It's quite obviously an extremely simplified schematic, devoid of the typical niceties of flow balancing valves and bypass pipework with flow restriction valves which lead me to taking a closer look at my own fully pumped Y plan system to compare against the various plumbing arrangements shown in the "Ideal Mexico Super CF.65, 75, 80, 100, & 125 Conventional Flue Gas Boilers Installation & Servicing guide", published November 1983  which had been left by the local Gas Central Heating firm we'd used.
There were a few interesting 'departures' from the fully pumped system plumbing arrangements for the boiler connections shown in that installation and servicing guide which raised a few questions I'd like to present to the cadre of Central Heating Experts that frequent this NG.
My system is laid out such that the boiler itself is installed in the basement with only 28mm flow and return pipes and the 22mm gas pipe plumbed into it and a multicore cable to the Potterton 2000 control panel in the ground floor utility room above.
There is no pipework associated with the header tank feed and vent connections as shown in the guide for a fully pumped system, those are plumbed into the airing cupboard pipework around the pump and 3 port mid position valve in the first half landing loo which is also the location of the switched FCU mains connection fed from the supply side connections of the immersion heater switched connection box.
The header tank, mounted as high as possible in the attic, accessible via a door on the half landing above, is about 5 or 6 metres higher than the pump which is itself a good 6 or 7 metres above the boiler. There are two drain down points in the system, an outside one to facilitate drainage of most of the system without resorting to the use of a bucket chain or a pump and one at the boiler itself to allow for a complete drain down whenever the need might arise.
Compared to the Y Plan plumbing circuit shown in the wiki <http:// wiki.diyfaq.org.uk/index.php/File:Y-Plan-Water.gif> there is a gate valve on the flow side plumbing between the mid position valve and the upper heat exchanger coil port on the hot water tank, obviously there to balance the flow when calling for both heat and HW. However, in addition, there is also a valved shunt (15mm pipe) tapped into the pump outlet to the AB port of the 3 port valve 28mm pipe and the HW H/E coil return which seems a little excessive of pump protection since the ground floor shower room (adjacent to the utility room) has a heated towel rail (previously a small radiator) with no TRV fitted to provide the required safety shunt.
I can't see a condition where such a shunt would be needed. The mid position valve can't block the flow unless the towel rail shield valve is closed *and every* TRV on all the other (12) radiators have closed when only heating is being called for. When only hot water is demanded, there is always a flow path via the H/E coil even if it may be restricted by the balance valve and, I assume, the boiler and pump will be shut off when the tank stat signals that the demanded temperature has been reached.
The only thing that might justify the presence of this additional shunt would be a misguided assumption on my part regarding boiler/pump shut down when only HW is selected and the tank has reached the set temperature.
The 15mm pipe header tank feed and the 22mm expansion pipe are both teed into the 28mm boiler flow feed to the pump (rather than, as suggested by the guide, the 2nd return and flow ports of the boiler itself) using a separate Tee adapter each, with a separation of about 5 inches along the 28mm pipe about 2 foot or so upstream of the pump inlet.
I'm not sure whether this departure from the guide (after some 30 odd years of service) is important. I can't see why there would have been any problems with such an arrangement even though I did have to push a length of pyro down the feed pipe from the attic to unblock it about a year or three after it had been installed. This was a one off problem that's never repeated in the subsequent 30 odd years so this plumbing variation does not appear to be of any consequence, at least not in my case.
So, my questions are:
Is my current header tank feed and expansion pipe arrangement something to be concerned about?
and, is there any good reason not to close the bypass shunt between the pump out flow and the H/E return?
Any other comments?
=========================================================================Maintenance History and additional notes
Whilst this was installed by a local company some 35 years ago, I've never had a maintenance contract of any sort. The only servicing it received was on those rare occasions when the boiler itself stopped firing up due, in one case, to a worn out gas valve (Nov 1998) and in another case a faulty thermocouple (Dec 2012).
Aside from the 3 port valve failure and one leaking TRV, all the other problems have been pump related where the original "shitty" Grundfoss unit failed after some 6 or 7 years of service when I started using a different pump type supplied by my father where the stator windings are separated by an aluminium sheet from the rotor/impeller in the wet half of the pump body which kept giving me grief over the next 5 to ten years until I finally gave up and caved into buying another "shitty" Grundfoss which, to my surprise, turned out to be anything but "shitty" being almost completely silent in operation, unlike its predecessor, and remaining so to this day some ten years on.
All in all, including two or three doses of Fernox MB1, I doubt I've spent more than 350 quid in repairs/servicing over the past 35 years or so since the system was installed so I can't complain. Looking at others' experience with "Modern Energy Efficient" Condensing Boiler systems, I've saved far more on expensive repair costs than any savings in gas consumption ever could.
Incidentally, this system was never endowed with a room stat, relying instead on TRVs on all but one rad and the boiler stat alone. Although I could easily wire in a room stat, ICBA with one more thing to be twiddled with and go wrong plus, I think a weather sensor would be a more useful feature than a room stat.
The extra energy wasted by relying on the boiler stat and TRVs alone isn't going as much to waste compared to more conventionally located compact lightweight condensing boilers where such control of the heat would result in hothouse conditions in the room that's been afflicted by such a boiler. Mine being in the basement puts that 'wasted energy' to good use in maintaining a base level of heat in the rooms above.
 About a year before the system was installed, coincidentally just before the 3 port mid position valve assembly design was changed to the current type whereby the valve motor head can be detached without having to drain down the system. I discovered this when I finally decided, last year, to do something about the 1st floor rads warming up when only hot water was selected. Initially I was seeing prices for the whole valve assembly in the region of 120 to 160 quid. In the end, I was able to upgrade to the later design for a mere 67 quid (new valve plate and matching motor head -retaining the original brass valve body).
 The immersion heater had originally been fed via a switch with neon indicator in the utility room with an FCU for the immersion element in the airing cupboard. The switch and the FCU were swapped round to save having to run a spur off one of the ring main circuits to provide mains power for the CH controller. The only casualty of this rearrangement being the matter of convenience in controlling the immersion heater which was now only required in the event of a CH/HW system failure.
 The extra height was needed to service the 2nd floor rads the tops of which are only a metre or so below the header tank water line.
 The basement was the obvious location as far as I was concerned since I was using it as a radio shack and a workshop which would benefit from the extra heat and save using up valuable living space elsewhere. Also, I realised that the higher static water pressure would hold any kettling tendencies at bay so a win-win situation all round.
True enough that it cost a little more in 28mm flow and return pipework (the heat loss into the ground floor above wasn't considered an extra burden on running costs) some of which was compensated for by the shortened run of 22mm gas pipe required to connect to the consumer side of the gas meter pipework located across the other side of the basement.
Johnny B Good
Johnny B Good