Glowworm Flexicom 18sx F9 fault

[AzzaT]

I’ve had intermittent problems for a while, mainly with the F9 fault code. I have the programs on the thermostats set for the hot water to come on early morning for about 15 minutes, then the heating to come on, and the same late afternoon.

It seems that every morning the boiler doesn’t fire up, when I check it’s showing the F9 fault on the display, so I reset. However, it doesn’t fault in the afternoon — which is a bit weird. The system isn’t losing pressure, usually showing 1.5-1.8bar on the display. We do seem to have enough hot water, plus it’s been the summer so we haven’t needed the heating on anyway.

With the colder months now upon us I asked a plumber to come and look at the issue. The obvious one was to replace the pressure sensor as the old one looked worn and the F9 code points to it being a pressure issue. He’s also replaced the AAV and has checked the pump.

But we still have issues. It mostly shows the F9 fault, but sometimes doesn’t. If I manually turn the heating on, the boiler fires up and runs for a short while, the radiators get hot, but the boiler then just stops running. It doesn’t show a fault, display is normal with the pressure around 1.5-1.6bar, but the thermostat shows the flame as if it’s still requiring demand from the boiler and the temperature isn’t up to what has been set.

It’s the same if I manually put the hot water on. It runs for a while and then just stops, and then does usually show the F9 fault.

So, seeing as we’ve replaced the pressure sensor and are not losing pressure in the system, is there anything else that could be causing the issue?

[Agile]

I would love to know how to see if a sensor looks worn!

Do you have TRVs on all the rads? Maybe these are closing and there is no flow?

Do you have an auto bypass?

Tony

[AzzaT]

Thanks Tony.

Yes TRVs on the rads, maybe I should check to see they're not stuck? Would bleeding them also help?

Not sure about an auto bypass.

[Agile]

You can usually unscrew TRV heads and see if the pin underneath will go down about 7mm when you push it down with something metal.

An auto bypass may be built into the boiler or fitted somewhere on the system. It is designed to continue some flow if all the rads are turned off.

[AzzaT]

I’ve checked all the TRVs, a few seemed stuck so I gave the pins a light tap with a hammer and they have some movement now.

Also bled all the rads but they didn’t have any air, water came out immediately.

Will see if it makes any difference. Thanks.