I'm no expert, but its been here a while, so I'll try.
The more the better. Too much is never enough.
It would be most efficient to leave the house at room temperature.
You are supposed to use the extra heavy CU.
A factory made wall/floor heater has good surface area, so it may be cheaper sometimes.
The more BTUs, the more surface area needed.
The 'propane guy' gave me heck for asking about something like this. Seems propane is absorbed or soaks into the steel of the tank. Something about explosions. He thought it was the dumbest question he was ever asked! Didn't mind telling me that either. A friendly sort. NOT!
Antifreeze 50/50 has rust preventitives and holds and carries more heat.
Some extra CU soldered to the heat exchanger would increase the effective surface area. Like CU washers every 1/2". Or a scrap plate soldered on like this O. Thats the plate on the pipe, best I can do. New CU plate costs more than more pipe I bet, but it doesn't have to be thick.
My folks house has something just like car radiators inside the ducts. Stacked and tipped to fit like this. // The original furnace blower moves the air.
The air moves from left to right. The hot water moves right to left through the radiators in series.
Car radiators are great heat exchangers. Could get a lot of heat out of a brand new $100 radiator and big, high volume 120v muffin fan or 4. Just bought a new one myself, and 2 core was only $20 more than single core, good deal for something like this.
This could be way off base... Logic suggests its a good idea is to have a LOT more water in the house than at the fire. Maybe 10X more. Hot water is trying to cool from 180' to 75'F. Cool water is trying to go from 75' to 700'F. The cool water is going to get hot faster than the hot water gets cool. So the house water needs a lot more surface area and time compared to the boiler water.
I think Nothing To Lose is working this area too. Check his posts.
Just 2 cents. Did anyone see this?
G-