Search the forum,

Discuss Gurgling and air in central heating in the Central Heating Forum area at PlumbersForums.net

Messages
86
Hello,

Around 2 years ago we had a very old, leaky Baxi combi replaced with a newer Baxi.

For background I live in a 3 storey terrace. First floor has 3 rads, second floor has 4 rads and third floor has 2 rads.

We noticed problems with noise and air in the system. One of the rads in the loft made gurgling noises. If the rad was bled, air came out. Also, at this time the pressure gauge was all over the place. Up to 3, back down to 1 and every number in between. The gauge didn’t appear to drop, though.

The installer wasn’t helpful over the phone so I ended up finding a different engineer.

Turns out the installer of the boiler didn’t do a particularly thorough job. He didn’t clean the old system, I found leaks on some of the nuts beneath the boiler, he didn’t fit a soak away outside so water drips out over our path (great in winter when it’s freezing).

He engineer found the expansion vessel was flat. He pumped it up, tightened the pipes, advised on a soak away. He told me to bleed all the rads individually for a few mins, top up the system and add more inhibitor. He also suggested power flushing the system. I did all this and also rented a machine and flushed the system myself.

I did all this but the problems remained. I ended up contacting Baxi who came out and replaced the expansion vessel.

This didn’t solve anything. More gurgling, more air.

We ended up getting the boiler serviced again (this is maybe a year after install) and I mentioned the problems to the engineer. He asked the usual questions (is there inhibitor, have you bled the system, etc) and mentioned we had an old radiator in the bathroom. He thought this might be the source of air.

We were planning on getting the bathroom refitted so I decided to wait and see if that solved any issues.

We got the bathroom refitted. Now, the gurgling and air does not go to the top of the house but is in the towel rail in the bathroom. I can bleed the rad once a day and there’s maybe a second or two of air and then water. The pressure does not drop on the boiler.

I contacted Baxi again who came out and replaced the expansion vessel with a bigger one. The engineer also suggested fitting an automatic bleed valve on the chrome towel rail because in his experience chrome towel rails suck in air. I bought one two valves (one for spare) and fitted it.

I noticed an odd thing while fitting the automatic bleed valve. I turned off the two rad valves, released the pressure. Once the air left the rad I could put my finger in the top of the rad and feel there was no water there at all. I’d expected that the water would be just below the top of the rad?

But you’ve guessed it — still gurgling and air in the system! The automatic bleed valve doesn’t appear to vent automatically, not sure why. Could be faulty. I can swap it over with the spare but haven’t gotten around to it yet.

The only other thing I noticed with the system is that if the system is off in the summer then there is minimal air in the rad. I assume because the pump isn't forcing water around the system?

If you got this far, thanks! I’d love to solve this issue and have a system that works! I’m not opposed to ripping out pipework and starting again (even though I’m nearing the end of a long renovation!). Ideally there are some simpler steps I can take to diagnose?

Any advice?
 
That's a bit of a length post 😂. When the system is cold what are you pressurising the system to?
 
Whopper of a post!
I don't agree with the auto air vent idea. A heating system should not be producing or introducing gases. Once they are installed correctly they shouldn't really need 'topping up'.

How far vertically (feet or meters) is the top of the top rad/towel rail from the bottom of the boiler?
What pressure is in the boiler when it is cold and not running?
Just for information.
 
Thanks, both! I went for maximum detail!

From bottom of boiler to top of towel rail it’s about 3.6m. There’s another floor above as well.

Pressure from cold? Not too sure now as it hasn’t been refilled in ages.

I know that when the pump stops the gauge hovers around 1.5. When the pump comes on it’ll drop to about 1.2. There’s a slight variance.
 
When filling a pressurised system you have allow for the lift/height of the property. So if the boiler and pressure gauge are on the ground floor and the highest pipework above it is say 5m then you will have to fill to at least .5 bar plus a bit extra so the pipes at the highest point of the system don't draw/suck in air.
 
@SJB060685 thanks. I’ll let the system cool and pressurise to say, 2.2? If that number seems reasonable I’ll go ahead and do it.

Out of interest, why would air collect in the bathroom towel rail and not the two rads on the floor above?
 
@SJB060685 thanks. I’ll let the system cool and pressurise to say, 2.2? If that number seems reasonable I’ll go ahead and do it.

Out of interest, why would air collect in the bathroom towel rail and not the two rads on the floor above?

When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?
 
Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts,
 
Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts

Gotcha.

I was under the impression that the flow and return didn’t matter? Like, doesn’t matter which way the flow and return is attached to bidirectional TRVs, etc?

I think I’m missing something! :)
 
Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts,

Or if I ask another way: how can I tell if the flow and return and round the wrong way?

All I can see is some pipe work going from the boiler. I don’t know which should be the flow, which should be the return…
 
The boiler won' be very happy if the flow & return are swapped over however unlikely that it was installed this way. Just feel the hotter of the two pipes and check in your users manual if correct.
 
When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

I jumped the gun a bit and pressurised to 2.2. So far no air in the bathroom rad.

I’ll check the flow and return when I have a sec.

So far the upstairs rads seem much hotter (a good thing).
 
When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

Boiler was at 1.5 when cooling. I pressurised to 2.2 (based on your advice of .5 and a bit).

The bathroom towel rail seems much hotter, which is good.

There is still a tiny bit of air in the bathroom towel rail every day. I want to say quite a bit less but there’s nothing scientific about it. I turn the bleed valve and I get a bit of air for half a second/second.

Other rads in the house, when bled, are just instant water.

Still need to check if flow and return on boiler have been flipped accidentally.

Any ideas?
 
When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

@SJB060685 checked system at about 9am this morning and there was a 1sec burst of air. Checked at 10:15am and there was another 1sec burst of air.

So air somehow being pulled/generated in the system?
 
Would appear so yes mate. Are you sure it's just air and not hydrogen?

@SJB060685 just reading How to | Circulation Issue Causes Central Heating | Sentinel - https://www.sentinelprotects.com/uk/support/how-diagnose-circulation-issues

  • If there is a reddish tinge to the water, it indicates that iron oxide is present, suggesting active corrosion. The system may be drawing in air.
  • If there are black particles present in the water, these are most likely to be magnetite. As this is the final stage of corrosion, large deposits can be expected within the system.

I'd say I've seen both of these in the system.

I bled the rads last week and the water was reddish in places.

I also bled the towel rail on the automatic air vent side (you have to unscrew the whole thing). The pressure was high and a jet of black water shot out of the rad.

Could it be as simple as the system needs a thorough clean?
 
That is an indication you have sludge (magnetite) in the system. This is caused where you have large amounts of air entering the system and it and the water reacts with the iron content on the steel radiators causing corrosion. Hydrogen may be produced as a byproduct of this corrosion as well. The lighter or match trick would reveal if hydrogen yes. Next time you vent a radiator vent a small sample of the gas into an upturned cup and offer a flame to it, if it pops it's hydrogen. Don't vent too much into the cup though.
 
That is an indication you have sludge (magnetite) in the system. This is caused where you have large amounts of air entering the system and it and the water reacts with the iron content on the steel radiators causing corrosion. Hydrogen may be produced as a byproduct of this corrosion as well. The lighter or match trick would reveal if hydrogen yes. Next time you vent a radiator vent a small sample of the gas into an upturned cup and offer a flame to it, if it pops it's hydrogen. Don't vent too much into the cup though.

Thanks @SJB060685! I'll give it a few days and vent into a cup so there's a bit more air/hydrogen.

All radiators in the property are less than 5 years old. Most are column rads.

Does the air/hydrogen in the system mean that air is entering the system now?

Or can the air/hyrdrogen be a consequence of a prior leak and subsequent corrosion?
 
Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.
 
Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.

Yes, previous boiler was a combi. New boiler is also a combi.

The installer added an inhibitor but didn't powerflush or add any cleaner. I remember asking him about cleaning the system and he said he'd do "everything required".

When I cleaned the system (a few months after install) there was quite a bit of black water coming out.

There is a filter on the return side (assuming the return side is correctly plumbed!).

The pipework on the bottom two floors of the property are fairly old as far as I know. From the second floor to third floor it's JG Speedfit.

If I understand I need to:
  • see if it's hydrogen in the system
  • if it is hydrogen, something is corroding in the system
  • if it is corrosion, then I need to powerflush the system and add inhibitor
If it's not hydrogen... argh!
 
That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.
 
That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.

Just checked and it's Barrier PB 15 x 1.5mm.

This pipework starts below the towel rail and goes upstairs in the wall and services two rads above.

I can isolate the plastic pipework fairly easily (I'll have to drain and cap off the system) to see if this is the cause of air? Not sure why the air would end up back up on the second floor and not on the third?
 
If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.
 
If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.

I was confused about that, too: but you say the entire system is positive pressure, not negative.

I'd been wondering if some part of the system was negative and somehow pulling in air.

If there was a leak, the pressure would drop, right?
 
If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.
 
Pressure won't drop with a leak if the filling loop has a PRV with the isolation valve open, so called auto filling.

A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.
 
If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.

Yeah, so if the system is at less than 1 bar and there's a weak point, air would be pulled in.
A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.

Isolation valve is a manual turn on/off thingy.

As far as I can tell there is no pressure drop based on the gauge.

Conversely, if air is being generated in the system, shouldn't I see a pressure increase on the gauge?
 
If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.
 
If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.

Pressure is now between 2.2 and 2.5 (I've not seen it drop below 2.2).

There's defo water in the rads at the top of the house.

When I pressurised to a higher bar I got the tado app to open all valves so everything was open. I only forgot about the two rads in the house in the hall that have normal TRVs.
 
Would air being admitted into the system cause the pressure to rise? Yes it would.

So the pressure never seems to drop or rise over time.

That said, I haven't looked too much at the gauge going up as I have been looking for drops and bleeding the bathroom rad.
 
2.2 bar cold?

Not stone cold, cool-ish. Because the heating has been on because of cold weather it's never quite cold.

As an example, when the pump comes on the pressure drops to about 2 bar. When the pump switches off, it goes to 2.2 or slightly higher.

Based on what @johntheo said I checked the flow and return pipes...

Facing the boiler, the left most pipe seems to be the flow. The filling loop goes from the cold water supply to the return. The return has the magnetic filter on.

I checked the schematics in the installation and service manual and this seems to match.
 

Reply to Gurgling and air in central heating in the Central Heating Forum area at PlumbersForums.net

Creating content since 2001. Untold Media.

OFFICIAL SPONSORS

OFFICIAL SPONSORS

Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock