Bit of a weird one, and can't find any other posts like this, so hopefully someone can help. We recently bought a house which has an old immersion boiler. After moving in, we started renovating the bathroom and swapped out the bath for a mixer shower. The switchover went great, the shower was working how it should, hot and cold water coming out perfectly. After a month or two, we noticed that upon turning the hot water on, it would take about a minute for the hot water to come out. As time passed, this delay got longer and longer, until now it won't come out on its own accord. We discovered to get hot water through the shower, we have to turn the shower's hot water valve on, then go over to the washbasin next to it, and keep pumping the hot water tap on and off quickly, before it comes out the shower head. Once this process is done, hot water comes out of the showerhead as it would normally, so it's almost if it needs some sort of pump to force the initial rush of water up the pipe to the mixer. Cold water doesn't have this issue; and will come out straight away. Any ideas what could be causing this? Could this be caused by low water pressure?