My trusty boiler that received a massive overhaul just 12 months ago with pretty much everything but the fan and PCB being replaced started playing up with F.22 (low system water pressure) leaving us with a cold shop to come into. A quick press of the "flame" button and it would fire up. Sometimes to stay on quite some hours and sometimes to stop a short while later with the same code. Everything on-line says it's a simple matter of topping up the system water pressure but the water pressure was always fine... So lots more digging and the only reference I could find was a chap being extremely cagey and saying it was a "gas" issue and needed a "gas engineer" to sort out.
Reading between the lines this was a "regulation" of the site/forum that no-one could discuss the gas-side of boiler repairs. A brief hint of replacing NTC sensors slipped out though. <lightbulb>
2 days later for the princely sum of £11.07 I had 3 x Thermistor NTC Sensors (252805) for my boiler.
Last night I entered the burner box (3 spring clips) and spied an NTC (sensor with a spade connector) on each side of the main heat exchanger. Both were only finger tight and against expectations no water came out when they were removed.
Screwed in the replacements, put the burner box back together and fired it up. This morning we came into a lovely warm shop and the boiler's been running without a hitch.
Only really posting this story as it was a bastard to find the right answer and the fault code is a bit of a red herring. Original Valiant part would have been £20 per sensor!
Now I'm just waiting for the "Gas Safe police" to cart me off for tampering with a gas appliance without a certificate of competence.