Thats not uncommon as these appliances will pull available water from your system and thus change the amount of either cold water or hot water running to your shower. When the shower goes cold and hot and then cold again its natural to blame the combi boiler for the problem. Sediment Is Clogging My Hot Water Pipes In The Whole House How Do I Flush It Out Hunker Slate Shower Stone Shower Shower Mold After a few seconds say 10-20 of decent hot water it turns completely cold for another 10-20 seconds even if I max out the control. Hot water keeps going cold in shower . Either take the shower head off and soak it in vinegar water for 12 hour to clean it or tap out the water restrictor screen in the center of it and see if that solves the problem. To explain this in detail we need to delve a little into combi boiler theory first. Some people may notice that their hot water fluctuates when other appliances are running. Remove the restrictor from the shower head. It could be caus...