Central Heating Staying on Full

[JF77]

I have a central heating system with a convential Baxi power flue boiler (not combi)
with a Programmer control, room stat, three way diverter value, hot water tank etc.

Every so often the heating sticks on full. Turning the heating off at the controller and/or turnig the stat down does not turn it off. The only way is to physically turn the knob off at the boiler. Turning it back on an hour or so later its fine.

I have changed the three way valve actuator, I was going to change the valve body but this moves freely and so appears not to be seized.

The programmer and stat are the only things that call for heat but these don't have any affect.

Could it be the PCB in the boiler or maybe even the programmer?

Thanks,

J

 

[jackthom]

I think you'll need to test a few mains voltages with a multimeter to get to the bottom of this. Do you have one and if so can you post a picture of the wiring centre?

 

[JF77]

Yes I have a multi meter. Which wiring? Behind the programmer?

 

[picasso]

Got a frost stat fitted ?

 

[jackthom]

Yes I have a multi meter. Which wiring? Behind the programmer?

The junction-box/wiring-centre where the thermostats, three port valve, programmer and boiler etc are all connected together.

That's a good question from picasso. Do you have a frost stat fitted?

 

[JF77]

No frost stat.

 

[picasso]

On some models of baxi the main thermistor acts as a frost stat, when it goes out of calibration it can bring the boiler on.

 

[JF77]

That would make sense. Before i start looking does anyone know where it could be?
Also the water is getting so hot and the tank stat makes no difference and imrecently replaced it.
I did my apprenticeship at Glow Worm so im determined to fixit!

 

[stem]

the water is getting so hot and the tank stat makes no difference and imrecently replaced it.

Did the problem of the boiler staying 'on' permanently occur before you replaced the cylinder thermostat?

 

[JF77]

I replaced the thermister before christmas and everything seemed fine. But this week it stuck on again.
I've noticed that when its stuck on there is no far left light on the boiler panel Boiler On i.e. no call for the boiler. Which makes me think its the PCB (again! I'm on my 4th)

and yes it did it before I replaced the cylinder stat.

Thanks.

 

[Agile]

Has anyone guessed what model of Baxi this is?

When boiler thermistors are used for frost protection they look for a higher resistance as would occur when the thermistor is very cold.

But a high resistance is exactly what occurs with a dirty contact on the thermistor or its lead back to the PCB.

When the PCB is replaced that involves replacing the connector to it. But the problem can just be caused by a high resistance contact.

So a conscientious engineer would check the continuity from thermistor to the actual pin on the PCB first.

An easy way is to measure the resistance of the thermistor at the connection to the PCB. But components on the PCB can affect the reading if measured whilst still connected.

Tony

 

[JF77]

Sorry thought I'd said earlier!
its a baxi solo 60pf

 

[stem]

There are several variants of the Solo. Like there are of the VW Golf.

If you have a Solo 2 or 3 then it may well be the PCB. The 'boiler on' light is illuminated when a 'run' signal is sent to the boiler from the external controls (Cylinder Thermostat for Hot Water / Motorised Valve for central heating). If the light is not lit, and the boiler is still running, then it will be something internal to the boiler that is keeping it running.

I'm assuming here, that the light works under normal circumstances.

I've got a 15 year old solo 3. It's been reliable, except for the PCB's and our's is on its third now. They've all had different faults, but none of them the same as yours. It does have the inbuilt frost protection.

 

[JF77]

At first I though it was the motorised valve because I'd had problems and replaced it.
As soon as I realised there was no call I suspected the pcb. As an ex glow worm apprentice I know how temperamental these are.
It's a solo 3 at 16 years old I'm now wondering if it's time to replace it.
Thanks everyone for the help!

 

[JF77]

I've now changed the PCB and the (intermittent) problem still persists.

I'm running out of ideas.