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