Please can anyone advise.
My house was built in 1983
The central heating pump fitted during construction was a SMC Commodore 2. The model number was 2-130-60 type Cast Iron pump case length 130mm.
At some time later due to a problem British Gas changed the motor head to a Multi-Head 11.
Please note my central heating and hot water is working ok at this moment.
I am considering changing my central heating controller from a Honeywell ST699 to a Honeywell ST900C.
I have checked all the wiring against the Sundial Y-Plan wiring Diagram of the ST699.
Everything is correct except...