if the 3 way valve is on the shonk, the 22mm flow pipe to the rads will get hot, turn the isolation valve off to this pipe then try tap again. if hot then correct. also watch flame or flame light on display if goes on / off / on and water hot/cold/hot/cold/hot etc then blocked/blocking plate heat exchanger due to sludgy water. remove n pressure wash for temp fix.
gilo
September 2009