There are a few simple test to determine if your central heating system requires a power flush or just a filter and chemicals. So first of all what you need to check is the radiators and search for any cold spots, usually these would be in the middle or the radiator where the sludge has mounted up. This only normally happens to old radiators.
The next easy check is do all the radiators get as hot as each other? if they do you probably don't require a power flush and may only need to have a filter installed, if they don't you need to check if the radiator valves are all open including any thermostatic valves you have fitted.Most thermostatic valves you can take the thermostat off and check the pin is moving up and down. Also it is good practice to balance the radiators so that they all take roughly the same time to get hot.
The last dry test which I will talk about is using a magnet to test if the steel particles are sticking to the wall of your pipe and either restricting the flow or causing a blockage. You can use any small magnet to do this test. All you have to do is run the pipe down any none magnetic pipe and see if it sticks. There are a few places on the system to do this test anywhere you suspect there to be a blockage/restriction also if you have an open vent system another place which collects lots of sludge is the cold feed from the expansion tank which is usually in 15mm pipe and positioned just before the pump or better known as the H because of how its piped.
If you have all these symptoms and have tested with a magnet the next thing you want to test is the water quality in your system and the only way to do this is by taking a water sample. So to take a water sample the best way to do this is switch the central heating on and let it run for about 10 mins then drain some water from a drain off point and inspect the colour of the water. If your water is black then a power flush will most probably be required but if there is only a small amount of particles in the water then you will probably only require a chemical flush and a filter fitting.
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