Thermostats and timers
These are often in the same location and can be responsible for heating or hot water being too tepid or switching on or off at the wrong times. If the settings are correct, you may need them replaced.
If a radiator gets partially hot, at the bottom or one side, the likely cause is an air pocket. Slightly unscrew the nut in a top corner of the radiator until either water or air hisses out. As soon as it’s water and not air, re-tighten it. Get a radiator key from any hardware shop.
If radiators in some rooms are hot enough but not in others, they probably need balancing. Adjust the water flowing through each with the nut on the pipe at a bottom corner. Often they already have a handle fitted. There can be a bit of trial and error to get the heat distributed how you want it.
If none of the above fixes your problems, there could be a rust or sludge blockage. The system can be flushed by attaching a hose with mains pressure water to a point near your tanks, but it’s a tricky job. Air or sludge may move to the pump or old radiators may still need to be individually drained. Get free advice and a quote before you set out on this from a company like http://www.rjplumbingandheating.co.uk/, who also do Woking boiler installations.
No heat in cold weather
Make sure the condenser pipe, the plastic tube underneath a combi boiler going through the outside wall, isn’t frozen up. Thawing out the pipe is all you need to do – https://www.britishgas.co.uk/help-and-advice/Boiler-HomeCare/Troubleshooting/Central-Heating-troubleshooting/My-pipes-have-frozen.html.
If the noise sounds like water boiling then it probably is. Often called ‘kettling’ because it sounds like a kettle, it can be caused by sludge or limescale building up in the heat exchanger. Plumbing systems have safety outlets to avoid pressure blowing up your house but you still need it fixed – it’s very energy inefficient. Chemical flushing is the solution but you may need a plumber.
Boilers that keep switching off
Unfortunately, this problem can have more than one cause. It can be due to low water pressure, a valve, the thermostat, the timer, the pump, or a pipe obstruction. You may need a plumber to tell you.