Bath tap not getting hot

If your bath tap is a hot and cold mixer combined and not two separate single taps then it could be down to the pressure of the cold water backfeeding through the mixer tap into the cold due to a pressure difference between the two if this is the case fitting a pressure reducing valve on the cold feed to the bath may help solve this issue

Hi, from your question I'm guessing you have individual taps on your bath. It could be a clogged pipe as old buildings can have this issue with different water and build up of minerals. 2) If you try turning the flow down when running the hot tap to the bath and see if slowing the speed of the water down allows it to become hotter. If this works then your combi boiler isn't providing you enough power to heat the amount of water needed for a hot bath. 3) if you have a tap with 1 handle I e mixer tap then I would swap it for a thermostatic bath mixer also with non return. This would allow the cold & hot to mix equally if your cold tap is off the mains. Hope this helps :)