Boiler Coming On Unprompted

[BenH1989]

That is only additional frost protection, your boiler still has internal frost protection and it could be that is faulty, when you say you disconnected the timer, which timer and how did you disconnect it ?

I disconnect the ribbon of the timer. Think it was a T10.

Timer below

 

[BenH1989]

Is it worth buying a brand new MT10 timer?

 

[ianmcd]

Yes that is the correct way to do it

Is it worth buying a brand new MT10 timer?

Not if it is coming on with the ribbon removed, can only be the frost setting or a false demand for DHW a sticking switch or turbine faulty cant remember what that boiler has

 

[CBW]

false demand for DHW a sticking switch or turbine faulty cant remember what that boiler has

Flow turbine.

Could it be a sticking stepper/diverter motor? Especially with him turning the eco (preheat) off.

 

[BenH1989]

To be honest I think what is occurring is what Eric mentioned below. This takes the timer completely out of the equation.

From what I remember that is a combi boiler with an Eco button, with Eco on the boiler should only work when there is a demand for domestic hot water or central heating, with Eco off it heats a small tank inside the boiler so the water is instant, this means the boiler will fire up every so often.

It seems the boiler changed over time, later models can have a special Bosch thermostat with modulates the boiler with connection to ebus, early models can only use return hot water to control output.

The theory was the boiler pumps hot water around the system, if all TRV's are open then the water is cooled so the return water is cold or cool so boiler runs flat out, as the TRV's close the return water has a smaller and smaller route and finally it will open by-pass valve so the return water gets hotter and hotter which causes the boiler to modulate (turn down flame height) and then cycle off/on.

So in theory it does not need a thermostat fitting. However in practice it will continue cycling on/off all summer, so normally a thermostat is fitted so as summer arrives it will turn off.

So in your case it would seem it's the heating of the domestic hot water causing it to fire up, try pressing Eco button that should stop it. I found Eco set on the taps can be cracked open to give a trickle of hot water, Eco off and taps must be wide open for boiler to fire up, Eco on resulted is shower getting warm then cold then warm again as boiler tank ran out, eco off once warm stayed warm.

With Eco mode turned on the boiler will stay off when the toggle is switched to off and will not come on at all. When the toggle is switched to on it will stay completely on. However, when the toggle is switched to timer nothing occurs and is unresponsive. So, say you have the toggle set to on, when you toggle to timer with a time set to 'off' it will remain on. This is the same vice versa. When the toggle is set to 'off' and you toggle to the timer with a time set to 'on' it will remain off. It's almost as if it is just completely ignored.

 

[BenH1989]

I've ordered a new MT10 timer. I'll let you know how I get on.