Vaillant problem??

[bobnew]

My Vaillant boiler fires up after opening, then closing the cold water tap,
only for a couple of seconds then stops. The boiler has been working fine
until I had a water meter fitted by the utilities water board.

Is there any kind of pressure switch or flow sensor that can be adjusted?
Thanks for any help you can give me.

Bob

 

[kevplumb]

you need a smaller one of these

 

[chrishutt]

I wouldn't worry about it.

What is probably happening is that the water meter is slowing the flow from the main and so the pressure in the hot pipes takes a few seconds to return to normal after opening a hot tap, hence continued flow through combi, hence continued burner flame.

As long as its not causing any overheating (which would trigger one of the overheat stats) you haven't got a problem.

 

[corgigazza]

Fit non return valve on cold water inlet to boiler but consult manufactures as some insist on small ex-vessel also.

 

[Agile]

As usual this person has not mentioned which of the many models of Vaillant boiler he has.

If its the old Sine T3 then it has a shock arrestor in it which can cause this action. It does not do any harm. Some later models also have it too.

It can also be caused by a dead leg full of air in the hot water pipework. This will probably dissolve after a few weeks.

If the boiler has a shock arrestor then the non return valve can be added as it absorbs expansion.

Tony Glazier

 

[chrishutt]

But how would a non-return valve solve the "problem"? It won't stop flow into the boiler from the cold supply, which is what seams to be triggering the boiler.

 

[ChrisR]

An air bubble in the pipework somewhere often causes this - something which may well have got in when the meter was changed. It acts elastically, allowing the water to "bounce" to and fro. If a one way valve is introduced (in the appropriate place) the water can move only one way, and is held there, hence reducing the movement or pressure pulse which the combi detects and interprets as a tap being turned on.

A good flow through an outlet (tap, float valve, etc) will usually clear bubbles like that. So try to achieve that through all outlets. The oxygen part of the bubble will dissolve fairly quickly if left alone, the nitrogen less readily.

 

[DP]

I would suggest follow what Chris says and install a double check valve in the cold inlet to the boiler.

It would seem the cold water tap (when run) is allowing the air in the hot dead leg to push the water back towards the boiler. This reverse flow would not activate the water section (higher pressure top side of diaphragm). But when the cold tap is closed, mains pressure compresses the air bubble, momenterily switching the boiler on as if a hot tap was run for a second..

Check valve will not allow reverse flow of water.