I don't know about the evap cooling the inside air. Logic tries to tell me that if the evap is coming from the tanks, that should be taking heat out of the tanks with it and thereby warming the air. But the laws of enthalpy say that when moisture is added to air, the air cools. (???confused???...as usual). I do know that my evap pads over the shutters definitely cool the air when they are running.
The more I thought about it, the more I started to think that adding more barrels of water shouldn't really make much difference. They are only 55 gallons and I have taken temp measurements in them through the summer and they definitely heat and cool MUCH quicker than the tanks. I just can't picture them radiating enough heat through the night to keep the air temps up enough. I know Calfo has experience in running a gh in a freezing climate and I have always valued his opinion, but if last winter, when it was like -2 F outside, I could crank up the furnace and get the inside air temp up to 90 F, it shouldn't matter how much (or how little) water is in the gh, it should be able to be kept warm enough. Even if it was only a ten gallon tank, if it is sitting in a room (or gh) with 90 degree air temp, the water should stay around 80(ish). The 2 times that I ran the furnace up that high last year to see how warm it would get in the gh were once at night and once first thing in the morning. I seem to remember that it only took about 20 minutes to get the temp up that high. I am starting to think that maybe there is an issue with the furnace if I can only get the inside air up to 71 at night with the thermostat set at 105 . I have noticed that the furnace sems to initially fire up, then when the exchanger gets hot enough, the fan turns on (normal). A few minutes later, the burner turns off and the fan stays running, but then the burner kicks back on before the fan ever stops running. So what is happening is the fan on the furnace is running all of the time, and the burner firing intermittently. I just don't remember that happening last year, but I could just be not remembering. It seems to me that the furnace ought to have the burner fire, then when the exchanger gets hot enough the fan should turn on (like it does), but it seems like the burner ought to stay fired until the thermostat says there is enough heat in the air.
It just seems like every day is a fight and like jnarowe's signature line, I have the knife. Get one problem taken care of only to have two worse problems arise. Better stop there before I get some rings on my butt from sitting on the "pity pot".