Here in Holland I was able to work for 8 full days between Dec 14th and Feb 24th...
Trouble with the temps is that if it is say -8 at night and does'nt rise that much in the day...you're going to have problems....hot will not work as that will freeze quicker than cold water.
If you hit -5 most days then working is going to be a real pain as the brush head will freeze and the jets also...more than likely before the hose.
Forget alcohols in the water for working as it will evaporate and if it gets that cold you'll need large amounts of it which is expensive...the idea that a coke can full in a 500litre tank is a myth...alcohols are good in 'enclosed' systems.
My advice is to save up and have a largish slush fund....we now have our holidays from Xmas eve to 24 Jan and then the slush fund to help out if more days off occur due to bad weather.
Salt...yup I use that more for my own safety than anything else...failing that table salt is very fine and hardly noticeable, those table salt containers with various openings are very good and do the job.
At the moment I am putting together my wintertime water making storage area..it's a 2mtrx 1.8m industrial racking...the 650 & 250 litre tanks will go in there along with the 4040 r/o and 2 x 12l DI Vessels all enclosed and insulate beyond reason
...along with the I have bought a pulse technology thermostat which will keep the enclosed space at a certain temp...
So at the end of the day the van tank gets emptied back into the storage tanks, hoses blown through with a compressor ( all pipes in van drain empty), pump box into shed storage area, so no need to heat van.
Point being I do not want to spend more time.money heating two places i.e van and ro system.
So my tips...take care of your gear and don't bother heating 2 area's...
Cheers
Dave.