We had a combi okay, hot water whenever you require it, but if somebody turns a tap on when you're in the shower, heavens you know about it! Bristol drains survey includes. Didn't unblock sink at all,if anything is was more blocked after applications, even when left for hours! Keep outlet pipes, drains, grease traps, waste disposal units, gullies & refuse chutes free from the build-up of grease & fat. Powerhouse installs boilers, especially.