Where oh where is this air coming from?

[Ric2013]

Check the feed isn't block as well

Vent a rad / bucket under mag and does the tank run ?

Feed not blocked. Drains down normally, so I assume feed okay anyway.

 

[Chuck]

I think that the problem is almost certainly the close-coupled 'H' arrangement you have at the inlet to the pump. If you want to connect the vent and feed in this way and this close to the pump I think you need an air-separator. See, for example, the diagram on page 316 of this text book:

http://www.pearsonschoolsandfecolle...FreesamplechapterPlumbing/Level2_PLUMB_SB.pdf

Another possibility to keep in mind is that 'smart pumps' can have start-up transients. E.g. coming on full for a second or two as the pressure rises before cutting back. If the amount of air depends on the number of on-off cycles rather than the total running time this is a somethin to investigate.

 

[Ric2013]

Thanks for that, Chuck. It seems your air separator is much the same as the Toolstation one.

I have close coupled systems myself without an air separator without having this problem, but the close coupling and pumps were on the ground floor, not on the first floor. I have also some reluctance to fitting one on the grounds that they have been described as a solution to a problem that shouldn't exist if the system is plumbed correctly, and, in fairness, I have yet to see a boiler installation manual that doesn't show close coupling to be simply a case of a couple of tees 150mm or less apart, ideally with the feed coming in at the bottom.

I think in principle the air separator is a good idea, to removing air, but if the vent arrangement is not pulling air in, then the air must be coming from elsewhere.

I can't help thinking that there may be a duff joint somewhere that could be drawing air in when the system is hot... even though the system is positive pressure - localised cavitation possibly?

I have noticed the pump starts faster than it runs, but would like to comment that the new smart pump post-dates the problem and, in any case, it still starts at a fairly slow speed.

Also that the system seems to run without air so long as the boiler is switched off (my system can still run if the boiler thermostat is set to the off position, but doesn't heat up). So I'm thinking it's a combination of steam or heat plus flow, rather than the flow itself. Certainly there is a lot of gas flowing through the system which the vent arrangement seems to get rid of, but I think it is mostly steam. I would imagine that once the system cools, this steam must become liquid once again, but the air obviously remains.

I'll try a few experiments tomorrow as I have the day at home and report back based on more careful observation.

Final question: Potterton Netaheat Profile 40e - any chance of air getting in through a heat exchanger? I think the cast iron heat exhanger is made from more than one piece and has a gasket on the side of it. Could this be letting air in somehow?

 

[Chuck]

I have close coupled systems myself without an air separator without having this problem, but the close coupling and pumps were on the ground floor, not on the first floor.

I think that's an important difference. You'll notice that the textbook examples where an air-separator is recommended have them near the top of the system.

I haven't got measurements to prove it happens this way but here's an idea for a mechanism for air getting into a leak-tight system:

Suppose your heating system contains 100 litres of water. If this is cycles between 20°C and 70°C the thermal expansion will be about 2%, i.e. 2 litres. As the system warms, water moves up the 15mm feed pipe into the expansion tank. Then, when the system cools 2 litres is drawn out of the header tank.

Rough values for the solubility of air in water:

at 20°C at 0 m H20 gauge pressure is about 1.8% vol/vol.
at 70°C at 0 m H20 gauge pressure is about 0.8% vol/vol.
at 70°C at 2 m H20 gauge pressure is about 1.0% vol/vol.
at 70°C at 4 m H20 gauge pressure is about 1.2% vol/vol.

When 2 litres of water at 20°C at zero gauge pressure is increased in pressure to 2 m H20 (as it moves down the expansion pipe) and is then heated to 70°C in the boiler about 16 ml of air can be released.

The volume of 15mm tube is about 130 ml / m. So, supposing you have 2 m of tube between the 'H' and the expansion tank, that's about 0.25 litre.

So each time the water expands and contracts maybe 1.75 litres that was in the system is pushed up into the expansion tank and replaced by 1.75 litres from the expansion tank when it cools. If the water in the expansion tank is saturated with air that's enough to pull 14 ml of air into your heating system each cycle.

I'm not sure whether the surface area of an expansion tank and the mixing due to the hot water flowing into it is enough to keep the water fully saturated with air but the argument still works down to about 50% saturation levels.

 

[Ric2013]

Thanks, Chuck. You've really made me see open-vented systems in a new light. I was of the opinion that a good open-vented system didn't have air in it, once it had been bled and any dissolved air had reacted with the steel in the system. But now you have me wondering whether a good open-vented system simply has less air in it, and appears to have no air in it because the air gets out easily.

I think that's an important difference[....] When 2 litres of water at 20°C at zero gauge pressure is increased in pressure to 2 m H20 (as it moves down the expansion pipe) and is then heated to 70°C in the boiler about 16 ml of air can be released.

This makes sense, provided, of course the water moving down to the boiler is already saturated with dissolved air. Not totally convinced by your comment about the textbook, as the ONLY close-coupled system it shows has an air separator, but I suppose the point is that the design of my system is such that microbubbles will always find their way to Magnaclean after the system is at rest, and from there they can only escape if the vent is opened.

I'm not sure whether the surface area of an expansion tank and the mixing due to the hot water flowing into it is enough to keep the water fully saturated with air but the argument still works down to about 50% saturation levels.

In any case, Chuck, the basic problem is, in your opinion, that the water in the F&E is moving into the system (by eddy currents, if nothing else) and will inevitably bring air with it as the water in the F&E is able to absorb air. So either I need to put a layer of paraffin on top of the water in the F&E (but oil in the system is bad news, so that rules that out), or a de-aerator on the boiler flow, or an air separator as you suggest.

In 25 minutes, I will have the result of an experiment I have been carrying out, which will, I think, suggest that no air is being drawn in through the vent.

 

[Ric2013]

Experiment to see if air is being drawn down the vent:

System bled at all points. Air only found in one radiator (very small quantity).

Turn boiler stat to OFF. Run system at DHW only for one hour. No noise of gases running around system noted, however it sounded like there may have been a bubble or two in the pump due to the pathetic pump-down arrangement. After an hour, the system was switched off for an hour and bled again. NO AIR FROM ANY POINT.

This would seem to indicate that the system is not drawing in air from the vent when cold.

At this point, a yoghourt pot approximately full of water was tied to the open vent so the vent dipped into it. and the boiler set to fire at the usual setting (4 1/2). The vent was checked soon after switch-on. It is possible that the vent may have caused pressure at switch-on, (it would have been better to have left the pot not totally full of water to see if the level rose and to have checked the level accurately) but all I can say is that the water seemed still when I got into the loft to check.

The pot was checked every ten minutes:

0.10 boiler started to kettle and noise of gases flowing around system. Slight see-sawing of water (1/16") in pot suggests some steam was rising up vent and condensing.

0.20 System less noisy (as boiler had begun to cycle and was cooler than at 0.10?) and water in pot seemed static.

0.30 As 0.20

0.40 As 0.20

0.50 As 0.20 (boiler now firing less due to reduced heat transfer as cylinder warms)

0.60 as 0.50. System switched off. Pump over-run did not occur.

Level in yoghourt pot within 1/4" of top. Had slight pressure previously caused it to overflow and now the level has dropped back, or was it like that at start-up?

After an hour cooling and settling, the yoghourt pot level was still the same, there was no air in radiators and the Magnaclean contained the usual amount of air that generally I find when I heat my water cylinder.


Would people agree this is a failed experiment as I have failed to prove that the vent has sucked in any air whatsoever due to the fact that I was not entirely certain of how full the youghourt pot was at fire-up, or were people suggesting the vent was drawing in air expecting an appreciable quantity to be being dragged in, and so the idea that the vent is sucking has been disproven?

Chuck - a thought: if the boiler is kettling then some of the water is boiling, thus its ability to hold dissolved air drops to zero. This will increase the quantity of air being released when the boiler heats up.

 

[Chuck]

Chuck - a thought: if the boiler is kettling then some of the water is boiling, thus its ability to hold dissolved air drops to zero. This will increase the quantity of air being released when the boiler heats up.

I doubt it is significant. It's the heating of the bulk water that will allow the air to be released and turbulence, e.g. in a pump or air separator will speed up the process of reaching equilibrium. The amount of water boiling during kettling is tiny compared with the total volume of water in the system. The noise is tiny bubbles of steam being created in hot crevices at the HX surface, moving out into the relatively cool bulk water in the HX and then collapsing as the steam condenses.

Did you observe how much the height of the level in the expansion tank rose and fell between the system being cold and the system being hot? My theory (well, more of a hypothesis really) relies on that volume difference being larger that the volume of the pipe connecting the tank to your 'H'.

 

[Chuck]

Thanks, Chuck. You've really made me see open-vented systems in a new light. I was of the opinion that a good open-vented system didn't have air in it, once it had been bled and any dissolved air had reacted with the steel in the system. But now you have me wondering whether a good open-vented system simply has less air in it, and appears to have no air in it because the air gets out easily.

I think that's probably the case. It's why unvented systems invariably fill up with of magnetite sludge after a couple of years operation. Fernox slows the corrosion but it doesn't stop it.

Not totally convinced by your comment about the textbook, as the ONLY close-coupled system it shows has an air separator, but I suppose the point is that the design of my system is such that microbubbles will always find their way to Magnaclean after the system is at rest, and from there they can only escape if the vent is opened.

The textbook wasn't supposed to be crucial to my point, just to show I wasn't completely making it up out of thin air.

In any case, Chuck, the basic problem is, in your opinion, that the water in the F&E is moving into the system (by eddy currents, if nothing else) and will inevitably bring air with it as the water in the F&E is able to absorb air. So either I need to put a layer of paraffin on top of the water in the F&E (but oil in the system is bad news, so that rules that out), or a de-aerator on the boiler flow, or an air separator as you suggest.

I think I'd call it a 'hypothesis' at the moment. It was just attempt to explain how the behaviour you observed could occur in a leak-free system.

I would definitely not try putting oil or anything like that into the tank.

It is my opinion that installing an air separator, as shown in the text book, where your 'H' is should be high on your 'worth trying' list. It's a conventional thing to have, will cost about a tenner and should be easy to install. If it doesn't work you can leave it in place and it won't be doing any harm.

 

[Ric2013]

I would definitely not try putting oil or anything like that into the tank.

It is my opinion that installing an air separator, as shown in the text book, where your 'H' is should be high on your 'worth trying' list. It's a conventional thing to have, will cost about a tenner and should be easy to install. If it doesn't work you can leave it in place and it won't be doing any harm.

I'll add the air separator onto my to-do list. I expect it can't prevent the air coming in as that happens when the system cools after use, but it will probably help get it out again.

Fernox only slows corrosion?! And there was me thinking that quality inhibitor is 100% effective in preventing rust and cures cancer if you rub it into your skin on a Tuesday . Or so Sentinel's online 'training' would probably like me to think lol.

Don't worry about the oil, I was just thinking it would probably work albeit with so many undesirable other effects I wasn't planning on actually doing it.

 

[Chuck]

Fernox only slows corrosion?! And there was me thinking that quality inhibitor is 100% effective in preventing rust and cures cancer if you rub it into your skin on a Tuesday . Or so Sentinel's online 'training' would probably like me to think lol.

I recently tried the 'nail through a piece of pipe in a small jar of water' test with water half-filling the jar so there was some air available to dissolve. One with tap water another with sytem water treated with the correct amount of Fernox F1. The untreated nail showed signs of rusting within 24 hours and was very rusty within a week. The Fernox protected nail showed signs of rusting after a month and was very rusty after six months.

So, yeah, in a vented system I'd describe F1 as 'slowing' not 'stopping' corrosion. In practice, if you can slow the corrosion rate by a factor of 20 it's not going to be a problem.

Don't know about Sentinel, I've never tried the same test with it, but I'd expect it to behave in a rather similar way. The laws of chemistry and thermodynamics are pretty consistent between brands.

 

[Best]

I found the original Fernox back in the day to be exellent on open vented systems, often with little corrosion present after many years. But the modern Fernox seems different.
What puzzles me is the 4 litre Fernox is not the same as the concentrate.

 

[ShaunCorbs]

I found the original Fernox back in the day to be exellent on open vented systems, often with little corrosion present after many years. But the modern Fernox seems different.
What puzzles me is the 4 litre Fernox is not the same as the concentrate.

Watering it down mate

 

[Ric2013]

I recently tried the 'nail through a piece of pipe in a small jar of water' test[...].

I assume that's to test the effect of dissimilar metals too?

I must say I took a water sample from a customer's system recently, having drained to add a new radiator, and did the plain nail in a jar test, just using the system water undiluted and, in spite of 2l Sentinel to 13 radiators, the nails, in 10 days, are not quite as bright as when new.

But better than whatever inhibitor the RGI had used two months prior, His let the nail go a dark orange and pitted in three weeks and seemed to encourage slime to grow in the F&E (which I had thoroughly cleaned prior to his installation). I was trying to show the customer what inhibitor did, but it didn't really work as a demonstration. When refilling, I asked the RGI which inhibitor he had used so I could refill with the same and he said, 'Oh, "Range" boilers don't mind. Just use any good-quality inhibitor.' I took this as an evasion, and let it slide.

 

[Chuck]

I assume that's to test the effect of dissimilar metals too?

It's to model the Galvanic corrosion that occurs in heating systems. I suppose one should include some aluminium in the mix these days but this was the quick and dirty test the scientists that developed Fernox used to decide which formulations were worth investigating further.

To demonstrate the effect of the protector, you need to use a second pot with tap water as a control. With a nail through 10mm of 15mm Cu tube there'll be a noticeable amount of rust after 24 hours.

The RGI should have specified the inhibitor used on the Benchmark statement for the boiler. If you've drained and refilled with something else don't forget to update the docs and any labels on the system.

 

[Best]

Watering it down mate

It seemed to be a different product entirely compared with the concentrate. Different colour and had a smell to it that the 500ml doesn't.