Some thoughts on gas/oil boilers and HW cylinders.
If you live in a hard water area, the cylinder coil/inside becomes furred causing slow heat up.
A water softener may help.
Most domestic heating systems can have 2 off 2 port valves or 1 off 3 port valve.
The 3 port valve can allow some 'reverse' gravity circulation when the system is off, allowing the cylinder to lose heat back to the boiler.
Both the above problems waste fuel, but fixing them costs more!
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