Hot water system

[crazypaving]

In the last couple of weeks, when we turn on a hot tap (not cold, cold's fine for some reason) at home, it makes sort of rumbling noises and spits water out in fits and starts for ages until it gets into the swing of things. Then it's generally ok.

We've been in the house for about 9 months and it's only just started doing this. The house is over 100yrs old - no idea how old the plumbing is!! - but we had a bathroom put in upstairs just before we moved in and no problems were identified by the plumber, except for a lot of sludge in the system. He cleaned out some of it.

Is this likely to be the start of a big, expensive problem? Or just a weird idiosyncrasy? And why has it just started now?!