Confused at Marford. The plumbers flew in and fitted the new boiler, associated heating manifold connections, diverter valve and coupled up the DHW header. For some reason they appear to have fitted a pressure operated relief valve directly across the tank coil! When I queried the purpose there was some mumbling about it being shown on the boilermate connection diagram. When I persisted they said it gave a route for the water when the valve closed. This is a W plan system using a Honeywell V4044C diverter valve. I suppose there may be a short period as the valve transits from hot water to heating when there is no flow. Even so, this diverter appears to be in the wrong position.
Sounds like a by-pass valve. There may be some point in having one between the pump outlet (before the diverter valve) and the boiler return. But if it's after the diverter and directly across the coil it will never do anything because the coil will always present a lower resistance path than the by-pass valve.
I supplied the valve definitely V4044C although they had fitted something else. When I asked why they said it was because they needed something to finish the connections having not *found* the valve I gave them. Either way the device is in the wrong place. If there is printing on the red knob, my eyes aren't good enough to read it.
From what I can recall of the image (it seems to have disappeared now), that valve didn't seem to serve any useful purpose such as to aid in drain down/refill operations. It's possible it might have intended to be in series with the flow to the H/E coil to save it hogging the flow when both HW and CH were being called for.
I've got a similar, if less complicated, plumbing arrangement in my own airing cupboard, also including a couple of extra valves which I couldn't initially explain their intended purpose. My setup, although similar, does vary in an important way, both valves do have logical reasons for their presence.
The first one, missing (or simply misplaced) in your case, being a valve in line to the 22mm feed to the H/E coil from the mid position valve DHW port. It's value, imo, seems rather questionable so I've got it backed off half a turn from fully open (wide open but parked so if it gets stuck through lack of use, I've got two directions I can 'exercise' it to free it off). The outlet of the H/E coil has its own 22mm pipe connection to the return manifold into which is teed a 15mm pipe from the other valve which is plumbed to the outlet of the pump, immediately after the pump's isolating valve in the 28mm boiler flow line.
This is a bit like that red valve of yours except it bypasses the flow to the return pipe before it reaches the 3 way mid position valve (IOW, it has nothing to do with bypassing the H/E coil other than utilising the H/E return pipe to bypass some of the main pump flow). Since we've always had one radiator (now a heated towel rail) to act as a bypass to guard against the unlikely event that the other 12 rads with TRVs all shut down, such a bypass control valve seems to be redundant, so much so that I keep that valve completely shut off.
After considering the existence of these valves, I came to the conclusion that the first valve's purpose was to balance the flow between HW and CH demand 'according to taste' (my 'taste' being that, "You're damned if you do and damned if you don't" so I "didn't"(throttle the flow, that is!)) and that the "bypass valve" is actually a drain down/ refill aid just like that little lever on the motorised valve's motorhead unit.
It's only when draining or refilling the system that you need to manually move that little lever so it drops into the mid position detent and you turn the "Pump Bypass Valve" from fully closed (normal operational state) to wide open. Whilst the lever on the motorhead unit will automatically unlatch when you've refilled the system and restarted it, you have to remember to manually close the pump bypass valve if you don't want to compromise the system's performance (it does no harm if you forget other than reduce pumping efficiency).
Hopefully, this description of those "Two Mystery Valves" amongst the DHW tank, 3 way mid position valve and the pump plumbing in my own airing cupboard, should give you a clue as to what your plumbers *thought* they were doing and give you a better idea about how to remedy the situation.
Except that it's a W-Plan system with a diverter valve which directs water EITHER to the radiators OR to the HW coil - but never both at the same time. There's therefore no need to balance the two.
During *diversion* is there a temporary fully closed moment? I wonder if they thought it necessary to avoid boiler pump pressure surges. Still in the wrong place!
I can't see it doing any harm but it is annoying to have ones wallet emptied unnecessarily.
It certainly doesn't do any *good*! If it ever opens it will reduce the flow through the coil, making it take longer to heat the HW - during which time there's no CH. So it *could* do harm.
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