Pressure in your heating system

If you have a combi-boiler, chances are your heating system is unvented and pressurised. If the pressure drops, this can cause inefficient heating and ultimately the boiler should detect low pressure and shut down.
Why is the system pressurised? Basically to allow air to be purged from the selaed system as the water circulates and to prevent the system taking in air which can cause the pump to run dry and burn out.
How much? In most domestic systems, 1-2 bar is normal but this can increase slightly when the water is hot.
How can I tell what the pressure is? Your boiler should have a small pressure gauge fitted, usually on the front panel and marked green and red for the acceptable range of system pressure.
How can I increase it/top it up? There is usually a flexible filling loop under the boiler with a valve at one or both ends. It should by law contain a non-return valve so that the water in the heating system cannot go back into the drinking water main. By opening these valves, water from the mains (usually 3-5 bar pressure) will go into the heating system and increase the pressure until it reaches 1-2 bar when you close the valves. When you first do this into an empty system. it can take a while to fill up, especially as you bleed radiators and any auto-vents release trapped air.
Why does my pressure go up a lot when the heating is on? There is a pressure vessel inside the combi which is supposed to balance the pressure of the water so that as the pressure increases due to heat, this vessel expands and avoids the system becoming over-stressed. If this vessel is broken or the "charge" in it has leaked out, then it won't do its job and any increase in pressure due to heating will be reflected on the gauge.
Why is the pressure dropping over time? Either the system was never bled properly and the air is slowly working its way out of the system or you have a leak. The leak might only be in one place and of course could be under a floor. It is also possible that a faulty boiler is causing the water to boil and the leak is actually steam being vented from a safety valve. This is hard to check but if the flow pipe from your boiler is stupidly hot, you might want to get a plumber to check it. Basically the flow temperature should be touchable briefly without burning yourself (about 70 degrees) and the return temperature should be virtually holdable (about 60).