LPG bottle getting hot

[lost in espace]

I bought an LPG powered generator and ran it for the first time yesterday for about 4 hours.

The propane bottle got very hot to the touch. I thought these bottles got cold as they were used, I am sure I have seen them with ice on the outside.

Anyone confirm what is going on here, I didn't spot it was hot until I turned it off otherwise I would have stopped using it.

[lost in espace]

Heating from the exhaust does sound the most plausible explanation, its just a big calor bottle with a regulator on it.