today i timed myself.apart from 30 mins for lunch and a 10 min coffee break this afternoon my intervals in between jobs/windows with my pole tap off was no longer then 3 mins(mostly 2 mins).usually it was to move my hose or trad the odd ground floor windows.
is this a short enough interval to keep the heater from going into "shutting down" mode?
My guess would be that it will shut down have a good chat to Oliver explain what you are trying to accomplish and he will be able to modify it to suite your style of work to a degree that’s what he did with mine anyway,
that means the heater will be firing up and shutting down many times over a day which is supposed to be bad for the burners(as well as fluctuating water temperature).i was hoping it would just keep running.it doesnt sound like these heaters are suitable for wfp really. 
still time to cancel your order, personally i would have asked all these questions first 
. I dont know why you just dont rig up filling your DI with hot water from home, you've said yourself hot doesn't make you quicker or is that necessary. If its supple hoses in winter you want then hot straight to DI will do the job and save you over 4k.
ive tried it and it doesnt work for me.also id have to refill in the morning before work.sod that.i do all that as one of my "end of the day" jobs ready for the next days work.ive also tried gas heaters.not interested in fitting an immersion heater so the only option left is diesel or stick with cold water.
hot water is a luxury and is not needed generally but it does make your working life easier and more comfortable.i also dont have to remember to put a heater in my van on freezing cold nights,worry about the water freezing in the hose,nice supple hoses,easier to clean certain dirt/marks off the glass and frames and easier add on jobs.
it looks good too esp for the customers and sets you apart from your competition in your area.