Worcester 280 combi-boiler

Can anyone help with this?
After we've run a bowl of water for say washing up, the overheat thermostat (OT) operates and has to be reset (by pushing down the button). The strange thing (at least to us) is that this only happens if:
1. we run the hot water tap for just long enough for the water to get hot and fill one bowl, and
2. the central heating system is off.
If the central heating is on, the OT doesn't operate and doesn't need to be reset.
However, if we run the hot water tap for somewhat longer, e.g. to fill a bath, then the OT does not usually operate (and doesn't need to be reset), regardless of whether the central heating is on or off.
We've had the system descaled (at substantial cost) but this didn't solve the problem. The engineer tested the boiler and found that it was registering a temperature of only about 65 degrees when the OT operated so there doesn't actually appear to be any overheating. The OT has in fact been replaced in the past (before descaling was carried out) but this also didn't solve the problem.
Anyone had the same experience or got any suggestions how to solve the problem?
TC
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On Sat, 07 Feb 2004 03:34:20 +0000, TC wrote:

And why has the OT not been replaced this time? It looks like it has failed again. Perhaps you should buy two so you have a spare? At the very least buying two will ensure that the OT won't fail again!
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Ed Sirett - Property maintainer and registered gas fitter.
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Thanks but replacing the OT didn't solve the problem last time so I'm not sure why it will this time around. Do you think OT replacement might work this time now that the system has been descaled?
TC
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On Sat, 07 Feb 2004 23:55:01 +0000, TC wrote:

OK the OT should not trip with a flow temperature of 65C. However, where and how was that 65C measured if it was not measured at the OT itself it might be the OT was doing its job right. In which case we are looking for some reason why the OT was too hot but the 'boiler registered temperture' was only 65C. There are a number of possible causes for that.
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Ed Sirett - Property maintainer and registered gas fitter.
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