Surging/glooping noise from central heating

[paul09]

I have a standard pumped system with a 3-way valve in the airing cupboard at the top of the house. The system runs normally, providing normal hot water and heating. When the 3-way valve operates, however, there is a surging/swirling noise like a sudden flow of water, or like a bottle being filled up. It is also very noticeable when the system starts up in the morning. It stops quickly and everything settles down normally. The boiler seems fine, inlet and outlet temperatures are normal. I've put the pump down to minimum speed (3-speed Bosch pump) to reduce noise in the house; I've tried increasing the speed but it doesn't seem to make any difference. System is 16 years old, was flushed out last year, new pump fitted, is treated with Fernox. There is no air in the radiators, the system has an air eliminator on the main circuit from the pump, and another on the inlet to the tank. Any thoughts on what might be causing this would be very welcome.

 

[45yearsagasman]

When you say standard systen do you mean open vented? if so,make sure the the fill/expansion tank in the loft is full.There is a possibility that you may be pulling in air on startup.If this is the case then you may well have a blockage near where the cold feed and vent are connected to the system just before the pump.Check around that area with a magnet to see if it sticks to the copper pipework.If it does then theres your fault and it will require cutting out and clearing restrictions.Just one possibility to try.

 

[paul09]

I've checked the expansion tank line and it seems to be ok. Hot water is moving up it as the water heats up and expands. There is plenty of water in the expansion tank.

The noise is particularly noticeable when the pump shuts off.