you are growing in your greenhouse.

It might seem wasteful but if you are using wood heat for a small greenhouse you probably will need to vent the excess heat. Having temps over 100*F is not good and needs to be prevented from reoccuring.

Although I use inside wood stoves in 3 structures the smaller super-insulated 15x40' is the only one where higher night temps become an issue. You have more control with your setup but you are still dealing with a fuel source that is best burned at higher rate. Since wood is relatively inexpensive compared to other heat sources, venting excess heat should not be a dealbreaker.

Well I moved the vent duct work back up. This way the heat will continue to vent into the greehouse. When it gets over 85 or 90 the vent opens up and the exhaust fans come on. So far everything is automated. heat with t-stat, cooling with 2 inlets and one exhaust fast , 2 back up electric heaters, all these are on the own t-stats and fan speed controller. And tonight I bought misters/foggers with a interval/timer. So I should be set.

bmoser as for wood, I go to the pallet recycler and burn CLEAN scrap pallet wood all for free.

Its an old photo and has been in use with a good tall chimney inside my partially sunken 600 ft.sq. greenhouse. I like the idea of having as much of the heat as possible inside. I do need air intake to burn the wood, but a lot of small leaks seem to do the trick.

Agreed Sand, I never saw a greenhouse that airtight that makeup air was needed for a woodstove. On the contrary I've started a fire on numerous occasions where the greenhouse smoke had to be vented until the fire got burning well. Usual conditions are rainy (low pressure), blocked flue pipe or air gaps in the pipe.

Fespo, the free pallet wood does make the heat much cheaper so venting should not be as much of a concern. However, I used to heat my house with the "Free" pallet wood and when I tallied up the expenses for carbide circular saw blades, extra handling and dealing with all of the nails in the ashes, I decided to return to cutting trees. You may have a better setup with your outdoor furnace. I remember the frustration of having to shut the fire down when nails would get lodged in the shaker grate of my Riteway stove and as time went along the pallet refurbishing Co. made everyone take the junky slat wood along with the better framing wood.

bmoser, about 95% of the wood is cut up already. Well today I added 2 of 3 the mist/fogger lines to today, It's pretty cool when I turn it on.

Now the new problem, the furnace is making my allergies go nuts. I removed all of the insulation from the flex duct on the inside of the green house. I checked all of the flex duct for holes or loose insulation. The inside shell of the furnace has insulation with a foil backing facing the stove or where the cool air passes over. I broke down the top outlet the other day and could not see any loose insulation. I even put the leaf blower down inside to see if anything would come out, nothing. Now with out a dust mask I can go in. Any ides? Fespo

My suspicion is that mold is growing in the crevices of the high moisture GH environment. You really can't do much about the moisture, in fact you rely on it. The mold spores are usually everywhere and just need time and temp conditions right for growth.

You could do periodic disassembly and sanitize everything. Bleach is probably the cheapest but you should use it in a well ventilated area as well. Don't worry about furnace contact areas that get over 200*F.