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Hey everyone still following! I replaced the valve actuator and the tank stat. On power on, I did the usual testing to make sure the valve swings properly under the right conditions and lo and behold (having hooked things up the same, part for part replacement), the tank stat was connected...

Thanks folks!! I had that suspicion and through my reading today with your generous help had started to test the voltages right when it was suggested, which is why it took me a few minutes to respond. :) Will order one in and update this thread with my results. Thanks again!

Lol, I'm getting that impression and per my last post, I'm measuring 240VAC on both terminals when heat is commanded/selected. I would agree the combo is cheaper, will likely go that route and just use the new head.

I am, and have just done so, 240VAC on both indicating the MA1 is not functioning correctly. I also measured continuity on the cylinder stat and it seems fine as I rotate the knob though I find the attachment strap very hokey. :p Next step is to replace the head.

Pinout, the connections to the boiler based on it's terminals etc. Pinout is more of an electronics term showing what each pin on a chip or system board is for. I just used the term for the boiler for simplicity.

Agreed though it does give me information on the connections to the boiler. This helps me understand that the wiring may not be properly done but depends on what the timer needs too.. I've not yet looked for that documentation

The boiler fires when it's supposed to. My problem is that the zone valve will never move to the middle or to central heat on its own, thus the house will never heat up. I only get hot water unless I move the actuator on top of the valve manually. All this other talk is more detail about my...

Water and Central Heat on, and room thermostat calling for heat: If I turn off central heat, the latched valve will swing to the hot water side. This is the state that will not recover, I cannot get the valve to automatically swing out of this position If latched and I turn off water heat, it...

I can do that, I'll get back to you

OK I drew out a diagram based on what is physically wired and it matches up with @ericmark diagram for system wiring except for some differences, but it's getting complex now and I don't want to tear apart a system that isn't mine as a renter. I think the honeywell timer/programmer adds some...

(oh BTW thank you everyone for your generosity so far)

As promised, here is a main view that I labeled to assist.. It doesn't appear that the installer used a layout that matches the diagrams above, and I don't have a dedicated mains feed; it must be coming up from the boiler or something but there's no cable from the mains panel to this closet...

Found my answer here https://www.diynot.com/diy/threads/how-to-test-honeywell-l641-cyl-stat.311552/ Tomorrow's project

I've wondered same, the tank thermostat, while held fairly tight to the tank, can be pulled out a bit AND I'm not certain if it's in the correct position. What I mean is that it can be rotated a full 360. A honeywell L641A1005, the dial on the front can spin a full 360. We did have a tech here...

I changed out the synchron motor in the head, the PCB etc are unchanged.. I wondered if there was something in the remains of the unit that might not be working right.

I can post pics, but haven't worked on any wiring. It's a matter of working last winter, staying on all summer except the room thermostat was turned down, and then turning it back up when it started to get cooler here but found the valve wasn't actuating. I'll look into posting a wiring pic...

Is anyone available, for a fair fee, to walk/talk me through diagnosing some behaviour that I think is incorrect for my system? In covid-times I want to avoid calling someone in if I can. I am quite competent with a great many topics, but being new-ish to the UK I'm not intimately familiar with...