central heating off BUT radiator on

[notnoweric]

Hello
we have just noticed this phenomena this summer.
I have a combi boiler and have a reasonable understanding of my heating system. At present CH is definitely OFF totally.
We run a mixer shower every morning and my wife noticed that one of the bedroom radiators was 'on'. Indeed I have felt the input pipe which also gets hot. No other radiators are on and infact I didn't think this radiaotr was closest to the downstairs boiler (but I am not 100% sure). In the winter we wouldn't notice this off course. Is this a natural thing whereby some heat is clearly being transfered to radiator system (separate closed system by the way). Or should I be worried by this phenomena.
many thanks for replies. I am hoping someone will just confirm this is natural - we do have our shower on for too long perhaps.
thanks

 

[ALCPlumbing]

The diverter valve within the boiler that switches the flow between the heating circuit and the plate heat exchanger (which heats your hot water) could be letting by allowing some heated water through to the radiator circuit.
Run the hot water and feel the furthest pipe to the left under the boiler to see if indeed hot water is passing by the diverter valve.

 

[45yearsagasman]

Common fault on most combi boilers. Needs to be fixed, and it's costing you more money to run. What make/model is it?

 

[notnoweric]

Thanks ALCPlumbing and 45yearsagasman (I'm actually only 45 by the way)
It's a BIASI M96 28 SM/C2
The 22mm left pipe is indeed the start of the radiator circuit. The right hand pipe must be the end of circuit and this contains the mains feed for increasing pressure. I put the heating on just to double check the above and yes the left gets very hot very quickly while the right stayed cool because I only turned it on briefly [I know all this is obvious to you guys]
Anyway when running the hot water taps, the LEFT stays cool and the RIGHT ie end of the circuit gets hot so ...
I am hoping it is not the diverter valve that you mention ...
if it still is however, how much will it cost me roughly
thanks