With the original spec DKO970 control box fitted (a new one, not the actual original unit that was fitted when the boiler was brand new 9 years ago) we noticed that the fire valve was letting air in, tiny amounts, tiny bubbles, every now and again. The transparent tube was full for a while, then a few minutes later there'd be tiny little bubbles appearing just where the tube is connected to the fire valve and soon the entire tube was practically all air, the burner was puffing like Puffing Billy and sending out puffs of smoke, whereupon the engineer said "That's going to lock ou!" and lock out it did. Then after locking out and reset pressed, the fuel started flowing again, after a fashion, allowing the boiler to fire again, and this just went on and on: Run for a few minutes -- air bubbles -- lockout -- recover.
However, the DKO970 seems much more sensitive than the TF 832.3 and made valiant attempts to let the boiler fire, but kept on locking out every few seconds. It was this that helped diagnose the fundamental problem.
The engineer said it absolutely should NOT be like that, so he fitted a new fire valve and pump. The old pump, well you cannot believe how shagged it was. He got me to turn the shaft and it wasn't just tight at one spot, it was practically rock solid! I held the pump in one hand and turned the shaft with my other hand and I really had to wrench the shaft past its tight spot. Bloody awful. I reckon the pump bearings must have been running hot with a tight spot like that, although how the motor and capacitor ever managed to start the pump turning, beats me. But start it did. I reckon the problem was compounded by the combination of the tight spot and the fire valve leaking in air. Somehow, the pump and control gubbins made every attempt to run, but that tight spot would nearly always be where the pump came to rest when the boiler switched off.
He reckons that if the pump has been sucking air over several months, even a year, it was pretty soon f***ed. The old fire valve was totally shagged. Sucking on the "to pump" side of it (as if siphoning), one could barely get anything through it. So maybe the pump, as the problem worsened -- remember how I said it started in July then got steadily worse, so that by October the lockouts were happening practically every day? -- had been trying its damndest to suck oil and so when air got in instead (through the duff fire valve) the pump somehow kept going, but was literally pumping itself into an early grave. With lack of a constant flow of fuel to lubricate itself, the bearings started running hot and the tight spot developed.
As for the other parts fitted (motor, solenoid, photocell, oh, well, at least they're not going to wear out any time soon! I'm just pleased to have a working boiler again at long last. Of course, in hindsight we should have checked the pump FIRST, but since it was only three years old, we assumed it must be okay.
My engineer and I have come to an amicable arrangement which involves draining and power flushing the system in the Spring and putting new inhibitor in.
MM