Do you use water barrels in your greenhouse?
How does that work for you?
Do you use an external heat source or just let the sun do its thing?
Do they freeze solid in mid-winter?

Thanks

I use 4 - 55 gallon steel drums that I connected to 2 solar collectors. I circulate the water through the system using a solar pump. You can see my system here:


I just ordered a metal drum heat strap that I will attach to one of the barrels for use when we get a stretch of cloudy days during the winter and/if the outdoors temperature gets or stays so low that the solar system is not enough on it's own.