Central Heating

Boiler firing up despite heating and hot water being off

Anonymous user 23/02/2024 - 3.50 PM

Hoping you can help me diagnose an issue with my heating system. I have a Vaillant Ecotec 630 boiler with two heating zones (one for loft conversion, the other for the rest of the house), plus hot water. It's a regular system with hot water tank upstairs plus a cold water tank in the loft. Despite both heating zones and hot water being off, intermittently the zone valve controlling downstairs heating will open and the boiler will fire up for 5 minutes then switch off again. When this happens the display shows code S04 - heating mode, burner on. A few minutes later it'll display S06 (heating mode, pump wake) and after 3 or 4 minutes further the boiler stops running. There doesn't seem to be a pattern to the behaviour but generally it happens in the evening/at night and when it's colder outside. I don't recall it happening during the summer. Recently it's been happening several times throughout the night (last night at 2250, 2344, 0330 and others too) I thought this might be the boiler's frost stat but at the time of this happening the temp inside the boiler cupboard is 19c. The zone valve in question is outside in an enclosed cupboard. Other than this problem the boiler is functioning fine. No faults are displayed on the screen, nor in the fault history. What could be causing this problem? The boiler was serviced last week and the Gas Safe engineer had no idea of the cause. Thanks in advance.