Potterton Profile 100e overheat trip - strange one

[Yorkshire_Paul]

Hope someone can shed some light on this...
Potterton 100e - motorised valves to hot water tank and to heating.
Over-temperature cutout trips about the same time every few days, more often if I turn the temperature on the boiler up (set about 3 now).
This has been bugging me for a while so I have temperature loggers in various places on the pipes - thermocouples on: flow; after valve to heating; after valve to tank
Heating is programmed to come on followed by the water 30 minutes later.

I can see the flow temperature cycling up and down every ~5 minutes, when the hot water program starts the boiler cycles twice then on the third time it doesn't turn off, just keeps getting hotter until it trips out with the flow temp around 92C.

After I reset the boiler it will work fine for a days or so, then do the same thing again.
It seems clear from the temperature readings that both heating and water valves are open, the issue seems related to the boiler control but seems a very specific circumstance.

Controller seems the obvious choice but possibly a sticking "gas valve"? Or the boiler temperature sensor sticking?

 

[JohnD]

Have you got a system filter?

Is the boiler about 30 years old?

When was the system last cleaned to remove sludge, sediment and scale?

 

[CBW]

I don’t know why you think a sticking gas valve, but it’s more likely a failing pump or something else causing it to overheat.

 

[Yorkshire_Paul]

The boiler is 20 years old, I treat the water every couple of years and when I last took a radiator off the water was fairly clean, no sediment to speak of (though it could just be somewhere else in teh system). I don't have a filter but its on the list. The reason I wondered about something sticking is that from the tempearture sensors I can see the flow temp cycling every ~5 minutes but then it stops cycling and gets hotter and hotter over the course of just 5 minutes. I would expect a sludged up system to just run hot permanently or get hotter over a longer period.

Take a look at the flow temperature plot below

At 06:30 the hot water valve turns on and the flow temp drops but after two heat cycles it just gets hotter and hotter (>90C) as if the boiler has stopped regulating - it just stops cycling.

 

[Ihavenojob]

What is the temperature of the return pipe ? Does the heating come on a t 9 AM, and is there a balancing/restricting valve on the cylinder return ? Could be the hot water circuit is restricted so the return to the boiler doesn't get hot enough for the boiler to shut down, but does when the heating runs.

 

[gas112]

Boiler should only get to 82 even on max
it could be
boiler stat old and out of calibration
gas valve not fully going out
pump not getting power everytime (faulty pcb /pump over run stat )
There is a switch by control panel behind bottom tray for gravity and fully pumped make sure someone has not put it to wrong setting

 

[DP]

Have you got a system filter?

Is the boiler about 30 years old?

When was the system last cleaned to remove sludge, sediment and scale?

If the system water had boulders in it, these would pass through the heat exchanger waterways without breaking in a sweat
Boiler is from an era where sedimentation was no problem same as eating deep fried Mars bars

 

[JohnD]

I used to have one, a huge amount of sludge came out when I cleaned it

and another huge amount when I converted the HW from gravity to fully pumped

 

[DP]

Hope someone can shed some light on this...
Potterton 100e - motorised valves to hot water tank and to heating.
Over-temperature cutout trips about the same time every few days, more often if I turn the temperature on the boiler up (set about 3 now).
This has been bugging me for a while so I have temperature loggers in various places on the pipes - thermocouples on: flow; after valve to heating; after valve to tank
Heating is programmed to come on followed by the water 30 minutes later.

I can see the flow temperature cycling up and down every ~5 minutes, when the hot water program starts the boiler cycles twice then on the third time it doesn't turn off, just keeps getting hotter until it trips out with the flow temp around 92C.

After I reset the boiler it will work fine for a days or so, then do the same thing again.
It seems clear from the temperature readings that both heating and water valves are open, the issue seems related to the boiler control but seems a very specific circumstance.

Controller seems the obvious choice but possibly a sticking "gas valve"? Or the boiler temperature sensor sticking?

what are you using to log the temperature

 

[jeff the gasman]

Flow pipe blocked where cold feed connects without doubt!

 

[DP]

Flow pipe blocked where cold feed connects without doubt!

Drat, boulders will be piling up there then
But as said, HE will be clear in that case

 

[JohnD]

 

[gas112]

.

 

[MeldrewsMate]

That's a nice trace that very clearly shows the symptoms - well done. I suspect the measuring thermocouples have a lot to do with the probable error in indication of temperature, so that can be glossed-over, but it's clear to see how the flow temperature drops temporarily when the hot water load comes on at 08:30.
1. Is your circulating pump driven directly from the boiler?
2. Have you considered that the pump may be sticking or failing?
3. Is the pump set to its highest speed? If not, increase its speed and monitor the system. If the symptoms go away with a higher pump speed then suspect a pump malfunction (which could simply be debris in the impellor).
4. How far away from the boiler are the two zone valves, in pipe metres?

If the gas valve was sticking on you'd see the flame through the viewing window.