I don't want to pay to heat the whole house? How can I completely shut off one zone?

Letting that space get that cold could open you up to moisture damage. I'd try to keep it, and the water in the pipes at least 40 to 50 degrees. Letting it get into the 20's is just asking for trouble.

jadnashua said:

Letting that space get that cold could open you up to moisture damage. I'd try to keep it, and the water in the pipes at least 40 to 50 degrees. Letting it get into the 20's is just asking for trouble.

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No, not this time of year. It's called 'dry winter air' for a reason, which you can tell by glancing at a psychometric chart if you can find one that goes far enough off to the left.

If it's 0F outside and my controlled space is at 27F it's typical to see RH at around 45%, which is in no way troublesome to me. When it warms up at all outside, the inside temperature stays 25F or so warmer that the outside temperature with no dew point in sight.

The only trick is being confident the radiator pipes stay comfortably above 32F. I've monitored the situation by placing one of those radio outdoor sensors at different points of concern and it appears than in my situation, goosing the loop 30 seconds every half hour keeps the baseboards up over 40F at all times.

Cheers, hba