I've bought a 30lpm cold , diesel, Zeta pressure washer and I will be using a 15lt hotbox with a 12lpm honda pressure washer, or I may even connect it to the zeta under low pressure, if that will work.
Heat will not be any better for render cleaning. As far as I can see, a doff will only be better than a hotbox for cleaning heritage stuff where it is required by the customer.
I hired a thermatech as they are only based around 10 miles away from me and carried out some tests. These machines are great, but I feel that for the type of work that I carry out a higher flow rate cold pressure washer was the better investment for my company.
There are some 'Falch guys' on here too, so I looked into these, but a trailer system would not be suitable for the majority of jobs I get in.
My new set up for 2015 I feel will be best for my line of work.