worcester rsf400 HW heating pipes hot and overtemp trip

[flatcaparmy]

can any body help i have a 8 year old worcester rsf 400
the problem is when i have the hot water running the heating pipes get hot as well in tandem with the hot water pipes, if i run a bath or a shower the water runs ok but when i switch off the hot water limit stat trips
the pump turns and continuous for the five minutes as spec
this has only started since switching off CH for the summer

 

[HarrogateGas]

I presume its the Highflow 400 you have got........try giving the filters and non return valves a good clean out.Works every time for this fault for me.

 

[Agile]

When you say "awitching off the CH, how did you do this?

Tony

 

[ollski]

Yep as harrogate says, could well be non return valve in ch return. I got a suspicion though that you may well find the dhw heat exchanger is blocked though 6 -8 years is about the mark.

 

[flatcaparmy]

thanks for a quick reply yes it is a highflow rsf 400
would a fernox type chemical cleaner do the job to clean the heat exchanger or do i need a strip and clean?
i have had a leak in the system last year and have used leak sealer. would this block the heat exchanger?
i might have confused you over the heating pipes only the central heating output pipe gets hot during hot water flow. if so sorry
and the Central heating is switched off on the timer