Boiler diverter valve

[Ian Nutt]

I have a Vaillant combi boiler and seem to have a problem with the diverter valve. The hot water only comes on when the heating is on. I have taken the diverter valve out of the boiler to study it and can see that the diaphram is supposed to push up a pin that forces a hammer to fall and make a contact that ignites the burner and diverts water into the hot tap system.

When I turn the hot water tap on you can see the hammer try to move down to make the switch contact, but it does not have the energy. It would seem that there is not enough pressure to push it hard enough.

Does anyone out there understand what i'm talking about, or, even better, know what the problem might be?

Kind regards.

[Ed Sirett]

Which model? They work by very different mechanisms.

[Ian Nutt]

Ed - thanks for getting back. I don't know! Cannot see a model number written anywhere and have no documents on it. Do you know how I can identify it?

Cheers.

[Ed Sirett]

Please describe the general appearance - eg. grey/black/beige control panel. Permanent pilot or electronic ignition? Fan assisted flue or balanced natural draught flue?

Anyway from the description is it most likely a split diaphragm in the water flow sensor.

If you look around and perhaps remove or swing-down a panel you will likely find the model type and rating plate.

[Ian Nutt]

Ed,

I have taken the right hand side panel off and there is no identification plate visible anywhere, just various stickers with nothing that looks like a serial number. The pump has 'Type VP5' written on it, but I assume this refers to the pump.

The control panel is black, with green dials and a pressure gauge bottom right. There are 4 dials (on/off/relight, water temp, ch temp, function setting) and 7 indicator lights that light up green to indicate when the burner is on etc. It is electronic ignition and naturally vented flued I think.

I took the brass diverter valve out over the weekend and took it apart. There was a water chamber above and below the rubber diaphragm, and the diaphragm didn't look out of shape. There is a pin attached to a plate which is supposed to push up when the pressure of the mains water is released. I made sure it was loose enough, and refitted it. It didnt do any good - when the hot water tap is turned on you can see the hammer try to fall to make the electrnic circuit that will ignite the boiler, but it is obvious that the pin inside is not moving up far enough to push this little white plaster hammer down to make the circuit.

Does this sound familiar?

[Ed Sirett]

Aha we have a vaillant VUW this is the model comes between the VCW and the latest Turbomax+ series. It is forced flue with a fan. IIRC.

In this model the DHW is fired from the microswitch mounted on the right hand side of the diverter valve/flow valve. The water flow diaphragm operates directly on the diverter valve and the microswitch.

You could have a problem if the diverter valve is stuck/sticky.

[Ian Nutt]

Exactly right! The microswitch is a tiny fellow which makes the circuit when this white plastic hammer I keep mentioning pushes down on it.

How do I ensure the diverter valve isn't sticky? Any tips?

Thanks again for your continuing attention.

[Ed Sirett]

There may be an inadequate flow of water to correctly operate the valve. There may be a split diaphragm in the valve. The diverter valve might be too stiff to move easily enough: i) failed lubrication ii) sludge etc. in pirmary iii) mechanical failure (unlikely)

The micro switch could be iffy - not enough to show when you operate it with a finger but enough when operated by the valves.

[Ian Nutt]

I have bought a new diaphragm and will fit that on Saturday - ill let you know!