Gas water heater vent cover attempts orbit

[Mel]

Hi, I have just come back from a pleasant long weekend at Plassey. I was sat outside my van enjoying the unseasonal sunshine when I heard a THUMPF and saw the vent cover from next doors van shoot a good 20 foot into the air and land at my feet. The very pleasant family explained that they were new to vanning and couldn't get their water heater working. I suggested that they leave the cover off (this worked). Whilst I was well chuffed with being able to give technical advice on site, I was somewhat concerned that the gas didn't cut out before ejecting the cover. Is this normal?
mel

 

[Sproket]

Something not right .....

Our water heater just runs on gas & if the cover is on it will not light....

 

[Damian]

Thats what can happen if the vent cover is left on when trying to light on gas.
When the control is selected the gas valve opens and lets gas into the burner, but as it is too rich it will not light, but during the time taken for the red light to come on and gas valve shut, and re trying the heater, some gas will have fallen out of the lower vent opening, and when the heater is tried again, the residual gas plus the new gas as the valve opens will ignite briefly,,,,result,,,,orbital vent cover.

This is a very good example of the need to actually read the operating instructions supplied with each caravan and understand what is supposed to happen, rather than just trusting to luck.

 

[Mel]

Easy mistake to make though. I was just glad that it went up and not sideways.....
mel