Modifying / extending S-plan open vented heating system

[andyscotland]

Not been about for a while - it's been a bit of a year - but getting back into my project now and the next step is to connect up the water underfloor heating in the extension into my existing heating system.

 

I believe the existing system is S-plan fully pumped open vented, with a couple of anomalies / issues:

• The pump is on the return, rather than flow - I understand this was common in the past.
• The system was extended when previous owners extended into the loft. The upstairs radiators are teed directly into the rising vent pipe, with no motorised valve - so even if running hot water only, the flow also goes upstairs. We've worked around this by turning the TRVs fully off upstairs in the summer, but it's not ideal...
• The heating 2-port valve has now failed (stuck part-open). The pump and both 2-port valves are buried behind / below the tank and almost impossible to access for maintenance. I'd basically have to remove the water tank to be able to work on / replace the failed valve.

 

So as part of connecting in the UFH, I want to take the opportunity to fix / reconfigure the existing pipework in the airing cupboard. I'd like to fit a magnetic filter, and relocate the pump & valves so that they're accessible in future. 

 

I've attached a rough drawing of the current setup and what I'm thinking of doing. For clarity, the boiler itself isn't in the airing cupboard - it's a Baxi Bermuda gas back boiler, mounted in the wall the cupboard backs onto. In future, we plan to get the boiler replaced / potentially move to an ASHP - but that is a whole other project that I can't start thinking about (it has a lot of knock-ons) until the extension is finished. So I'm looking to do enough now to be able to connect in the UFH and keep the system running in the short-to-medium term, I can cope with it not being a perfect solution.

 

Any advice / comments would be very much appreciated!