A combi boiler is a closed system which is slightly pressurised. Combi boilers will only work correctly if the pressure is right.
Check the front of the boiler for the pressure dial. If pressure on modern combi-boilers should usually be between 1mb-1.5mb but check the boiler instructions to make sure. If the dial is reading low, you need to top up the system with water.
You should see a valve on the incoming water pipe going into the boiler. This will either have a small handle on it or alternatively simply a slotted valve which requires a screwdriver to open it. Open the valve carefully whilst checking the pressure dial. It won't take much at all to get the pressure to the required level. Be very careful you don't over do it. And make sure you turn the valve off as soon as the required level is reached.
NOTE: a valve is normally closed when the knob/screwdriver slot is at 90 degrees to the valve and fully open when in line with the valve.
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