pump grinding noise when inclrease boiler temp

[PJF220]

Hi,

I've seen various questions relating to pump noise etc and I've narrowed down my fault, but wondering if there is any advice as to cure?

Background:

Potterton Netaheat 30 boiler
open vented system
Grundfos Selectric 15 50 pump
Drayton controls incl Drayton mid position valve

When the Potterton is on control setting 1 it all works fine, but if the temperature drops down to the -5 degrees outside, the boiler can struggle to keep the house at 21 degrees. Therefore increase Potterton to 3 and when it all fires up, the grundfos pump draws in air and makes a horrible grinding noise. Air can be heard circulating round system for a bit. The open vent pipe is 2m in height so tall enough I understand not to be drawing air in, though not had a chance to try the pint glass test yet.

Seems like it's the boiler causing the problem, but I don;t yet understand what it's doing or what is causing it.

Any ideas?

Obviously, if I need to supply more info, let me know and I'll confirm.

 

[45yearsagasman]

It's not the boiler but restrictions/blockages in your system close to the cold feed and open vent.Need more info on system.Can you upload a pic of your airing cupboard installation?

 

[PJF220]

Ah ok. I'm off out now, but will post up a photo later/tomorrow.

Thanks

 

[PJF220]

Photos (sorry they are sideways):

Untitled

• PJF220
• 10 Dec 2010
• 1

Untitled

• PJF220
• 10 Dec 2010
• 1

Untitled

• PJF220
• 10 Dec 2010
• 1

Untitled

• PJF220
• 10 Dec 2010
• 1

[/img]

 

[oilhead]

Possibly scaled up heat exchanger restricting flow to pump, which causes cavitation to pump, which with hotter water means the water boils, leaving the pump short of cooling/lubrication.

 

[45yearsagasman]

Second picture down will be where the problem is.Called the H section just above the pump and will be restricted in that area.Needs cutting out and cleaning.Blockage usually in 15mm cold feed.Forget about scaled heat exch.