Some on here will say that having a hot water heater is the pinnacle of window cleaning.
We mainly use our diesel heater in winter.
With our Heat Wave diesel heater, each of us gets around 50 degrees of heat leaving the boiler. By the time it gets to the brush head with the hose lying on cold/frozen ground, the temperature of water leaving the jets isn't much more than lukewarm.
A single operator will be able to increase that heat output a little more.
It's lovely to work with hoses that are supple, so that's a plus. If you don't wear gloves, then you can feel a little warmth holding your pole if working for an extended period of time.
With a soft hose, the pressure of water swells the hose a little. This stops that lag which we sometimes experience as the controller pauses for its 3 or 4 seconds before it pulses the pressure in the hose to identify if we have switched the tap/Univalve on to start cleaning again. I get frustrated with that lag on occasion.
Even with lots of pollen and bird poo on the windows, I haven't really noticed a marked difference in cleaning ability between hot and cold water. But that's my opinion.
If you want to work in freezing weather, then I'm sure a diesel heater will aid you doing this. I don't work when it's freezing, as I worry about slipping and breaking bones at my age.
We don't do much mileage a month as most of our customers are local. The heater uses more diesel a month running than the van uses.
I purchased this unit as a spares or repair sale. Would I buy a new heater? No, as I don't feel the cost justifies the benefits. To me, it's a luxury which isn't totally necessary. But again, that's my personal opinion. Many others will disagree.