Hot water problem

[lambeek]

I have a Ferroli combi boiler (not sure of model), which has problems with the hot water - it gives warmish water for 30 seconds, then totally cold water then back to warm water again. This goes on the whole time the tap/shower is on. Both taps and the shower in bathroom exhibit this problem, however the tap in the kitchen (a mixer tap) has boiling hot water all the time. I've seen other posts here with similar problems but I'm not sure why the downstairs hot water is ok but the upstairs is not?

The central heating is fine.

I haven't called anyone yet as I'm not sure if I need a boiler engineer or a plumber. Any advice would be appreciated.
Thanks
lambeek

 

[ChrisR]

First call ferroli Their tecjh line is quite good.
COuld be thatthe temperture regulation on your combi doesn't work too well when the flow is restricted a bit by particular tap(s) Measure the hotflow rate in ltres/miin.

Usual culprit is scale in the heat exchanger but can be a temp sensor or scale around it.