intermitant hot water runs hot then cold its making contact and boiler fiers up all ok but still no hot water would the heat exchanger be blocked and it going through bypass

g harden, February 2012

What bypass?
HW flow sensor will sense when HW required, if flow just on limit, due to restriction in heat exchanger, it could switch on/off, depending on type of flow sensor, you may be able to watch it, some use a microswitch operated from a diaphram, if that goes in/out flow varying, Needs exchanger seen to.

SD, February 2012