Quick advice please - Unvented Cylinder on Y-Plan

[BournemouthPlumber]

Ive got a retro fitted Megaflow, running on a Worcester Bosch Greenstar RI gas boiler. When the Megaflow was install it appears they kept the Y Plan heating system in place, however, they have not added in a thermal cut out valve on the primary flow to the coil.

My question is, once a thermal cut out 2 port valve is added to the primary flow to the coil - should there also be an automatic bypass installed between the hot side of the 3port valve, and the new 2 port valve, linking to the return to the boiler? My thinking is that in the event of a fault and the T&P valve not working, and the thermal 2 port valve being called into operation, should there be a bypass to enable the water to continue around the system, just avoiding the cylinder?

Thanks for your thoughts in advance.

 

[Gasguru]

The cylinder overheat stat is normally wired in series with the cylinder thermostat.

An auto bypass valve should be fitted after the pump to allow pump overrun and the 2 port wired in to work in conjunction with the 3 port. There are diagrams on the web. IMHO it would be much better to dispense with the 3 port and use 2 x 2 ports since fault finding in the future will be a PITA.

An alternative stance would be to ignore the requirement and leave it with just the 3 port, in the past Worcester have taken the view that there are sufficient temperature controls on the boiler to prevent cylinder overheating...and the building regulations are out of date (nothing new there).

Of course this whole requirement for a zone valve on the primary flow is mis-guided. On an indirect setup the safety systems already exceed those for a direct setup even without the zone valve. In addition, many zone valves particularly the shoe type (Danfoss, Sunvic, Potterton) have an inherent weakness in that they often seize fully open once the spindle O rings stick so rendering them pointless and less safe...the microswitch then stays on regardless of the whether the cylinder thermostat is satisfied.