Hi
I have an old Honeywell boiler system at home. The programming unit is ST6400c.
From last few weeks we are having an issue that heating is not working. It turns ON the led but actually it doesn't turn ON heating.
But hot water works without any problem
If I turn ON hot water and heating together then heating works even after hot water turn OFF.
Can anyone please help me ton understand what could be the problem?
Is it programming unit or boiler?
How much it could cost to fix?
Many thanks
Noor
