It never rains but it pours

Scornful as ever of the cheap sort of scaremongering from BG about making sure to run your CH boiler during the 100F heat in summer to prevent problems later, I switched mine off for 24 hours at the weekend to replace a radiator in one room. As I had to relieve the pressure in the system, it also gave me an opportunity to recharge the expansion vessel.
Next day, repressurised it, bled the radiator, switched on and... nothing. The pump ran, but not much else was happening. However, the high limit stat button appeared to be out, so I pressed it back in. It felt oddly resistant and stayed out when released. Boiler still wouldn't light with it held in, nor with a temporary wire bridging the contacts. Ho hum, out with the boiler manual and a multimeter and to work. Removing the boiler front revealed the fan was not spinning, so at least it wasn't another ignition module failure like the last outage (more later). Followed through all the wires, mentally replacing pins A, B and C in the flow diagram with 4, 2 and 3 on the unit, to find the water flow switch apparently not operating. The pin operating the microswitch lever was very stiff indeed and seemed to have stuck out (off). I pushed it back in with a screwdriver and the boiler lit. The pin now remains in all the time...
How do two components decide to fail simultaneously ? Worse still, with all problems coming in threes, what nasty remains lurking, waiting until the first frost...
[This boiler also packed up in the coldest week last winter, right at New Year. I sourced a new ignition module and installed that with success, but a persistent "pulsing" remained, where the burners would start to cut out, then fire up, then cut out again, and so forth. Having eliminated what I thought were all reasonable possibilities, I finally replaced a fan, having found a speedy online supplier. To great irritation, it was no better. Finally decided to dismantle the external flue figuring there really had to be some invisible blockage, to find that one joint collar had rusted apart internally, allowing the 1.5" inner exhaust pipe to dislocate about 1/8" at one spot. This obviously upset the fan enough for the pressure switch to trip. Still haven't located a replacement, so the flue is 2 feet shorter.]
Mail john rather than nospam...
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