Vaillant ecoTEC Plus 938 - Low Flow Rate

[gary333]

Hi guys. Hope you are able to help me, and thank you in advance for anyone who can contribute.

I've got a Vaillant 938 and notice that the hot water output isn't near the claimed flow. I've tested the pressure at the stopcock and it's around 3 bar. I've tested the cold flow rate from the outside tap and it's around 21-23L per minute.

With one bath tap turned on max (Aqualisa Shower/Bath mixer) with JG Speedfit 15mm flexi's i'm getting 10.5 - 11.0L on D.36. If I open 3-4 other outlets I can get D.36 to report a maximum of 13.1 - 13.2L.

The manual states 15.9L output (however I thought this boiler could output up to 20L as that's why I bought it). So if the boiler is only capable of 15.9L - how can I acheive this, and where does the 20L figure come from?

For a little bit of detail about the pipe work as I think i made it far to complicated, however it's:

-Hot = 15mm DHW outlet is converted straight in to 22mm for a distance of 1.5m and run in to the ceiling. From here it's reduced to 3x15mm pipes. 1 pipe goes straight to the bath, the other pipe goes straight to the shower and the third pipe goes to all other outlets including basin, sink and outside hot tap.

• The stopcock was replaced with a 25mm > 22mm full bore stopcock
• Boiler cold feed is in 22mm direct feed is first tee off the stopcock going to the boiler (stopping off at the cold for the bath and shower only on it's way).
• 15mm pipe is used as second tee above to feed all other outlets including toilets, basins, sinks etc

The 21-23L flow rate was tested from the second tee'd 15mm pipe.

Cheers guys

 

[Scott_D]

Is this a new boiler?

 

[gary333]

Is this a new boiler?

Hi Scott,

The boiler was fitted around 1.5 years ago, however the house has been in a stage of renovation so we only moved in a couple of months ago. The boiler was used very little since fitting until a couple of months ago so the problem wasn't noticed. Although the person who fitted the boiler did note down the flow was 12L per minute on the benchmark form.

I'm also, having problems with the return temperature delta being low (5-7 degrees), however thought I would ask just one question at a time as assumed these would not be related
[automerge]1589884197[/automerge]

Hi Scott,

The boiler was fitted around 1.5 years ago, however the house has been in a stage of renovation so we only moved in a couple of months ago. The boiler was used very little since fitting until a couple of months ago so the problem wasn't noticed. Although the person who fitted the boiler did note down the flow was 12L per minute on the benchmark form.

I'm also, having problems with the return temperature delta being low (5-7 degrees), however thought I would ask just one question at a time as assumed these would not be related

Additionally the boiler is a Revision 6 if this makes any difference.

 

[oz-plumber]

The flow rates are probably with a 25C temperature rise, with no pipework or fittings connected to the outlet of the boiler.
You are probably requesting more than a 25C temperature rise from the cold water temp to the hot water temp and you also have pipes, fitting and tapware restricting the flow.

Take it up with the manufacturers regarding their claims.
Not that it would bother them, you have already purchased and installed one of their boilers

 

[gary333]

The flow rates are probably with a 25C temperature rise, with no pipework or fittings connected to the outlet of the boiler.
You are probably requesting more than a 25C temperature rise from the cold water temp to the hot water temp and you also have pipes, fitting and tapware restricting the flow.

Take it up with the manufacturers regarding their claims.
Not that it would bother them, you have already purchased and installed one of their boilers

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.

 

[Scott_D]

I think the 20lpm requires a blending valve under the boiler

 

[oz-plumber]

Boiler manufacturers!!! - dishonest about their abilities of hot water on their combi's.

You'll be barred from this site - forever - with a statement like that....

They mean 35C temperature rise from the incoming water supply will give you 15.9 l/m
And 13.2 l/m @ 42C temperature rise.
So depending in the temperature of the incoming water will depend on the temperature of the water selected.
Higher temperature of water selected = lower flow rate of hot water.

I would only ever put a combi boiler on a property with 1 bathroom, no matter what the manufacturers specify their units can do.
I have installed them to a property with multiple bathrooms, but only had a couple of fixtures fed off the combi.

They have their place in the market, but I only find it a limited place

 

[gary333]

I think the 20lpm requires a blending valve under the boiler

Hi Scott. I think yo

Boiler manufacturers!!! - dishonest about their abilities of hot water on their combi's.

You'll be barred from this site - forever - with a statement like that..

They mean 35C temperature rise from the incoming water supply will give you 15.9 l/m
And 13.2 l/m @ 42C temperature rise.
So depending in the temperature of the incoming water will depend on the temperature of the water selected.
Higher temperature of water selected = lower flow rate of hot water.

I would only ever put a combi boiler on a property with 1 bathroom, no matter what the manufacturers specify their units can do.
I have installed them to a property with multiple bathrooms, but only had a couple of fixtures fed off the combi.

They have their place in the market, but I only find it a limited place

Would I be correct to say that in this instance I am not being unreasonable in expecting more flow? I set DHW to 35oC. At this time of the year with outside temperature around 18-20oC so I wouldn't expect the water to be under say 8oC thus the delta should would be a lower differential than the manufactuer have used to give 15.9L?

 

[king of pipes]

It will only do 15 litres a minute I would add restriction to the shower heads 8 litres a minute is plenty for a good shower any more is a waste and we should all do our bit to save water. Kop

 

[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
[automerge]1592573671[/automerge]

 

[gary333]

Have you got a Big C on your display
[automerge]1592573671[/automerge]

Hi Dave,
I do have comfort mode on (C logo on constantly) although the flow rate is the same whether it’s on or off and regardless of temperature set.

 

[Davefal]

Check d12 if cylinder pump is on off

 

[gary333]

Check d12 if cylinder pump is on off

It’s off at the moment when no hot water demand. However it does show as “on” when hot water is run.

I tried it with comfort mode off and the charging pump does show as on with hot water too.

I notice if I go into the “function menu” and select “ cylinder charge pump” test (T.04) I cannot here any noise at all though. This is the same when selecting T.05 “Circulation Pump”. T.01 “Internal Pump” is audio-able.

Thanks for helping
[automerge]1592577403[/automerge]

Check d12 if cylinder pump is on off

It’s off at the moment when no hot water demand. However it does show as “on” when hot water is run.

I tried it with comfort mode off and the charging pump does show as on with hot water too. I notice if I go into the “function menu” and select “ cylinder charge pump” test (T.04) I cannot here any noise at all though. This is the same when selecting T.05 “Circulation Pump”. T.01 “Internal Pump” is audio-able.

Thanks for helping

 

[Davefal]

So u got to suspect charging pump or it’s control
Talk to Vaillant it should still be under warranty

 

[gary333]

So u got to suspect charging pump or it’s control
Talk to Vaillant it should still be under warranty

Luckily it’s got a 7 year warranty and isn’t 2 years old yet. I did mention this to Vaillant before and they said there is a pump in the tank but it just pulse and might not be audioable from outside. However I have no idea it they meant cylinder charge or circulation pump. Do you know if this of the case, or should I be able to hear one or the other?

cheers

 

[EvilDrPorkChop]

Doubt there's anything wrong with the boiler as said previously. Down to incoming pressure and pipe sizing. It'll only be able to perform to what's incoming and if the hot is piped up in 15mm from the boiler to the rest of the property then you aren't going to get 20lpm.

 

[gary333]

Doubt there's anything wrong with the boiler as said previously. Down to incoming pressure and pipe sizing. It'll only be able to perform to what's incoming and if the hot is piped up in 15mm from the boiler to the rest of the property then you aren't going to get 20lpm.

Hi ya, I have a 22mm hot pipe from the boiler split in to 3 separate 15mm feeds in ceiling. One to bath/shower, one to another shower and third to all basins and sinks. Incoming is 28L at stopcock, 24L at bath and 22L at point before it enters boiler. Static pressure is 3.2bar at moment. Would this still not be good enough?

 

[EvilDrPorkChop]

Hi ya, I have a 22mm hot pipe from the boiler split in to 3 separate 15mm feeds in ceiling. One to bath/shower, one to another shower and third to all basins and sinks. Incoming is 28L at stopcock, 24L at bath and 22L at point before it enters boiler. Static pressure is 3.2bar at moment. Would this still not be good enough?

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.

 

[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
[automerge]1592595914[/automerge]

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.