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You should be able to control hot water temperature by adjusting the thermostats. Either or both may have failed, causing heating to increase to the maximum. The "spitting" is caused by boiling water. Likewise, the pressure relief valve is operating as it should resulting in the overflow you observe. You need to get your heater serviced immediately!!
When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.
This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.
What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).
When the water cools the pressure drops very low as the volume of water contracts.
The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.
Check temperature off the water for proper functioning thermostat. Look at set points on heater's thermostat. It's made to let water out when water get's to a certain high temp. More than likely it will be the pressure relief valve going bad and not holding.
T&P releif valve can be replaced on heater. Turn water off and remove with a pipe wrench. It will come out and any hardware store will carry it. T & P pressure relief valve is 3/4 threads.
When installing new one use teflon tape on threads.
Whining = pump bearings Pressure reduction = pressure relief valve diaphragm either perished of gunged up - look for water coming out of the overflow pipe from the valve leak in ch system look at all joints
That overflow is the Pressure Relief Valve.usually they are set at 3-3.5 bar but over time they can be gunged up inside and leak/allow water to flow out. Dismantle & clean or replace. Turn off mains water and ch pipes before removing - operate valve remove & allow for some water to leak from it