High Pressure In The Boiler
Check the pressure valves to ascertain if the pressure is within the 18-25 PSI. Anything outside this range should be approached with trepidation. If higher than 25 PSI, water will start to leak. Rule out any obstructions from debris and limescale. The pressure relief valve should help to reduce the high pressure, hence prevent a possible blow up. The valves can be replaced if need be, or alternatively, reseal them.
Temperature Probe Is Defective
Sometimes you will observe that the pressure valve has a high temperature, leading to the boiler leaking water. In such a case, have temperature probes inspected and the fault rectified as soon as possible.
Corrosion
The tanks and pipes get corroded over the years. This corrosion, as you will observe, permit water to leak. The valve as a component could lead to leaking as well. Where the boiler is entirely corroded, fixing individual components will not do much to alleviate the situation. It will just be scrapped down and a new replacement sought. Otherwise, valves can easily be replaced. The cost of replacing the entire boiler could be high, hence the need to insure it.
Faulty Installation
When you notice water leaking around the pipe fittings, it is not necessarily that the pipes are broken. The installer could have carried it out the wrong way. You will need a qualified contractor inspect it and find out how it could be removed and installed afresh.
Leaking From Boiler Pump’s Seal
The seal of your boiler pump could be problematic. In such a case, you would need to reseal them. If they are beyond resealing, a replacement should be arranged at the earliest opportunity.
As a word of caution though, you should always remember to turn off the gas as well as the water. This way, you will be able to diagnose the problem safely. Involving qualified engineers is highly recommended and especially if the problem is a bit too technical for you. There should not be delay in fixing the leak problem. This would prevent serious structural damage occurring.