I'm going to bet a virtual £ that somewhere under the flooring, a flow is mistakenly plumbed into a return, as per my previous post.
Please let us know, so that I can give you details of my virtual crypto account for my virtual £.
A light bulb went on when you mentioned possible DIY install, what's the betting it's a combi conversion, and Mr DIYer has got the pipework wrong under the floor, so that for example the flow is going down what was the old cylinder return, or something similar?
This happened to me when...