How do you replace heating element in central heating?

no because an heating element can cause electric shock.

Sounds like the thermostat is set too high so the unit is calling for heat... If the thermostat is set normally then the unit has a fault. Repairing an air conditioning unit is not a Do-It-Yourself...

welll yes it fucvkign is

You will need new injectors on the boiler and some additional controls on the gas line. Pipework for LPG is smaller than nat gas so your pipes should be fine.

Alice h Parker