Problem with heating system (Vaillant boiler)

[JW118]

I have this on going problems for a couple of months with my heating system.

When the timed heating is suppose to come on, but not come on unless I turn the thermostat down and then up until I can hear the click or switch the timer off then back on to auto or constant then heating will started ! But only lasted for 2 or few days.

First I thought it’s the thermostat faulty and changed to a new thermostat (like for like) it’s works for few days, but happen again! Need to do the turn on and off myself and then the heating can come on.

Hot water is not a problem with megaflow same boiler.

Boiler is Vailliant Eco Tec 831 Plus Combi.
Honeywell Timer ST9400C
Honeywell home MT1 Thermostat THR830TEU
Mechanical Room Thermostat

Can anyone give me some kind of advice, it will be greatly appreciated.

Thank you.

 

[stem]

You state you have a Megaflow and a ST9400C which suggests that you have a separate stored hot water system. To have this connected to a combi boiler is a bit unusual as combi's produce hot water without the need for a separate hot water system.

Anyway with the system you describe usually there are motorised valves that control the boiler. One motorised valve controls the heating of the hot water cylinder and at least one other for the central heating [Some homes have multiple heating zones each with a separate room thermostat. e.g. upstairs and downstairs, so may have more than one motorised valve]

The room thermostat is only responsible for opening and closing the motorised valve that's all it does. But inside the motorised valve is a small switch that is operated when the valve opens, this switch controls the boiler. Sometimes these switches become intermittent so work sometimes and not others. When this happens, it can create the symptoms you describe.

 

[Stuckinarut]

Which motorised valve(s) do you have?