Having visited the recent show at the NEC looking for fridge vent covers I was advised by one of the Thetford guys not to use covers when using the fridge as the will reduce the air flow for the flame, use covers only when the van is in storage was his advice. We will be wintering in the van this year and have found the advice in all the mags to be misleading.I agree with Mikey but would add that a problem with gas is that spiders webs can cause the air flow to decrease and cause sooting when using gas. So in spring to autumn I take them off on site. Winter and when in storage they go on. (the truma fire and water heater can also suffer spiders webs- hence the cover on the boiler output.)