boiler hwc cylinder

Really hoping you good people can help. My set up is a standard vented HW and CH system. The other evening my HW failed (made known by Daughter moaning how cold the shower was). The cylinder thermostat is a really old Honeywell type so I used a screwdriver to move the dial down to see if it...

Have old gravity Heating system, old Potterton (32 ys, in good shape), HWC and small and large water tank in loft above. May need to change HWC as F&E tank has overflow, suspected hole in coil. Happy to get new HWC, vented/indirect again, but would like to know if this like for like change will...