hard to tell but it looks like 22mm to me going into the HW cylinder, which should mean it's not gravity HW but i can't see anything suggesting it isn't.
where is the pump pictured in relation to everything else?
presume its in another room?
there doesn't appear to be any controls on the system other than the dial in the middle of the boiler which controls it's target flow temp, try turning this down a notch or two and see how you get on.
you could look at getting someone in to upgrade the system to fully pumped and on S Plan, then you can add room stat and HW cylinder stat to control the heating and hot water correctly.
Its a gravity hot water system with pumped heating, room stat wired directly to programmer/ pump.
You can have hot water without heating, but you cant have heating without hot water
Looks like an old basic set up,you have no control on temp of hot water only by the Boiler thermostat, the higher you turn that up will increase temp of hot water
To sum up,probably temp on boiler set too high, room stat/ programmer
knackered.
the programmer will be wired up incorrectly or is unsuitable for gravity hot / pumped CH
basic operation of mine was this
HW = boiler on
CH = Pump on
CH was mechanically linked so turning the CH switch on would always turn the HW on as well.
if you can get them separated then you can run it like that as long as you need.
i'd seriously consider getting the system changed to fully pumped S or Y plan to get more control over it, or possibly even C plan.
C plan would be simplest to retrofit as it'd involve fitting a zone valve on the HW return controlled by a HW tank stat, then the pump controlled by a room stat.
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