This applies to systems which use the “Y” plan for control of central heating and hot water
from a gas boiler.
BEWARE
too many people use the Y plan diagrams from the internet.
Some are very wrong , and unless you have a secondary form of heating the hot water would not be detected.
On the hot water tank/cylinder there is a mechanical thermostat
the Common connection MUST go to HW demand, many diagrams incorrectly show the common going to the boiler control.
If you have an electrical heating element in the tank powered from Solar PV panels
when the water temperature exceeds the setting on the tank/cylinder thermostat , if it has been wired incorrectly with the common going to the boiler control, the three port valve will move and the central heating gets turned on, regardless of the room stat settings.
So on a nice sunny day when its nice and warm outside, it will get even warmer inside the house.
The Y plan diagrams from the major manufacturers are correct, Honeywell, Danfoss etc
The mistakes made are often not detected as most of the time the hot water temperature will not exceed the cylinder setting.
If automating, as I have done, by checking if there is liable to be enough energy from the sun to heat the water, I just use a 2 pole change over relay to swap HWon and HWoff and control the relay with a remote switch.