Vent Pipe Sucking In Air


Postby Diy3r » Fri Jul 29, 2011 10:02 am

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.
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Postby plastic_man77 » Fri Jul 29, 2011 10:11 pm

@ Diy3r

To check whether your feed pipe is blocked, get your mouth round the CH F&E vent pipe (after cleaning it a little) and blow as hard as you can. See if you get anything come back up the cold feed pipe. If you don't, more than likely, you have a blockage.

You can also try draining the top half of your system and seeing whether the F & E tank drains at all or is very slow to drain. Or, when you come to refill the system, see how easy the rads bleed. If not at all, then you either have an air lock, which a blast of the pump may sort, or you have a full bore or partial blockage of the cold feed.

The difficult bit is finding where the blockage is. It means cutting pipe and seeing whether it's clear or not. The blockage, to be honest, is nearly always where the cold feed tees in to other pipework- so on the return pipe before the pump, in your case.

If you don't have a blockage in the cold feed pipe, make sure the feed and vent are definately the right way round- cold feed closest to the pump and that there is no greater than 150mm between the cold feed pipe and the vent pipe, where they join the return pipe.

The pump was nearly always put on the return pipework when it was near the boiler as the flow temperatures used to cause damage to the pump. It's location has no baring on whether you can run CH or Hot Water seperately, this is down to the fact that there are no thermostats or motorized valves wired to a programmer in order to control them seperately.

If you are worried that a power flush could make your rads leak, then you can clean your system in other ways, including the use of chemicals and by removing radiators individually and blasting them through with mains cold water via a hosepipe (but this alone wont clean your pipes or the boiler's heat exchanger etc). Either way, you need to sort your other problem first.

Once you have sorted your problem and cleaned your system, don't forget to protect it with inhibitor.
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Postby Diy3r » Mon Aug 01, 2011 10:14 am

Hi plastic_man77

Thank you for your reply. Due to the layout of the vent pipework and location of trusses in the loft I think blowing down the vent may prove to be a bit tricky, I will perhaps try attaching a hose and doing it that way, or alternatively, and far easier will try bleeding the rads and see if they let any water out. You are right about the system, it is very very basic, I have half a mind to upgrade the whole thing however I am thinking of moving house, so may leave the upgrade.

Thank you for your advice, much appreciated!!
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