Just looked again and the air/water model only raised the water from 14 to 69 , fine that’s a 55c jump but it was circulating 10L and took 30 minutes !
I can go 10 to 60 instantly 
Ok I see what you’re saying, the air & water heater is splitting the heat output between air heating and water heating.
What if you simply disconnected the fan that blows the air through?
Then all of the heat it produces would be available to heat the water. This is a 5kW heater, same as yours. The only difference is that this has been designed to heat water, whereas yours is designed to heat air and you have adapted it.
And, using a heat-exchanging loop (ie with header tank etc) is, in my opinion, by far the best way to safely get heat from a unit like this. Plate heat exchangers are not expensive, and there’s no problem stopping and starting the cold-side output water flow, as the hot-side water circuit is unaffected.
I would rig it so that once the hot-side water circuit was up to operating temperature, if the cold-side flow was dead ended then it flowed back into the tank. Easy to do with a thermostatic valve in series with a pressure relief valve.
