About 6 months ago, our heating system started losing pressure and i was having to fill the system up about once a day. I called in BG, and they put a leak sealant into the system, to no great effect. Since then, I have had about 8 different plumbers in who have come to the conclusion there is a leak under the concrete floor - the only trouble is there is no visible signs of a leak anywhere. BG then called in Dynorod with thermal imaging and sonar tracing equipment. They found nothing, so Polygon were called in. They came and rather than finding a leak, they found something unusual with the heat exchange vessel/condensate pipe - basically, there was an awful lot of water being released in one go (at least a pint or two). Therefore BG returned and replaced the heat exchange vessel (?). 5 hours later, the pressure was back down to zero. Since then I have had a gas tracer done, and that did not indicate a leak - no gas escaped in over an hour. I am now at my wits end. I am now filling the system up about once every 3 hours when the heating is on, but my gut instinct (but I am no plumber as you probably guessed!) that there is no leak - but what else could it possibly be? It seems that the pressure is lost mainly as the system heats up and cools down. Any help would be appreciated so much you have no idea.