A chemical flush circulates a cleaning agent around the system using your existing boiler pump, usually left in for a few hours or a few days before the system is drained and refilled. It works on systems that are only lightly dirty — discoloured water, no real cold spots.
A power flush connects a separate high-flow pump to the system. That pump moves far more water than your boiler ever will, and it can reverse the direction of flow through each radiator, which is what actually lifts settled iron oxide off the bottom of a panel rather than just rinsing past it. Radiators are agitated individually and flushed one at a time until the water runs clear.
The honest test is the water. If it comes out of a radiator black and gritty, a chemical flush will not clear it and you are wasting money trying. If it comes out slightly cloudy, a chemical clean and a magnetic filter is usually enough. We will show you what comes out before recommending either — you should not have to take our word for it.