potterton puma 100e too hot ?

[spaglemon]

i have a potterton puma 100e combi boiler , i did think for a while that the expansion vessel was going but now i'm not so sure.

it runs away fine until intermitently when running hot water ( particularly the bathroom tap ? ) the pressure in the boiler will start to rise and if i'm not quick enough to switch off the boiler the pressure gets so high the prv activates.

we have the temp control for the hot water turned down but the hot water is still boiling hot !

as a trial i rechecked the pressure in the vessel and also drained the kitchen radiator and deliberately left it half full of air incase the pressure vessel was not functioning properly but the same fault has kept occuring since doing this so i'm now wondering if the water is overheating somehow and causing the pressure to spike and activate the prv ?

any thoughts

 

[gazthepottertonengineer]

Sounds to me like the expansion vessel hose is blocked!

 

[Agile]

Oops....

The OP is one step in front of the pro!

 

[spaglemon]

??

 

[Andygasman2010]

Sounds like the wax capsule is sticking maybe...?

 

[Agile]

I went with a landlady to one with exactly the same symptoms. I took great pleasure explaining my diagnostic processes to her as it was a very good example of how logical diagnosis finds the fault.

But there can be many causes.

He can check that the DHW demand stops immediately the flow ceases. See the red demand light on PCB.

He can measure the resistance of the DHW temp sensor at the grey wires at the PCB.

If everything seems normal he can measure the voltage across the grey wires during a DHW cycle. It should be inversely proportional to the measured temperature.

Tony