Worcestor Bosch 240 modulation problem?

[Andrew Collins]

I have a worcestor bosch 240 boiler. An excellent piece of kit that keeps my house toasty right through winter.

The problem is with the DHW side, you need to have a tap on full for it to kick in and starting heating the water. It also has a nasty habbit of switching off mid-shower and then switching back on again a few seconds later.

I had assumed that this was all down to poor water pressure. However, a friend of mine has just bought a house that also has a WB240 in, and the water pressure there is significantly *worse* than mine, yet the boiler kicks in when the tap is just dribbling and you get lovely, piping hot water.

So I'm guessing there's something up with my boiler, perhaps some sort of pressure sensor somewhere?

The diverter valve was replaced a couple of years ago and had no effect on the problem..

Any help would be greatly appreciated. Andrew Collins bflfcbb hotmail com

[Ed Sirett]

This symptom is alway due to the hot water flow sensor. As often as not these involve measuring a difference in water pressure and applying this to a rubber diapragm (which can split) which moves stuff including a microswitch (which can and do fail).

Users who find that they need to open the HW taps more and more to get the boiler to fire will very likely find that the rubber diapragm is splitting as the split get bigger it takes more pressure to move it.

I'm beginning to get the idea it might be time to write a Boiler Problems FAQ (with their causes and solutions). This could be a collaborative effort ... I might start something off in the New Year.

[stuart noble]

I have a new Bosch 24i Junior and, although I wouldn't say the tap has to be full on, it needs a lot more water than my previous combi to fire it. It has to run cold for a set period, which is bloody annoying if you're trying to add extra hot water to the sink. I put it down to some crazy Bosch philosophy. A friend has your cutting out mid-shower problem and she has only just realised that she's on a shared mains supply with next door. I suggested she apply for a water meter, which presumably would result in her own mains supply, possibly at a good price.

[Dave W]

Im sure a lot of people would really appreciate that, it would also mean that Dimm would then be able to correctly diagnose boiler faults as wall as everything else.:-( BTW girlfriends BAXI fault turned out to be a dry joint on the control board.

[ritchieaber]

In my humble experiance, with this boiler it is the thermistor that measures the DHW temp that intermittently goes open circuit. It can be proven by swopping it with the ch one, will need to depressurize first. The hot water flow sensor on these models is a travelling magnet in the flow and reed switch.

[IMM]

What! Maxie, another one at it. They are after the love of your life, Dim Lin the far eastern luscious lovely. Maxie, these ponses should lay off your chicky.

[raden]

In message , Andrew Collins writes

It could be the pcb, but more likely, the DHW temperature sensor (as it works after a fashion). The temperature sensor is a temperature dependent resistor, and it it's characteristics (resistance for a given temperature) have changed, then it will be giving the pcb a false temperature reading.

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[raden]

In message , IMM writes

That's it, you tell them

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[Andrew Collins]

Aah... so I'm guessing the boiler thinks it's producing hotter water than it really is, so switches off when it needn't?

Some sort of temperature sensing problem would make sense, as the DHW temp adjust knob seems to do very little (if anything).

Time to see if I can find a schematic for the boiler then!

Thanks!

[raden]

If you can't (it is in the installation manual), the pcb connectors are fairly well labelled IIRC

I probably have one at work if you're really stuck