There's a wiring diagram here which a quick glance at seems to show the BDR91 firing the boiler.?
They are known for sticking yes but they wouldn't normally cause one to switch when the other was disconnected, they normally either switch or they wouldn't. That being said it may be the SL from one that's causing a problem in the other. The microswitches may be getting sticky/intermittent, if they continue to be problematic then best approach would be to replace the head(s) if they are replaceable.Its going to be hard to do any more because its working normally now. Which is what makes me think something mechanical was stuck/sticking. And zone valves are prime candidate for this.
Honeywell used to be the best, but they’re not as good as they once were. Others still rate them.They are this type, don't know if they are good ones or not.
With valve in manually open position, microswitch will not operate. It only operates under power.With a multimeter, on ohms setting. System powered off and manually opening closing the valve to check function of the microswitch.
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