Vaillant 937 plus Hot water flow

[Agile]

1) The problem - house is luke warm in winter not hot and water flow to hot taps is poor. Also if you turn 2 hot taps on then 1 goes cold (the 937 should allow 2 or 3 hot taps on at a time).

2) Identifying the problem - we measured the cold water flow from the water main with a funnel device (I think it is called a Weir gauge). This measured in excess of 25 litres per minute. Then we cut the hot water pipe just below the boiler and measured the water flow. It gave 12 litres per minute. This proves that it is the boiler and nothing else giving the problem.

Please note I am not a plumber but have had to become an expert in this as very few people seem to know about this issue.

You do not seem to be such an expert on boilers. I have explained three years ago on this thread as well as many others that the open pipe flow rate is not what is required. Its the dynamic flow rate. For your boiler about 20 li/min @ 1.0 Bar is needed.

You dont say if you cut the cold in or the hot out pipe. I can only guess the hot out.

Both supply pipes and boiler have a flow resistance and this reduces the open pipe flow rate to about half in your case.

I agree this is not well understood but its only GCSE physics involved and anyone working on boilers should fully understand it although many dont.

You need to upgrade your supply pipes etc.

Tony Glazier

 

[vaillantassisstancefroman]

Thanks for the reply.

I am not a plumber - I work in an office.

I measured cold in to the boiler >25 l/min
and hot out <12 l / min

Understand the issue over resistance but surely this could not reduce the flow by more than half (the pipe run between the 2 is less than 1 metre). Both were measured as open flow rates i.e. water flowing from an open pipe.

Will know more in a few days when the flow rate limiter and filter have been changed.

 

[Agile]

Understand the issue over resistance but surely this could not reduce the flow by more than half (the pipe run between the 2 is less than 1 metre). Both were measured as open flow rates i.e. water flowing from an open pipe.

I dont know why you think the flow reduction should be any particular value.

The reality is that a boiler can add a very considerable flow resistance.

Thats why only the dynamic flow rate is relevant when specifying a combi boiler! Search on the forum for further explanations and try to understand them.

It would be better to remove the flow restrictor in your case.

Tony

 

[Beerlover]

The 937 has a filter - if this is blocked it is an installation fault not the boiler. It also has a 14 litre flow limiter. To achieve a flow rate of 20 l/min it has to have a blending valve fitted to the hot water outlet. Bet you 10 pence to the pound it doesn't have one.

 

[vaillantassisstancefroman]

Thanks to you also.

I thought there was a blending valve fitted in the boiler. maybe I am wrong?

If not then what do I need and how do I ask someone to fit it correctly.

 

[vaillantassisstancefroman]

Are you referring to an "opt. accessory 0020057235 for integrating a circulation pump" which is referred to in teh manual or somethign else?

 

[Bunnyman]

Vaillantassistanceman u have had to become an expert ffs! lol! 15 l/m max outlet what makes u think u know better than Vaillant engineers? Could be blocked filter or dhw pump or known problem of dip tube deteriating in the tanks this causes poor hw flow rate is a bastard job but factory engineers would find this u got no chance ps if your house is cold with a 37kw boiler ur rads are to small or u got no windows,,,

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• Bunnyman
• 26 Mar 2010
• 499