Vaillant ecoTEC Plus 938 - Low Flow Rate

[gary333]

Well that's your Static Pressure, what's the dynamic? As that'll be the big factor.

Are you sure you're getting 24l at the bath? Have you used a proper flow cup.

Ive been using a 12L bucket and timing how long to fill them and converting. So might be a little out. However Yorkshire Water came and used a weir cup on my outside and He said it was off the scale his only went up to
25L
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Ive been using a 12L bucket and timing how long to fill them and converting. So might be a little out. However Yorkshire Water came and used a weir cup on my outside and He said it was off the scale his only went up to
25L

Unfortunately I don’t know how to measure dynamic pressure. Additional to above the Yorkshire Water measurement was at my outside tap not the boundary.

 

[EvilDrPorkChop]

Ive been using a 12L bucket and timing how long to fill them and converting. So might be a little out. However Yorkshire Water came and used a weir cup on my outside and He said it was off the scale his only went up to
25L

Where in relation is your outside tap to the point where it enters the property then?

You probably need to buy yourself a flow cup if you're wanting a better idea of flow rates.

 

[gary333]

Where in relation is your outside tap to the point where it enters the property then?

You probably need to buy yourself a flow cup if you're wanting a better idea of flow rates.

Outside tap is close to the entry point of the mains water pipe so totally appreciate itl’ll give a better flow even though only on 15mm compared to the 2.5m higher bath that’s on 22mm priority feed from the mains stop tap. Is the bucket test not good enough to give idea? For hot water I am using D.36 on boiler.

 

[EvilDrPorkChop]

Outside tap is close to the entry point of the mains water pipe so totally appreciate itl’ll give a better flow even though only on 15mm compared to the 2.5m higher bath that’s on 22mm priority feed from the mains stop tap. Is the bucket test not good enough to give idea? For hot water I am using D.36 on boiler.

The 15mm won't give much resistance in reality if it's near incoming.

You have to remember the water has to pass through the Plate HEX which will cause a bit of resistance so you'll never get what your incoming is through the boiler.

You also have to remember your showers and bath mixers maybe thermostatic too (Said you had a
Aqualisa Shower/Bath mixer?) so they'll be mixing cold and hot together to give you a combined flow rate. So even if d.36 is only showing 11/13lpm it's possible that's because your valve is mixing X amount of Hot with X amount of Cold. Hotter the hot the more cold it will use hence less flow required from the boiler.

Bucket method isn't brilliant. If you're so concerned spend £15 and get a proper flow cup.

 

[gary333]

The 15mm won't give much resistance in reality if it's near incoming.

You have to remember the water has to pass through the Plate HEX which will cause a bit of resistance so you'll never get what your incoming is through the boiler.

You also have to remember your showers and bath mixers maybe thermostatic too (Said you had a
Aqualisa Shower/Bath mixer?) so they'll be mixing cold and hot together to give you a combined flow rate. So even if d.36 is only showing 11/13lpm it's possible that's because your valve is mixing X amount of Hot with X amount of Cold. Hotter the hot the more cold it will use hence less flow required from the boiler.

Bucket method isn't brilliant. If you're so concerned spend £15 and get a proper flow cup.

Appreciate what your saying mate. I measured the cold 24L from the pipe feeding the aqualisa. The aqualisa itself seemed to go to about 18L.

Vaillant have agreed to come out but I don’t want to risk a charge if they say nothing wrong

Do you think I should ask Vaillant to remove 14L flow restrictor on this model. On the 832 you fitted, did you remove the 12L flow restrictor as it’d be great if that’s a simple way of getting flow up?

 

[Davefal]

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 ??

 

[gary333]

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 ??

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.

 

[Davefal]

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

 

[gary333]

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
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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 ?
[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.

 

[EvilDrPorkChop]

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.