This may be of interest to those who have one. Combi boilers that supply central heating radiators, work with the water in the system under pressure. It's not a lot, between a bar and a bar and a half. Sometimes the system leaks a bit, usually from the connection to a radiator. These are often hard to spot, as the leak is so slight, it evaporates before it can be noticed. There's never damp carpets. But over several days, the leak can be such that the pressure drops below half a bar. Much below that and the boiler will trip out and won't work until the pressure is restored. It's easy enough, just a question of turning on a tap under the boiler and turning it off again when you see the needle on the pressure gauge reach the required level. Takes less than a minute. We've a fairly new boiler but, the radiators are 22 years old. Our system started to leak a couple of months ago and I was losing half a bar of pressure over three days. It was no bother topping it up with a couple of cup fulls of water each time, to stop it getting much below one bar, but it was annoying. I was certain the leak wasn't from any of the radiators, as I'd checked them thoroughly, so it had to be in the pipework. Looking for it would be as nightmare. It would mean carpets and floorboards up in every room and there's no guarantee I'd find it. So I'd rather put up with topping it up every few days. Anyway, I found this stuff called Sentinel. For those who can remember, It works like "Radweld." The stuff you used to put in leaky car radiators, but it's designed for combi boilers with very small leaks It looks like PVA. Just a question of turning down the thermostat to turn the boiler off, turning off the valves on the bottom of the bathroom towel rail and removing the plug on the top on the other side of the rail to the bleed valve. Then syphoning some water out and replacing it with a litre of Sentinel. Putting the plug back, then turning on the towel rail and the heating again, to flush it through. Then bleeding the radiator and adding some water via the tap under the boiler to bring up any lost pressure. It takes 24 hours to do its job. No more leaky system for nearly a week now! The Sentinel stays in the system and doesn't harm it. Checking on-line, people who've used it say it's fantastic. It's around £15 for a litre and well worth it.