Vaillant ecoTEC Plus 938 - Low Flow Rate

[EvilDrPorkChop]

Flow is going to be dependant on your pipe sizing too not just pressure. If it's a 15mm supply to the boiler then 15mm hot supply pipe around the house then even at 3 bar you're not going to get 15lpm. To get the full flow rate you'd need a 22mm supply to the boiler and it coming off in 22m then sizing down accordingly. Also take into account Aqualisa will flow limit the processor - it hasn't been set in Eco Mode has it?

I've just done a 832 with a 22mm supply and we were getting 15lpm on a 45oc rise.

 

[John.g]

Hi Oz,

Thanks for looking . The manual states the output is based on a DT = Δ35K which I assume mean 35oC. I have tested the output by setting the boiler at it's lowest DHW setting of 35oC. However, the output is the same if I raise to any temperature.

I think you've probably hit the nail on the head though with regards to the 20L per minute. I bet they expect a customer to keep water in the hot tank @ 70oC and then the 20L is only possible at an outlet when it's mixed with cold. Seems very dishonest as surely any boiler could be set to 70oC and then claim it has a much higher output if the cold is to be mixed in.

Boiler shows in manual:
Domestic hot water flow rate (at DT = Δ35K) = 15.9L
Domestic hot water flow rate (at DT = Δ42K) = 13.2L
No mention of the 20L at all.

What is the boiler output?
15.9 LPM @ 35C deltaT requires a boiler output of 38.83 kw
13.2 LPM @ 42C deltaT requires a boiler output of 38.68 kw (the same)

I think you have a 38kw boiler so a flowrate of 20 LPM will give a deltaT of 27.23C which will give you "hot water" at 35.32C from cold water at 8C or 37.23c from cold at 10c and so on.

 

[gary333]

Flow is going to be dependant on your pipe sizing too not just pressure. If it's a 15mm supply to the boiler then 15mm hot supply pipe around the house then even at 3 bar you're not going to get 15lpm. To get the full flow rate you'd need a 22mm supply to the boiler and it coming off in 22m then sizing down accordingly. Also take into account Aqualisa will flow limit the processor - it hasn't been set in Eco Mode has it?

I've just done a 832 with a 22mm supply and we were getting 15lpm on a 45oc rise.

Hi, It's piped in 22m from a full bore stopcock direct to the boiler. I don't know if this boiler has EcoMode, how would I check please? I've tried it in both "Comfort Mode" so the storage tank is energised, and also in hot water mode so the tank is just holding cold water and run boiler at 35oC, 50oC and 75oC but the flow appears to be limited.

I've spoken with Vaillant installer helpline a couple of times now and it appears they don't really know much about this boiler. Some of the them have said that it will run at 20L for first 10 minutes regardless of flow as it's using the tank, others have said it'll only do 15.9L, others have said there is a 14L restrictor on the cold monoblock feedpipe in to boiler so won't ever go above this.

None of the people at Vaillant have mentioned a blender valve, which after searching on the internet the 937 needed to get 20L and I believe the 938 is just a slightly tweaked version of the 937.

 

[Davefal]

It’s a storage Combi so it needs to be set to C mode then it will give supply from inlet also from the storage cylinders to give u the higher flow rate ,when they run out flow rate will drop

 

[John.g]

I think the 938 has a 15L storage tank? so if its storing the water at 70C and the user requirement is 45C then it looks as if it could only boost the normal 15.6 LPM to 20 LPM for approx 5 mins.

 

[gary333]

Apologies guys for the gap in responding.

I've been on to the "installer helpline" at Vaillant a few times as they are the only people when phoning Vaillant who seem to know about this model, and even then I've had contradictory information so still not sure what to believe.

Person 1 advises:
Boiler should be able to deliver 20L per minute for the first 10mins. He says no need for a blender valve as that's not something Vaillant even supply. He was under the impression that even if the incoming flow rate wasn't at 20L the tank would still allow it to work at this level until it goes empty. When I pushed him that I cannot get anywhere near that level regardless of temperature set he looked at the schematics and said there was a 14L flow restrictor on the incoming main "monobloc". He advised to remove this and see what happens.

Person 2 advises:
Straight away said you need a blender valve and that was the only way to get 20L by cooling down high temperature boiler output with the cold water. I asked him why the literature said 20L is possible and why there was no mention of this, he said he understood but said installer should know this (no idea how, if no material website, installer manual etc mentions this in any shape or form). I asked him about the 14L restrictor. He said Vaillant don't recommend removing this. He also said the flow rate was simply based on what's achievable technically and that the temperature has nothing to do with output, this will either get hot or not get hot. I asked about the 15.6L quoted in the manual, and how that's possible if you limit output to 14L but he just seemed to repeat you have to use blender valve.

Super confused. I have asked for a Vaillant warranty claim. I spoke with my plumber and he had never heard of the whole house blender valve and said he only fits at each appliance usually.

The water company have increased pressure to my house (as I raised with them a faulty pressure reducing valve in the street) and if I turn on all taps I can get for fleeting moments 14L passing. This seems to confirm there is a 14L restrictor as over 25L is available to the boiler. Should I get tech support to remove the restrictor or could there be unwanted consequences.

Thanks once again people.

 

[Davefal]

Have you got a Big C on your display
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