In 2007, we actually had a bit of a catastrophe. In February, there was a blizzard that caused the primary heating source to fail. At the time the entire greenhouse was heated by a big natural gas furnace. Even the tanks were heated by it. We would pump water to a set of heating coils near the output of the furnace and back. Anyhow, snow piled up on the roof of the greenhouse so high that it blocked the furnace's chimney and caused the air to back flush and blow out the pilot light.
Here is a pic of the furnace:
While it was no fun to lose everything, it did give us an opportunity to fix some of the things that we would not have been able to fix otherwise because we had 3000 gallons of reef systems in operation. We took everything out of the greenhouse in 2008 and installed a new heated floor. For the first few years we had a gravel floor seen below:
It was great for drainage, but it really allowed in a lot of cold even though it was about 6"-10" thick. We installed a 4" foam pad and poured in an additional 4" on concrete. The red tubes are the heating coils that provide the radiant heat.
The new heating system is a tankless Rennai that was 40% more efficient than the previous gas furnace:
It looks pretty complicated, but it's surprisingly easy to use. Not only did it cut our bill significantly, it also did a much better job of heating the actual greenhouse. Before we were pretty happy to get 70 degrees tank temperature in the winter, but after installing the floor we were getting 72 degrees.
Here is what one of the tank sets looks like now:
