Mike4 wrote:UncleEbenezer wrote:It's a Baxi Combi 80Eco.
Ok I've had a look in the manual, and this was Baxi's most basic "Ford Escort" of a boiler, and comes with a wide variety of different badges on it including Baxi, Potterton and Main. A massive seller and a non-condenser killed off by the regs in 2005 which made steamers mandatory. A cheap and cheerful boiler which generally works quite well but is subject to a number of faults that should have designed out 15 years earlier. It is generally easy to fix and all parts are freely available.
It has a hydraulic diverter valve that is particularly troublesome which needs to fully divert in order for the HW to work. If it is sticky or has a perished diaphragm it only partially diverts and the boiler fails to fire in HW mode. Turning the CH on disguises the problem by making the boiler fire anyway so I reckon this is your problem causing the cold showers. There is a possible alternative but related cause. There is a pin which extends from the diverter valve when it diverts into HW position and presses a microswitch to fire the burner. The water seal around the pin degrades with age and water gets in the microswitch stopping it working. You could have either problem, or both. I prescribe a new diverter valve and switch assembly. Parts are about £150 plus mebbe 90 mins on site to fit and test.
Regarding the pressure loss, this could be a failed or flat expansion vessel causing the pressure to rise excessively then discharge via the pressure relief valve to outside. This should have been checked in the service you had done though, particularly if you reported the pressure loss to the bod. He should really have been able to tell you about the diverter problem too as is is really common on this really common boiler. If this is the problem, a plastic bag tied over the end of the PRV discharge pipe will collect some tell-tale water after the next time you re-pressurise the system. I think though, you may have mentioned NOT getting wide pressure swings so this still points to a slow system leak.
At the same time I suggest demanding a new domestic water-to-water heat exchanger if you are feeling flush as it is hardly any more work to fit this at the same time, and yours may well be contaminated which causes the HW temperature to swing up and down over about a 45 to 60 second cycle, which I think you also mentioned happening.
Finally, these boilers have a single NTC thermistor to control all the temperature functions and this too is a common fail, so at only about £25 for the part I recommend getting this changed at the same time too.
Cost to do that lot will probably be £500-£600 by a local bod, unless you decide to cut your losses and get a new one.
That is a masterly remote diagnosis, I am very impressed. It reminds me only too well of UK-designed & UK-manufactured cars from the 1970s given the known but unaddressed faults. I am encouraging my boiler, and my GF's boilers to last as long as possible so that ASHP get beyond the equivalent stage !
regards, dspp