Boiler doesn't start - Bosch Worcester 24Si II

19 Dec 2016
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United Kingdom
I have a Bosch Worcester 24Si II boiler (see control panel below), but there is no hot water and the boiler isn't turning on. It's housed in an external housing.

I've followed the instructions in the user manual: Initially the fault light was flashing slowly and after resetting it (holding the button down for five seconds) the fault light went out. However, there was still no hot water and the fault light came back on after running the tap. After a few resets, the fault light started flashing quickly.

I turned the power switch off and on and the fault light went out. When I turned on the hot tap the boiler made some sounds as though it was starting, but it didn't, and the fault light (fast flashing) came on again.

The flame detection indicator is off and has not come on at any point. There are no blockages in the flue (that I can see) and it appears that there are electricity and gas supplies to the unit. The gas black needle is on about 0.7.

The fault occurred today (which was quite hot) if that's relevant. The boiler was last tested about 18 months ago and hasn't had any faults before.

Any ideas what is wrong with it? Are there any other tests or fixes I can do before having to call an engineer?

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Doesn't work in heating or does it have the same fault for both heating and hot water?
Good point. The boiler initially worked for heating - the boiler flame detection light turned on and exhaust came out the flue - but a few minutes later the fault light came back on (flashing quickly).
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The pressure showing on the gauge is a little low,increase that reading to 1.5 bar.
Then my first thought would be flow switch, this switch tells the boiler you are running a tap so it knows to fire. If you run the tap and nothing happens, it's probably that the boiler doesn't know you've turned the tap on.
By trying the heating, you've established that most of the components are fine, such as fan, air pressure switch, gas valve etc
Thanks for the links - the images in the Bosch Worcester page are not displayed, but it doesn't look as though it is a key-type filler. I can find the silver braided filling loop but there is only one tap and it is already partly-turned (i.e. not fully open or closed). The tap is on the right in the image. Is this the filling tap to increase the boiler pressure? If so, which way do I turn the tap to increase the flow? It's in quite an awkward space and is a bit stiff so want to make sure I'm turning it the correct way.
You open it like you would a normal tap. This allows water into the system. When the gauge shows about 1.5bar, turn it off.
Bleed air out of the system and then check the gauge. Top up again, as required until system is at 1.5 bar cold and all air has been bled out.
Hopefully the tap will operate as that is not a usual setup for a filling loop and I hope you have clearance between tap handle and floor to be able to turn it on and off. Maybe spray it with some lubricant before trying to manipulate it.
Good point. The boiler initially worked for heating - the boiler flame detection light turned on and exhaust came out the flue - but a few minutes later the fault light came back on (flashing quickly).
I'm not a pro, but in my experience if it stops working after the heating has worked a short time, it's unlikely to be low static pressure, normally it would refuse to fire up the pump in that situation. Once it's warm it would keep going. Topping it up is still a good idea though to make sure you can bleed upstairs.
i doubt the problem is your system pressure , i find this boiler has a weak part but needs an gas engineer to attend to sort this part (y)
OP: same as has been said on 'another forum' - you need an engineer in. And they're gonna love how close that boiler is to the floor :confused:

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