1. Every time you top up you introduce fresh water with dissolved oxygen into the system. This will dilute the inhibitor, and, in time, rust the radiators from the inside out.
2. The leak will either be within the boiler or in the radiator / underfloor heating / hot water system. You'd be very unlucky to have both sides leaking.
3. Determine which side the leak is as follows:
3a. Let the system go cold and top the pressure up to the normal (between 1.0 and 1.5 bar). Make a note of the pressure as accurately as you can and turn the boiler off.
3b. Turn off the isolation valves for flow and return under boiler. See manual for which is which. Note that there is a possibility the valves may leak after being used. Shouldn't, and its fairly rare, but it does happen.
3c. Leave for twenty four hours, or longer if feasible.
3d. Check the pressure. If its the same, then any leak is likely to be outside the boiler. If its gone down, the problem is inside the boiler.
3e. Open the isolation valves, and re-check the pressure. If it falls, its a pretty good indication that you have a leak somewhere in the radiator / underfloor / hot water systems.
4. If the leak is outside the boiler:
4a. Try tying a bow of tissue / toiler paper on the pipes just under each radiator valve. A very slight leak might not be visible, but may be enough to make the paper go soggy and droop.
4b. If that doesn't work, you may need a leak detection service. They are generally expensive, and you pay for their time, not by results.
5. If the leak is inside the boiler you will need a gas safe registered engineer. Could be:
5a. Simple, like the pressure relief valve passing or a damaged expansion vessel. See if the pressure relief pipe is dripping. (Copper pipe, outside, end turned back to wall).
5b. More complex with a leak from the heat exchanger(s).
The above supposes a gas boiler with a sealed heating system. Could be combi, system or open vent boiler (running a sealed system).
