Why is my boiler pressure low?

Here are several reasons why the pressure in your boiler is too low.

1. Overtime, it is quite normal for air to get into the system. You can tell if this has happened because you can feel cold spots on your radiators. Try bleeding the radiators, then topping up the pressure on the boiler.
2. There’s a leak somewhere in the central heating system. Check pipes, radiators and radiator valves for signs of a leak, such as rust, corrosion or staining. You may not notice any water, because it may evaporate due to the heat of the pipework before it has the chance to drip. 
3. The expansion vessel inside your boiler may need recharging or replacing. This is a job for a qualified, Gas Safe registered heating engineer.
4. There could be debris inside the pressure relief valve. This is the safety valve that usually discharges water if the pressure in the system gets too high. If it is dripping with water and the pressure gauge is too low, your Gas Safe registered engineer will need to clean out or replace the safety valve.