Boiler runs hot and cold

[BSW1]

My Ideal Isar HE 35 runs funny, turn the hot tap on and after a min or so it runs hot then cools down to luke warm, then gets warmer, then red hot then warm, then cold, and so on

Any body any ideas?

Thanks Brian

 

[tininho]

DHW sensor or Div/Valve cartridge fault most likely.

Have you noticed if your radiators begin to heat while the tap is running?

 

[Andygasman2010]

Could be a blocked plate heat exchanger.

 

[BSW1]

I've not noticed the rads getting warm when the hit taps on as the central heating is usually on when we're in anyway, I will try without it on.

Thank you both for your info, are they expensive to repair, can I do it myself?

Thanks Brian