Thanks Trevor as thought the flow would be very low.
Think I will now need to buy a suitable rated pump (prefered electric driven). Can anyone recommend a decent one that is not going to cost silly money.
Can i just ask, why do you want to build something like the Thermowhatsit? Do you constantly do heritage rated restoration/cleaning? or do you do some gum removal, graffiti removal etc?
If you start to get into steam, as Chris pointed out, you need specialist hoses/guns/etc as the highest rated jetwas ones will melt, warp and blow.
I might be missing the point of your exercise here, but wont your hotbox, running with your current machine, be suitable for the work you do?