I pretty much always use the heater.
If there is no hot water available I use the heater to heat the water for the pre-spray - e.g. 5 litres and then pump it out into my pre-spray pumpup sprayer - works great.
I then refill and it will then take maybe 20 mins or so, to heat a complete load (around 20 litres).
The in-tank heater I have fitted is adjustable and works really well.
