For those interested, here's an update...
After exploring the various options kindly suggested here and elsewhere, and after meeting with a Corgi engineer/inspector/trainer on site, the following solution was adopted for this project.
I worked under the supervision of the CORGI man and will be undertaking full training with him, so that I can get qualified myself. After inspecting the existing gas meter, it became apparent that this would not be sufficient to supply an additional shed boiler as well as the current domestic system boiler, for the house.
The decision was made to remove the current boiler and replace it with a Keston C55. The old boiler (Ideal brand) was rated at 25kw and the pool needed a heating source in the region of 23.5kw (80,000btu), thus a boiler with a total output power of about 48.5-50kw was the ideal, while 40kw would probably suffice. Concerned that my client's expectations should be met (preferably exceeded), and with a good price quoted for the C55, I chose that model as a suitable replacement for the 10 year old 'Ideal' model.
The C55 has a variable output, ranging from 14-55kw, depending on demand, and could easily cope with the additional requirements of the pool's Heat Exchanger, as well as the current domestic HW and CH, with output power to spare. There are also the obvious economy benefits to be gained from the variable output when the demand is less, plus the energy savings that come with a condensing boiler such as the C55.
To incorporate the pool zone onto the primary system, I branched off from the flow and return ports of the C55, to serve the new loop, as well as reconnecting to the existing primaries of the domestic system. To allow isolation of either the house primary or the pool primary, avoiding unnecessary water circulation, I fitted actuating 2-port MVs on both branches of the flow primaries, one on the house side and one on the pool side. Likewise, I fitted gate valves on the returns.
Other changes made to the existing domestic system included: removal of the, now-redundant, circulation pump, located in the airing cupboard... the C55 has an integral pump that has the oomph to push water round the house and pool loop combined. To avoid the C55's pump discharging water into the CH expansion tank, via the vent pipe, in the loft, the system has been converted over to a pressurised one, incorporating a suitably sized pressure vessel.
I had a great time measuring the volume of water in the system, by draining it into a 3 gallon bucket and counting the number of times I had to empty it! Adding to this the volume of water held within 50m of
22mm barrier pipe and contained within the C55 (when full) gave me the total volume of the new system (in gallons), and enabled me to select a vessel of the appropriate size, with a bit extra for good measure... in this case, an 8 gallon vessel was required and a 12 gallon was selected.The new loop for supplying heated water to the pool's heat exchanger, in the shed, and back, was run using 22mm HEP barrier pipe. This was sheathed in consecutive lengths of foam insulation and then wrapped with 4" Denso tape along the full extent of the run. The 2 pipes were then buried in an 18" deep trench, backfilled with pea shingle and will eventually be patio'd over.
With most of the project now completed, I do have some uncertainty on the configuration of the electrical wiring of the additional system. The pool's thermostat will need to call for heat via the MV on the pool branch of the flow primaries. If the pool's programmer (which I have purchased but have yet to install) confirms that this call occurs within an "on" period, the boiler will fire and heating commences. Exactly as would happen if a call for HW and/or CH came through to the domestic programmer, during an "on" period. However, the domestic MV needs to be closed if pool-only heating is called and the pool MV has to be closed if domestic-only heat is called. If calls are made to both zones, during coinciding "on" periods, they both need to be open! As these valves are "normally closed", it follows that either will only be 'open' during an 'on' period, as allowed by their respective programmers. That's all fine, but what about the need for an "open system" to facilitate pump-overrun of the boiler's integrated pump? If both circuits have switched off, following a heating period, then both MVs will close and water cannot be pumped around the primary to cool down! How can I get round this? I am wondering if this is accounted for within the boiler itself, as the C55's manual states that all system wiring is external to the boiler, with only L,N,E and switched L needed internally.
Cheers deano.