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Colleen,
I believe the warning is there because the heat tape is designed to heat a fluid filled line, whereas the fluid absorbs the heat. If you take away the fluid, the heat tape will overheat and either melt the line or cause a fire.
In an ideal world, I believe they assume that a hose or fuel line all have potential to be empty and the heat tape would then start melting into the flexible line, and a drain line is empty by design.
I haven't used the tape, but I'm sure there are plenty of experienced suggestions out there.
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