[williamx]

With gas heaters the bottles of gas freeze up if the tempriture drops too much,

Butane freezes at  - 1°   -------------------------blue bottle

Propane freezes at about - 42°      ------------ orangey red colour bottle

so go for a propane gas heater 

My local pub uses the orange bottle to heat up the outside smoking area, when the tempreture drops, the staff have to pour hot water on the bottle, because the gas is freezing and the flame is dying down.