Worcester Bosch boiler probs with hot water

The downstairs hot tap has to be on before hot water fires up or comes through to upstairs taps. Any suggestions as to problem? Could something have got knocked under boiler where pipes come out? Like a valve or something? Has just suddenly started happening last 2 days.

 

Which model of worcester bosch ?

 

28CDi

 

How old is the boiler ?

 

which one

 

We are trying to find out if its an old boiler or the newer greenstar.

 

It's old, about 15 years. But it's fine apart from this. Heating fine, water piping hot when heating on, so strange. Don't really want to replace whole boiler if possible, what could it be?

 

Does the front look like this.....

 

You say you knocked a pipe, which one? Able to post a photo?

 

No, I don't remember knocking a pipe but just wondered if something like that could've possibly happened. Yes the front does look like that.

 

It's the old cdi...pre 2005 for sure.
Sounds like it could be the diverter failing.
It's relatively easy to check yourself if you felt inclined.

 

Have the hot water diaphragm changed, it will probably sort the problem.

 

Yep new DHW diaphragm time, surprised it lasted this long without having it changed