Hi, any advice given would be much appreciated, I’m at my wits end! It all started last August when all the sudden I had the red ‘lockout’ light on my system boiler (Baxi Solo 24 HE). I would reset it and it would be fine for around a week then would need resetting again. I put up with this for around 6 weeks until I went away for a week and switched off the boiler altogether. When I come back from holiday and switched back on, the lockout never occurred again until last month. I just assumed that it had cleared out any air in the system naturally whilst the boiler had been off for that week in September. Now since March the lockout light has been a nightmare. Sometimes it will reset, sometimes it will take 5-6 attempts. When I leave it overnight then reset it in the morning it seems to reset first time which points to an overheating fault. I have bled the radiators, the pump and the manual air vent on the flow to the hot water cylinder coil in the airing cupboard. Although there was air one of the radiators the first time I bled it made no difference to the lockout fault. After the initial bleed it did seem to cure it for a week then it would lockout again. I tried bleeding again but water comes out of all radiators and vents straight away. After looking at various forums and YouTube videos, I decided to replace the pump. Although it appeared to be working fine I just wanted to rule it out. I replaced a Grundfos UPS 15-50 with a UPS2 15-50-60, and at the same time I checked over the diverter valve (Honeywell) and am pretty confident that it is operating correctly. I manually turned the spindle on the valve to make sure it wasn’t seized but it is opening and closing when it needs to. After I replaced the pump it ran perfectly for 4-5 days then the lockout faults started happening again and is becoming more of a pain to reset. I had to drain the system down to change the pump as both isolator valves on each end were leaking. Since it has been topped up the boiler sounds like there some water slowly swirling around (sorry hard to explain!) I had not noticed this before. Wrongly or rightly I put the original pump back on as I did not want to waste money on a new pump. The next step I tried was to run some Sentinal X800 for a couple of hours. I drained the system, topped up, drained again then added the X800 on the 2nd top up. The header tank was full of brown sediment which was obviously getting in my system as was evident on the original drain down. I cleaned the header tank, ran the X800 with everything on for about 3 hours, drained down, topped up, drained down again and I have now left it with some X400 in the system which I intend to leave in there for a couple of weeks. The water looks a lot clearer now. I have had the X400 in the system since Thursday but the lockouts are still happening. When the boiler is working everything is as it should be it only seems to happen when the boiler fires up after it is calling for heat. It never locks out when the heating and hot water are on but when it starts to call for hot water. The boiler thermostat is turned about ¾ of the way up. The tank stat is set to 60. The tank stat appears to be working in the sense that it comes on during the day when tank temp drops and turns off (if boiler actually fires up) when it has reached temp. I have tried both on a lower heat but makes no difference. Is there anything else I can try before calling out a plumber? I have not tried backfilling the system as I want to give the X400 a chance to work but is it even worth it? No air seems to come out when I bleed anything just water.
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