[user_12345a]

Heating a vented crawl space is a losing proposition, even if you close the vents, will still be very leakly.

How about heat tape?

https://www.homedepot.com/p/Frost-King-12-ft-Water-Pipe-Heat-Cable-HC12A/100032792

 

[user_12345a]

Just insulating the pipes and between floor joists, as you've experienced isn't a good idea. too bad it was done that way.

If you insist on a heater, it should be a hard-wired baseboard one controlled by a line voltage thermostat.

Those plug in heaters are for temporary attended use and pose an extreme fire hazard.

 

[user_12345a]

Baseboard, hard wired is the safest.

You can put a line volt thermostat and set it to like 45 degrees

 

[user_12345a]

The wattage of heat tape would be much lower than a baseboard and should be self regulating, i would think.

But i share reservations with respect to being a fire hazard.

The key with a baseboard is to properly control it, having it only maintain the space just above freezing.