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Discuss Vaillant ecoTEC Plus 938 - Low Flow Rate in the Plumbing Forum | Plumbing Advice 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 ??
 
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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
[automerge]1592604515[/automerge]
Fit a tap within 1M from boiler see what flow is ??

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.
 
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 ?
[automerge]1592641332[/automerge]
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
 
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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 ?
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.

This seems to imply that the cold water will be diluting in the tank all the time so the burner would have to ramp up as time passes on, eventually getting to a point where the storage tank is only cold.

I didn’t think it worked like this, however would explain my issue to some extent. This also means the tank wouldn’t be “recharging” where there is a constant drain as that’d not be possible. It would have to wait until demand has gone then recharge.

Any other views?
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How do I put it in to status mode? I haven’t seen an option for this. Cheets
[automerge]1592644481[/automerge]
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 ?
[automerge]1592641332[/automerge]
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

Found it :)

Comfort mode off sequence = S.31 > S.21 > S.23 > S.24 >S.20 > S.27
Comfort mode on sequence = S.31 > S.21 > S.23 > S.24 > S.27

So it appears to S.20 labelled "DHW Demand" that isn't present on comfort mode off.
 
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I think the most simple question here is if you run your shower at 38oc, what's the flow rate? (Is your shower a standard thermostatic or a Aqualisa Digital Box?)

I think you're really just overthinking it.
 
I'll give it a go and come back to you :).

I just want to make sure I am getting the performance from this thing that I should be. It cost significantly more for this model along with the work to have it's own cold supply, having separate feeds to each main hot outlet and balanced etc so felt a little let down when the flow wasn't really any better than I had on a 30CDi in my last house.

Cheers muchly
 
So what from what u say supply to boiler is good the o/p is 14l which in c mode should be 20l
All indications s mode ok on c mode
But your not getting the flow rate u should
Just to rule out the pipe work to taps etc connect a tap or washing machine v/v about 1M from boiler if still not flow rate it’s the boiler if flow rate is 20l it’s pipe work
 
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.
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.

Oil fired stored combis sometimes do qualify their boosted performance, for example Firebird quote a 35 kw boiler as supplying 120 litres at a deltaT of 40C & 18 LPM with a primary stored volume of 44 litres @ 78C., this translates to a boosted time of ~ 7 minutes, the normal unboosted flow rate should be ~ 12.5 LPM @ a 40C deltaT.
 

Reply to Vaillant ecoTEC Plus 938 - Low Flow Rate in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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