Thank you Spruce.
He says in the link that there's a 20 min cut out on these heaters. Will this affect me if I'm constantly turning the water flow off by means of a tap in between windows?
No, it won't be a problem. It will reset itself each time you switch the tap off and the heater switches off.
These gas heaters have been used for a number of years and by far the majority have had excellent service from them.
The only issue that I have heard of is from frost damage. Users must drain the unit in winter to stop the innards being destroyed by the water inside freezing. There is a drain plug especially for this next to the connectors. So if you are fitting this in your van its a good idea to fit it up a bit higher so you can gain easy access to this drain.
Spring also make a Frostat controller that will kick your pump on and fire the heater up if the van temperature drops to 2 degrees. The pump will circulate water for 10 minutes and then switch off. You must redirect the water from the heater back to your van tank.