Gurgling boiler + Air in CH + FE tank

Dear Forum readers,
I require some help on a issue that might have been covered before but like to bring it up with context to my current problem.
I had a new Rad fitted upstairs and at the same time TRV's installed on all rads except one in the house.
F+E tank was cleaned out. System drained down and sludge remover placed for 2wks then redrained, cleaned and filled with inhibitor.
It has now been a month but am continuously finding gurgling noises in my boiler and magnafilter when the HW or CH is turned on. Sometimes the boiler cuts out and needs a resetting from the overheating error and flow errors that are received. The boiler also shorts cycles very frequently (every 30s on/off when the HW is on, longer when CH/CW combo)
It is worse when the HW is on itself and when the CH is on too, trickling noises can be heard from several of the rads and I find that I have to air an upstairs rad nearly everyday. We also getting a knocking sound in lots of the rad whether boiler is on or hours later after cooling down.
I also noticed that the F+E tank looks murky again.
What really worries me is the gurgling noise that occurs when the boiler is on and when it turns off, there is gushing water sound into the magna filter.

It sound like air in the system but not sure how and where to get rid of it.
I have been told by a plumber that the piping is a little odd with the pump on the return side and that this could be the root cause of the air. However prior to this plumbing work, we had no air or noise issues at all. All rads were working very well, except the lounge which took longest to heat and cool at bottom. hence the callout of plumber to investigate this (sludge?) and installation of rad and valves.
We have a S-plan system, F+E tank, a cold water storage tank in the loft and a condensing boiler that is non-modulating.
Any help or advice is greatly welcomed! Thanks in advance!

when the boiler is on is water coming out of vent and back into F+E tank?

In my opinion a condensate trap are blocked , but I just guess

Thanks for your reply. I can't see any water returning back. The water level in tank is fairly low and not very high. It's not warm either. I did notice that when I place a glass of water under the vent that it sucks up water then pushes it out a few times. And when I switch off the boiler that some water is sucked in. Does that make sense?

Norbert said:

In my opinion a condensate trap are blocked , but I just guess

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Is condensation trap the same as a air vent?

SJB060685 said:

when the boiler is on is water coming out of vent and back into F+E tank?

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Thanks for your reply. I can't see any water returning back. The water level in tank is fairly low and not very high. It's not warm either. I did notice that when I place a glass of water under the vent that it sucks up water then pushes it out a few times. And when I switch off the boiler that some water is sucked in. Does that make sense?

The cold feed could be partially blocked and not allowing the system to fill and vent correctly . Kop

spatel said:

Is condensation trap the same as a air vent?

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The condensate trap is part of the boiler, it removes moisture from the exhaust gases and should be checked for proper operation by a competent, qualified person. It should have no bearing though on the F&E tank problems.

I assume that it's a oil fired combi if no modulation.?

You say that the F&E tank is murky again, does this mean that it was murky before any changes were made to the system?

Vented systems can be very sensitive to pump head/speed with regards to pulling in air through the vent. Its possible that someone increased the pump speed to its max when circulating the chemicals and didn't return it to its original settings.

Was the pump renewed with the changes?
Can you check if set to speed 1,2 or 3, if set to 3 then reduce it to speed 2 and see what happens.

Can you also please post the make/model.

king of pipes said:

The cold feed could be partially blocked and not allowing the system to fill and vent correctly . Kop

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Thanks KOP.
Is there anyway of telling of the cold feed is blocked? Where would I find the cold feed pipe. When the system was drained, it did seem to refill at a decent rate. Not too sure if this info is useful? For sure there seems to be a blockage of some sort, air or otherwise but not sure how to eliminate it

John.g said:

The condensate trap is part of the boiler, it removes moisture from the exhaust gases and should be checked for proper operation by a competent, qualified person. It should have no bearing though on the F&E tank problems.

I assume that it's a oil fired combi if no modulation.?

You say that the F&E tank is murky again, does this mean that it was murky before any changes were made to the system?

Vented systems can be very sensitive to pump head/speed with regards to pulling in air through the vent. Its possible that someone increased the pump speed to its max when circulating the chemicals and didn't return it to its original settings.

Was the pump renewed with the changes?
Can you check if set to speed 1,2 or 3, if set to 3 then reduce it to speed 2 and see what happens.

Can you also please post the make/model.

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Thanks John.G.
I did have the boiler serviced 2wks back during these issues and he said that the boiler was fine and there is air in the system from all the draining and refilling. He did say that the pump was on the wrong location and could be drawing in air. The odd thing is that as far as I know it has been like this for yrs ( we have moved in to this house 6mnth ago) and prior to the work last month there was not one jot of a problem with air and knocking noise etc.
I didn't look at the F+E tank before all the work, but during the sludge removal process the tag looked really murky. So I cleaned it out with the numbers help and the water was crystal clear. He then added Fernox to the tank then refilled the system. Now, the tank looks murky again.
I have Main Heat only High Efficiency condensing boiler and is suitable for the open vented system that I have. I am at work right now but I have a grundfos pump UPS (I believe). I have an image of it attached. The pump is set on Max setting 3 though to be honest I didn't pay attention to whether it was on that before the works or not. But I will try to pump setting 2 if you think that could help.
Thanks!

Read more: https://www.diynot.com/diy/threads/boiler-on-off-short-cycling.517330/#ixzz5gX9FUJdp

There are probably hundreds of thousands of vented systems with the pump on the return (including my own) that have no problems with air ingress.

When you get home, take a snapshot of the pump label to identify which exact model you have, there are a lot of UPS pumps with various head/flow rates, depending on age, and running on speed 3 could definitely be a recipe for drawing in air.
Has the pump got a push button to change settings with indicating LED lights or is it just a simple rotary switch?.
I assume also that the magnetic filter shown in photo wasn't fitted in conjunction with the above changes.

John.g said:

There are probably hundreds of thousands of vented systems with the pump on the return (including my own) that have no problems with air ingress.

When you get home, take a snapshot of the pump label to identify which exact model you have, there are a lot of UPS pumps with various head/flow rates, depending on age, and running on speed 3 could definitely be a recipe for drawing in air.
Has the pump got a push button to change settings with indicating LED lights or is it just a simple rotary switch?.
I assume also that the magnetic filter shown in photo wasn't fitted in conjunction with the above changes.

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Hi John.G
I attach a pic of the pump that is currently installed. It is a UPS 15-50 130. Currently on setting 3 with the switches on the side as seen. I will try switching to speed 2 as recommended. You might notice that the pump appears to be upside down?...
The magnafilter was already installed previous to the works and our arrival. It was recently removed during boiler service to check for sludge. There was a little but not a massive amount. Plumber also placed a lever upstairs next to the hotter cyclinder upstairs in airing cupboard.
One thing I have noticed since the system drain, clean and refill is that the Rads get superhot (although the one furthest away is lukewarm - used to be hot previously). I have turned down the temperature on the CH system at the boiler. Not sure if this is due to limited water flow or that the system is much cleaner now that the it is being heated more effectively? However this does not jive as to why the boiler overheats and shows up with the flow error. In any case, would appreciate any feedback you have.
Thanks!

That particular pump, UPS selectric 15-50 130 isn't particularly powerful, its a 5M (4.75) pump on speed 3 and shouldn't IMO cause pump over to the F&E tank even on speed 3 but do try it on speed 2 which is very close in head to speed 3, you could then try it on speed 1 but I don't think this is the root cause of the problem.
The very hot rads IMO is caused by the now poor circulation rate, you are also getting a flow error.
If the pump is pumping downwards and to the left into the boiler return?? it just means that the pump head is "wrong" way up.
BUT just check that the pump (arrow on pump body) IS pumping in to the boiler via the return.
It probably makes no difference but I assume the filter was replaced with the proper orientation.

What is this "lever" that the plumber installed.?

It's possible that the pump body ports are partially restricted with sludge after the chemical clean, this would mean at the very least removing the pump head to have a look if you are happy to trust that the pump isolation valves won't leak after re opening them, otherwise it requires system drain down.

Can't think of anything else just now.

Those pump isolating valves should be ok re leaking afterwards as they are glanded gate valves.

Other possibility but hope not is that the boiler heat exchanger has become partially blocked.

I can see a blue ball (Lever) valve which is the one he installed for throttling the flow to the hot water cylinder?, I can't make it out clearly but the cylinder coil MV seems to be on the vertical pipe (return?) its normally installed in the horizontal section on the flow to the coil, it really does'nt matter as long as it isn't interfering in any way with the vent. The vent pipe is normally a continuation of the vertical flow to the cylinder coil and should have no restriction of any kind.
You might have a look at that set up around the cylinder and satisfy yourself that it is more or less as I described and perhaps open that lever valve fully after making a note of its approximate opening.
What I am trying to say (poorly) is that IF that vertical flow pipe up past the cylinder is continued on as the vent (which is the normal) then there must be no restrictions of any kind on it.

John.g said:

I can see a blue ball (Lever) valve which is the one he installed for throttling the flow to the hot water cylinder?, I can't make it out clearly but the cylinder coil MV seems to be on the vertical pipe (return?) its normally installed in the horizontal section on the flow to the coil, it really does'nt matter as long as it isn't interfering in any way with the vent. The vent pipe is normally a continuation of the vertical flow to the cylinder coil and should have no restriction of any kind.
You might have a look at that set up around the cylinder and satisfy yourself that it is more or less as I described and perhaps open that lever valve fully after making a note of its approximate opening.
What I am trying to say (poorly) is that IF that vertical flow pipe up past the cylinder is continued on as the vent (which is the normal) then there must be no restrictions of any kind on it.

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Thanks John for your insight on the matter.
I have another plumber coming in this thursday to have a look at the issue. The previous plumber did indeed install the blue ball lever. The pipe connected to it (not sure what that one is) is the one that is making the trickling noise and the pipe connected to the pump is also doing the same. I am not quite familiar or what to look for in terms of pipe systems so apologies for my lack of knowledge in terms of the various flows/returns mechanisms. I have had added another pic of the airing cupboard piping system if that helps but can appreciate that without being there physically the sherlock holmes styled logic approach is all that can be done here!
The plumber who did the work is very methodical and is very cautious. I live in the Cambridge area and the pipes installed in these types of houses are micropore and v.narrow. I am hesitant to do a power flush as this might lead to a leak. Whilst installing the TRV in one of the far rads, he said the pipe might have kinked. He has not ruled out a possible leak but I can't see any dripping into the F+E tank when the CH is on. The gurgling noise is most prominent when the HW is on. The trickling noise is heard in the rads when CH turned on.
Would you still say whether there is a possible restriction somewhere and if so where you think it is most likely? I will relay this info to the new plumber. The old plumber is wondering to fill the system with a sealant, to fill any pinholes but I am worried by this as I recently had my car cylinder head blown due to the previous owner doing this with the coolant! cost me 4K to repair as all the sludge built in the car. So as you can see, I have already been burnt by this type of issue before.
Thanks a lot John.G for all the advice you have given so far!

Hard to second guess re what's going on there, I'd let the "new" plumber do his own thing.
Under no circumstances would I use leak sealant in my own system.
There are two simple things that you can do yourself before he comes, wait until the system cools down and say first thing in the morning see if the F&E tank ball cock is making up or dripping or if the level is high in the tank, that will give you a good idea if you have a leak or not.
The other, with system running, slacken the brass screw on the pump,
and see if any air comes out, you need a old towel to catch the small water leakage, if you feel up to it then remove the screw and ensure that the pump shaft is turning, if you are satisfied that it is from feeling/hearing the pump then do not do this step.

You could also try speed 1 or 2 on the pump while awaiting the plumber.

John.g said:

Hard to second guess re what's going on there, I'd let the "new" plumber do his own thing.
Under no circumstances would I use leak sealant in my own system.
There are two simple things that you can do yourself before he comes, wait until the system cools down and say first thing in the morning see if the F&E tank ball cock is making up or dripping or if the level is high in the tank, that will give you a good idea if you have a leak or not.
The other, with system running, slacken the brass screw on the pump,
and see if any air comes out, you need a old towel to catch the small water leakage, if you feel up to it then remove the screw and ensure that the pump shaft is turning, if you are satisfied that it is from feeling/hearing the pump then do not do this step.

You could also try speed 1 or 2 on the pump while awaiting the plumber.

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Will do John thanks!.
Pump setting is on 2 today. The 'old' plumber did try the pump shaft a few wks back . he opened the screw and quickly plunged in a screw driver to see that the pump was working and it was. But I can do that bit again.
I will check the F-E tank to see if there is dripping. On the contrary, if anything the tank volume looks low rather than high (no danger of overflowing).
Interestingly early this morning, when the HW came on timer, I heard some gurgling noise coming from somewhere in the attic. I will need to check this out. I noticed when I went up there a few days ago that the cold water storage tank was quite full. It was not quite up to the overflow but there is a pipe that goes over it (like a vent pipe) but that it was submerged into the water. I don't know if it should be like that.?
Thanks!

Why did he fit the lever valve?

No, the vent pipe shouldn't be dipping into the water, it should be a little higher than the tank overflow level. I think that the air in your system when hot is displacing some water and pushing it back up through the cold feed pipe but then one would think that this pipe would be at least a little warm.

I was looking at those pump curves again and the pump head at speeds 2&3 are very close so try it on speed 1, I assume it was on speed 2 this morning with high tank level.

John.g said:

Why did he fit the lever valve?

No, the vent pipe shouldn't be dipping into the water, it should be a little higher than the tank overflow level. I think that the air in your system when hot is displacing some water and pushing it back up through the cold feed pipe but then one would think that this pipe would be at least a little warm.

I was looking at those pump curves again and the pump head at speeds 2&3 are very close so try it on speed 1, I assume it was on speed 2 this morning with high tank level.

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Not too sure about lever valve.
I just want to double check with you that this setup sounds correct to you.
I have 2 tanks in the loft.
One is the F+E tank that serves to feed the CH system and and this has an vent pipe over the tank.
The other tank is the cold water storage tank, that will serve to provide supply H20 for the hot water cyclinder and the F+E tank (if it runs low). This cold water tank also has an air vent too? (which in my case is currently submerged in H20).
So when you refer to the cold feed pipe, is the pipe that associated to the Cold water storage tank? And if this is partially block, is that why the levels are quite, there is air in the system somehow and also why the vent pipe is submerged in H20?.
Thanks for your patience, am just trying to clarify the basics of home plumbing 101!

spatel said:

Not too sure about lever valve.
I just want to double check with you that this setup sounds correct to you.
I have 2 tanks in the loft.
One is the F+E tank that serves to feed the CH system and and this has an vent pipe over the tank.
The other tank is the cold water storage tank, that will serve to provide supply H20 for the hot water cyclinder and the F+E tank (if it runs low). This cold water tank also has an air vent too? (which in my case is currently submerged in H20).
So when you refer to the cold feed pipe, is the pipe that associated to the Cold water storage tank? And if this is partially block, is that why the leve
If the system has been emptied then water is allowed to enter the Feed & expansion tank

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spatel said:

Not too sure about lever valve.
I just want to double check with you that this setup sounds correct to you.
I have 2 tanks in the loft.
One is the F+E tank that serves to feed the CH system and and this has an vent pipe over the tank.
The other tank is the cold water storage tank, that will serve to provide supply H20 for the hot water cyclinder and the F+E tank (if it runs low). This cold water tank also has an air vent too? (which in my case is currently submerged in H20).
So when you refer to the cold feed pipe, is the pipe that associated to the Cold water storage tank? And if this is partially block, is that why the levels are quite, there is air in the system somehow and also why the vent pipe is submerged in H20?.
Thanks for your patience, am just trying to clarify the basics of home plumbing 101!

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The feed & expansion tank is dedicated solely to the heating system, it is normally filled or topped up from its own ball cock which in turn is normally connected to the cold water mains, it is not (or shouldn't be) fed from the cold water storage tank.

The cold feed from this feed and expansion tank goes to the boiler hot water circuit and may join it in various places such as on the return to the boiler or tied in just before the pump suction, the vent pipe is normally Teed off where the cylinder coil flow (top connection) pipe connects to this cylinder coil and then carries on up and over the feed & expansion tank.

If the system has been emptied then water is allowed to enter the Feed & expansion tank via its ball cock and starts filling the system, as its filling most of the air is expelled through the vent pipe until the feed and expansion tank is at its normal working level (maybe 1/3rd of tank), there will also then be water at the same level in the vent pipe.

As the system heats up the water expands and depending on the system contents will rise the level in the feed and expansion tank by ~ 2 or 3 litres, it will expand back up through the cold feed (hence the name feed and expansion) and also expand in the vent pipe so that the level in the feed and expansion tank and the vent pipe level are equal, they act as a U tube.

If everything works as designed, then when the pump starts up everything stays as it is but again depending on where the cold feed is tied in and any blockages/restrictions in the boiler/pipework etc water can be forced up the cold feed, air can then be drawn in at the vent, water may also be forced up the vent pipe ( often if the pump head is too high) and flow into the feed and expansion tank, when the pump stops then air can be drawn back in through the vent.


The cold water storage tank is used to supply cold water to toilets, baths, showers & wash basins etc. It is also used to feed cold water into the bottom of the hot water cylinder, this cylinder is heated by the boiler water circulating in the cylinder coil and generally has a electric immersion as well.

This water also expands when hot and a separate vent from the top of the hot water cylinder is taken up and over the cold water storage tank.


I think this is the submerged vent that you are referring to, it shouldnt be submerged but should be (well) above the level of the overflow from the cold water storage tank.

I don’t do plumbing, I leave that to others, I prefer oil fired appliances. However there is a formula for working out the height of vent. I tried looking back through my college books but couldn’t find it, I did find a post on this forum about it though from a number of years ago.

Height of vent above water in CWST = height (metres of static head) x 40mm + 150mm

This result is the height in mm the vent should terminate above level of water when the system is cold.

That sounds similar to what I remember.
Your vent being below water line is not installed correctly.

To get over the problem of pump over through the vent (pitching) or back up through the cold feed (in vented boiler heating systems) various systems are used, mine, (from new, 42 years ago), has a combined feed&vent and never had a problem)

I’ve heard of combined feed and vent but never seen one. I know they’re supposed to prevent pumping over but I know they are prone for trapping air on initial filling

I have a 1/2" gate valve on the ball cock supply, I just barely crack it open when filling my system, it might take 40 minutes or so to fill the system but I have very few problems by doing this.

SJB060685 said:

I’ve heard of combined feed and vent but never seen one. I know they’re supposed to prevent pumping over but I know they are prone for trapping air on initial filling

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Thanks to you and John for engaging with me and for all your help. I have attached some images of my system in the loft. What I noticed when taking pics was that bubbles popped up from the F+E cold feed. Am not too sure if this should be the case or not and whether this explains why the F+E tank has got dirty again given I gave it a thorough clean 2wks ago. In any case from the pics, it looks like a traditional system though can't rule of they combine lower down. I can't make heads or tails from the piping in airing cupboard. The exposed copper pipes I think feed into the 2 showers we have upstairs. I would imagine that another 2 should be air vents for the F+E and the CWS tank.
Would be great to see what you think. Cheers.