Discuss Vaillant ecoTEC Plus 938 - Low Flow Rate in the USA area at PlumbersForums.net
Switch off the C mode is the flow the same if it is then u not getting the increase of flow from storage cyl if u are then it’s not the boiler
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Fit a tap within 1M from boiler see what flow is ??
I think valliant were trying to say that the tank is always the feeder for the heat exchanger. So it’s not extra capacity being added in after the incoming cold water is added, it just means the water goes back through and requires a little heat.But u haven’t got just a normal combi u got a combi that as u say is giving u restricted 14l then you got the xtra storage supply from internal cyl adding xtra litres
And this is what your not getting
The only thing I need to check out is d36 giving u the o/p of combi only and not both ?
But u haven’t got just a normal combi u got a combi that as u say is giving u restricted 14l then you got the xtra storage supply from internal cyl adding xtra litres
And this is what your not getting
The only thing I need to check out is d36 giving u the o/p of combi only and not both ?
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Put boiler into c mode check your status reading should be s21 to s28 c mode
If it’s s10 to s17 normal dhw mode
If not changing to S21 to s28 boiler faulty
Re flow rate and temperature control, IMO the temperature control will only come in to play when the hot water demand is less than the thermal output of the boiler, for example if you set the DHW temperature to 50C and you only needed a flow rate of 10 LPM then the thermal demand (from water at 10C) is 28kw so the 38 kw boiler will simply modulate down to 28 kw to maintain the 50C required, if you open up the hot water tap or whatever then the hot water temperature will fall depending on the max flow rate obtainable, the max flow rate through the boiler is only determined by the mains pressure and the various losses through the pipework, boiler, tap(s), shower or any restrictors fitted. So as stated above you can measure this and IF the boiler manufacturer hasn't fitted a (14 LPM) restrictor, then its up to you to get the required flow rate, if its 20 LPM then depending on the store type (primary or secondary), volume and temperature, you should get this 20 LPM for a fixed time only since 38 kw will only give a 35C rise at a flow rate of 15.6 LPM.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.
Reply to Vaillant ecoTEC Plus 938 - Low Flow Rate in the USA area at PlumbersForums.net
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