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Single pipe system's such as yours are more inefficient and difficult to balance than a two pipe system. This is because cooled water from the radiator output is mixed with hot water in the "bypass" pipe, as you referred to it. If you have the opportunity, I'd replumb it to a 2 pipe system.

Just to add to ianmcd's comprehensive post above, you may find if you activate and deactivate the microswitch a few times, it becomes unstuck (assuming that is the fault). Might be worth trying before ordering a new actuator.

It's easy to check with a multimeter and access to the wiring centre. However, if you're not confident in doing so (as there will be live electricity flowing when doing the check) then better to call somebody in.

If your boiler is fired by the SL (orange wire in honeywell 2 port valves) in the motorised valves, it's possible one of the microswitches is sticking. This is quite a common fault.

Try resetting the heat link by holding down the button for 20 seconds. Google to see what lights should show. On the nest thermostat you can also re-pair the heat link. Check here for instructions: https://support.google.com/googlenest/answer/9251846?hl=en-GB

N and L are self explanatory. 1 is HW OFF 3 is HW ON 4 is CH ON 5 is Common

COM to 2 NO to 3

Yes, the Nest 3rd Gen heatlink will take those connections. The Nest thermostat itself will need to be powered, though - either by USB, or by running 12V to the wall plate.

You should be able to change just the power head (older valves required a full drain down to remove the head): https://www.amazon.co.uk/gp/aw/d/B003EJCDDQ It's a fairly straightforward swap over but will require the new one to be wired in. Depending on your competency, you may want to attempt...

It's a protocol - and one which is not always implemented properly or completely by all manufacturers (boiler or controller). One example would be that of a boiler manufacturer (I forget which one, possibly Veissman) who default their max temperature to some extreme amount (90 degrees plus?)...

Yes! As I said above: :=)

To be fair to the boiler manufacturer, shouldn't the company developing OT controls for both heating and HW design it so that it is capable of working with both? Evohome manages to do it just fine. I also agree that the 230V input could override the OT controls - seems like a simple thing to do.

Put a brown/black sleeve over it to identify it correctly.

The nest thermostat does have an anti legionella program, so it has obviously been designed with the hot water tank in mind to a greater degree than just bolting it on. However, as above, whether it is smart enough to boost the flow temperature when the HW zone is on, you'd have to just try...

Yes, I only used one secondary flow/return for all 3 of my zones. I also only have a single secondary pump.

If it helps - I used a Worcester Greenstar LLH, which is good up to 70Kw, and is relatively low priced. I have a Vaillant 438 boiler. If you use a single secondary pump, it can be wired into the Zone Valve's orange wire so that it comes on only when at least one zone valve is open. The primary...

The 637 has a built in pump, but you can add an additional one in series. I had this setup on my system for a while and it "worked" - in that it improved the flow. I had a 438 boiler, which means the pump was external - so I then took an intermediate step of replacing the pumps with a semi...

The wiser install diagram for the 2 channel is misleading. Terminal 3 is "HW ON". Terminal 4 is "CH ON". Also, the live is internally linked. There's no need to link to terminal 1, unlike the single channel which has a "Common" terminal.

Yes, it replaces your programmer. The stat is wireless so can be mounted anywhere. https://salus-controls.com/files/iT500BM-Installer-Manual-V003.pdf

You are, of course, right. I have (or had!) that thermostat, but it was connected via a programmer near the boiler. I suppose the point I was making is that OP would be better off wiring his heatlink close to his boiler, rather than using the existing thermostat wiring which would usually be...