Grant Vortex Boiler

John.G

Came across this problem on another site, someone may have words of wisdom.

The system is Y plan with a MPV, (Mid Position valve), if either CH or HW is selected then the boiler cycles normally, if both CH and HW are selected, MPV to its mid position then the burner switches on/off every few seconds until CH or HW is again selected, it seems to have all the symptoms of micro firing. I thought it could possibly be due to the MPV hunting in its mid position interrupting the power to the boiler but a few tests on my own Firebird (S plan) seems to rule this out, when I interrupt the power to the burner for a second or two, the burner cuts out then does its 12 sec (pre) purge and refires, if I interrupt the power while its purging then it again just does a full 12 sec purge, if I interrupt the power immediately after refiring but before the extra 5 sec for flame detection has elapsed, it does exactly the same, a full 12 sec purge and refire, all as expected really?. A voltage test may show something?.

10-10-20

It's not a pressure/blockage issue? I'm sure you checked the pump pressure.

Lenar3556

I think verifying the voltage supply to the boiler is the first step. Sounds more like a controls power supply to the boiler issue.