Lee!
OK. 3-4 weeks, I think will be a bit optimistic for 19kg. You can do it however depends how hot you want the water. Luke warm 30deg at brush will prob last 3-4 weeks. However 50deg @ brush in winter will use alot of gas.
One thing I have found with the heaters is they can be a bit temperamental. Depending on if you plan on using one heater per pump or one heater for two pumps it will take alot of fiddling to get the setup just right.
I use quite a high flow rate but I still found myself needing a new low detetor that turns on the spark every time. However you may find you don’t need this at all.
The fittings you use will make or break you. Stay away from PVC hose from boiler to hose reel. It will swell up, and you will use more water and the pressure will burst the hose. If you can, use pluming fittings. I rigged mine up with John Guest fittings, copper pipe straight from the boiler, and then flex push fit connectors into the reel. You really need to invest in the equipment that can take the pressure and wont leak.
Also tell you guys to be prepared to tighten things up. O rings and jubilee clips that were adequate before will not be adequate when using hot.
Gardiners orange hose is an absolute must. It’s the best hose I have ever used, and I'm a serious user. My gear has to last and work properly. I only work 2-3 days a week so when I’m at work everything needs to go smoothly, so listen to my advice improve where you need to, but I cannot stress enough test test test. Make sure everything is secure.
Any questions just ask.
Kind Regards
Alex