Problems on HW only

[halton1957]

I've posted recently regarding noise problems, further investigation has narrowed things down a bit.
It's a 20yr old microbore system with Gloworm Fuelsaver, 3 port valve, etc.

When the system is running on CH + HW all is well. When I shut off CH the system clatters and bangs around the air separator pipework. This lasts a few minutes.

When it's running on HW only it appears to cycle on & off with the Boiler stat before the tankstat reaches shut off temp. HW does get hot though.
The HW is only used once or twice daily for washing up.

It appears to me that I may have a flow restriction in the calorifier coil? or build up of calcium in the HW tank restricting heat transfer? or something else I haven't thought of?
Any thoughts please?

 

[Agile]

Much more likely its a very old cylinder and unable to absorb the full heat output of your very old boiler.

Your boiler does not modulate down so on/off modulation is what is to be expected.

Sounds as if there is no pump over run. I am not sure if your boiler has it available, probably not.

Not very difficult to add a separate delay relay to give pump over run.

Tony

 

[halton1957]

Thanks Tony, Boiler has fan over run. I'll try to check pump over run. Any comment on the loud clattering banging please?

 

[halton1957]

Just checked boiler manual, the circuit diagram shows combined thermostat and pump over run.
Would over run be relevent when I shut down CH but HW is still running? - That's when it makes a lot of noise.

 

[Agile]

Just because boiler has pump over run it does not mean that the pump has been correctly connected to the boiler as it should.

Sounds to me as if the pump over run is not working.

Even if correctly connected sometimes the over run on the PCB can be failed.

Tony

 

[halton1957]

Thanks for continuing to help Tony, But what I am saying is that when CH and HW are running, I turn off CH only with the roomstat. The HW remains on. The pump does not stop because HW is on and calling for heat. This is when the noises happen for 2-3 mins, boiler and pump continue heating the tank.
The 3 port valve is switching over correctly.

 

[mustbeunique]

It was the pump. A Grundfos automatically variable type. Must have been varying randomly when the back pressure changed. Fitted new pump - no further problems. Thanks for the help.

PS Different username. Forgot my original one and had to re register.