Cavitation / Over running pump in Open Vented System

[UncleJJ]

I have open vented central heating system for my 3 bed house. The boiler is in the kitchen, hot water cylinder , pump & 3 way valve in airing cupboard upstairs, and feed & expansion tank in the loft.

I have recently replaced the microbore with 15mm copper pipe to all the rads (there is a feed to all rad inlets and return circuit from rad outlets). The system has been filled and bled of air throughout and works (to a degree).

When the pump starts it sounds as if it is drawing all the water from the over flow pipe (to the expansion tank) and drawing a lot of air into the system. It then either runs OK for a while and then the pump seems to "over run" and not push water round or over run straight away. I still have to do the test with a jar of water over the end of the over flow pipe to confirm if it is drawing the water down when it the pump starts. The pump has 3 settings and it is on the slowest (750 rpm) setting.

This issue was partially there before I changed the pipework but now seems to be worse. I thought it was due to pressure loss / inability of the pump to push water round the whole system due to the microbore pipes.

I have read that that this may be due to too low static pressure for the pump, incorrect placement of pump, or there being a need for a non return valve on the return to the boiler.

Appreciate thoughts / comments from the forum.

 

[Agile]

I dont think you should be adding non return valves!

Without seeing a layout of your system its difficult for me to guess whats wrong but obviously there is a design flaw.

The critical distance is that between the vent and feed pipes.

Tony

 

[UncleJJ]

Thanks Tony ..

Do you have any information on what the distance should be between the vent and feed pipes or where I could get this from?

 

[Agile]

Vent and feed pipes should be as close together as possible and never more than 150 mm apart along a horizontal pipe!!!

Tony