The overall setup comprises an S-Plan system with gas boiler and unvented primary and secondary circuits.
The DHW part is based on a Santon Premier TP120B (120 litre unvented indirect) cylinder and the other components are broadly as per the relevant Wiki - http://wiki.diyfaq.org.uk/index.php?title=Unvented_DHW The cylinder was installed by the previous owner in about 1999.
When I arrived at the flat yesterday, after a period of absence, I turned on the cold water and set the heating/water programmer for DHW as usual, and the boiler fired up as expected.
However, the boiler was still running two hours later - far longer than it should take to heat a tank of water. I then found that water was running into the tundish from the T&P valve - so I was using gas to heat water and simply pouring it down the drain!
The T&P valve is only supposed to open if the temperature exceeds 90-odd degrees, or the pressure exceeds 6 bar - but I'm pretty sure that neither of those was the case. The boiler was cycling on its stat, which was set far below 90 degrees - so the boiler couldn't heat the cylinder to above its own temperature. If I turned a hot tap on, the water coming out wasn't excessively hot - and there was no steam released into the tundish, just hot water. I don't have any means here of measuring the HW pressure, and the only gauge is on the primary circuit - which was reading about 1.2bar cold and 1.4 hot. I did measure the air pressure in the DHW's expansion vessel, and that was about 2.5 bar - so I don't see how the water could have been at a higher pressure.
This morning, I tried heating the water with the immersion heater rather than the boiler. That took a long time because it's only a 1.5kW element - provided for backup purposes - but that seemed to be ok without discharging through the T&P valve.
Water being discharged usually indicates a failure of one of the control systems - thermostats, etc. - resulting in over-temperature/pressure. Since that doesn't seem to be the case, another possibility is that the T&P valve is faulty. I did open the T&P valve manually quite a few times to try to flush out any crud which may be caught in it, but that made no difference. If it *is* faulty, I would expect it to leak when using the immersion heater - which it didn't. I'll try that again tomorrow in case it was a fluke.
Meanwhile, if anyone has any bright ideas about what to do in order to get an accurate diagnosis, please let me have them!
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