Hi Dave, The flow is the same whether C is on or off, or whether the temperature is 35oC up to 65oC.
I have been thinking (doesn't happen often lol ). Is my way of understanding how a combi works just totally flawed. By example, I assumed that changing the temperature on the boiler changed the amount of hot water the boiler pushes out i.e the litres per minute and that there would be a correlation between the two. So asking for more heat leads to less water coming out below the boiler and vice versa.
Is it actually the case that the flow rate is fixed regardless of temperature you ask the boiler to provide, and it's simply a case of whether or not the boiler can achieve this flow. If this is the case then this explains why people like me see a DHW litres per minute advertised for a boiler and in our heads see this as the litres the boiler pushes out of the pipes. (the same as what D.36 is measuring). However, is it really the case that the DHW LPM figure being quoted is only a relative figure always requiring mixing with cold to cool down to hit the delta T of say 35oC for all combi's?
If this is the case, it seems logical that having a 14L restrictor in the boiler is there to prevent situations where someone could feed the boiler 25L. In this example the person might want 40oC but the boiler with 38kw would never be able to provide this level of heat and thus you'd end up with tepid water. This in effect means that the absolute flow rate from the boiler at least from Vaillants can only be equal the flow restrictor. Thus for other Vaillant models the absolute flow would be around 8.1L for the 825, 10.4L for the 832 up to the maximum of the 838 & 938 of 14L and we should just be looking to see if we can hit the flow limit which would show the boiler could operate at it's maximum heating capability
Hope someone can put me straight and tell me I'm barking up the wrong tree or not. I think this is what the last guy at Vaillant might have been trying to tell me but with a very bad explanation as he said the boiler DHW temperature set point is irrelevant to flow.