Air Being Sucked Through Vent Pipe From Expansion Tank

[Diy3r]

Hi all,

Looking for some advice regarding an issue I have regarding strange set up on my heating pipework. I noticed a bubbling gurgling sound from the pipes in the loft when the boiler kicks in and then when it kicks off. I have checked the F & E tank when boiler comes on/off and can see nothing, nothing coming out of vent pipe, and by the looks of it, nothing being drawn down the cold supply pipe from the F & E tank. Checking other forum posts, I thought it might be related to when the pump turns on / off, so I held a glass of water under the Vent pipe and turned on the boiler, and some of the water was sucked up, hence I am thinking that air is being sucked into the system via the vent pipe when pump kicks in. I have not seen any 'pumping over', and the vent does rise more than 450mm over the F & E tank. When I checked the pipework I noticed that the pump is actually attached to the return pipe to the boiler, not the flow. Also the vent pipe, and the cold feed are also attached to the return pipe. The layout goes - Boiler, flow pipe, then off to cylinder upstairs and central heating pipes, then when the return pipe comes back into the kitchen there is the vent pipe, cold feed then the pump, then boiler again. The arrow on the pump (Grundfoss) is pointing toward the boiler, which I guess is right in terms of it being piped into the return pipe.

From what I have read the pump should be on the flow pipe, so it goes, boiler, vent pipe, cold feed, pump?? Is my set up normal for older installations (house is about 25 years old) I have been told that the set up is a bit strange, as you cannot control hot water and central heating seperately, so for summer months we have the rads turned off from the TRV's (looks like a botch job when a new boiler was put in at some point). I know that air in the system is a bad thing, leading to corrosion etc, should I get the set up changed so that it is boiler, vent, feed, pump, will this stop the pump sucking in air via the vent pipe? Also should I check if the cold feed is blocked where it tee's onto the return pipe (I tried the magnet test but magnet didnt stick). I have a feeling that this set up has been in place for a number of years, I was thinking of getting a power flush done but want to make sure that the pipe work is set up correctly first, and also now worried that continued air being sucked into system has corroded rads to degree they will leak if / when power flush is carried out. All replies, hints and tips appreciated.