I had the same issue with the Sterling heater for my GH. The BTU calculations indicated a need for about 35,000 BTU, but the heater was available only in 30,000 or 45,000.

Our home H/AC contractor recommended the larger unit, explaining that during especially cold weather, we could not increase the output on the smaller one, which would run continually at full force just trying to attain the desired temp, but would never be quite up to the task.

but if i get the larger one, won't it be cycling on/off to much?
-renee

Renee:

An oversized heater continually cycling is a good point and might be one that would cause you to go with the smaller unit. You could, because if and when you fall victim to a real bone chiller, with your GH attached to your house, you can always open the door and set a fan to blow warm air into it, but I can't.

My GH heater did cycle on and off a lot last winter, but we didn't have it set very high, because we went into winter with only the building up, sealed and wrapped with a solar pool cover, and the ceiling fans had not yet been installed, so it was set to only 50 deg. most of the winter.

The only time I raised the thermostat to 74 deg, was during a very cold spell in Jan. when I set a 20 gal. tank in it with some plants and red ramshorn snails, just to see how they would do with no artificial lights and if the heater was up to the task of my committing the fish for this season, and when I did, the heater ceased cycling.

Frankly, keeping it warm in winter proved the easy part. It's keeping it cool in summer that's caused the real headache and additional expense.

There is one other thing you might try for winter though.

Right before the holidays, my husband drives around a nearby suburb and always comes home with more than enough discarded and often quite good carpeting to cover the concrete floor in the tractor shed. He discards it for good in late spring, after it's been saturated with grease and oil, but it helps keep his feet warm.

That gave me the idea of doing something like it in my GH, and it did make a difference when I had them laid down, but I scrounged Amvets for throw rugs that can be tossed into the dryer when they get wet.

Anything you can do to barrier against the cold ground will help and if it's a passive barrier like rugs you can wash/dry or toss in spring, it's cheap to free.

Another option would to be get the smaller size heater, the 7,000 btu one, for your main heating system, and use your old electric for the extra boost you will need during the extra cold weather. In my last gh I used 3 electric heaters (before I went to LP). Two were set at 55oF and the third was set for 52oF. It only kicked in when we had a very cold or windy night.

that is what I have decided to do. The larger heater isn't much bigger but it is 20 pounds heavier, so I thought we would go with the smaller one.

In emergencies I always can use the electric heater.