I also have a problem with pipes freezing in a crawlspace. I own a duplex that I live on the second and third floor of and rent out the first floor apartment. Most of the house has a warm basement (oil heater) beneath it but the kitchen for the first floor apartment was an addition with only a crawlspace below. As long as I have a tenant, the pipes don't freeze. Not because I turn the heat off (I don't) but because (I think) the water is run a few times a day to keep things thawed. My first thought is to get some warm air from the basement into the crawlspace but it's an old house with stone basement walls with brick above that. To get a big enough opening for the air exchange wouldn't be easy. Then I thought why couldn't I run a loop from the hot water system that feeds the cast iron radiators that heat my home? I could put the loop in the same joist cavity that the fresh water supply lines run in. My first concern is where to put a bleeder valve but other than that I think it would throw off enough heat to keep things above freezing down there. The added bonus is that it's not wasted heat because as the heat rises, it would heat the kitchen floor.
Any thoughts? Do you think it would work or can you improve on it? I thought I would use cast pipe (galvanized or black? I don't know) because I thought it would continue to hold and radiate heat better than copper when the circulator is not on.
Thanks for any help.
Dave