aplan insure the van with a gas heater,
any gas appliance needs to be signed off by a gas safe engineer, at a guess 99 percent of window cleaners using these gas heater don't bother
if your going to fit one you must fit a flue, don't think that leaving the back door open is adequate ventilation, and get it checked,
they work very well much better than a diesel heater for a fraction of the price, you don't have to use it everyday, I only use mine to stop the water from freezing so it hardly ever gets used