HoneySuckleDreams Posted October 29, 2013 Share Posted October 29, 2013 I have a constant problem where just one radiator keeps collecting air. I can bleed the rad OK and it fills up with water, but within 20 minutes it is burbling away like a running stream. All the other rads are OK.Any ideas ? Link to comment Share on other sites More sharing options...
Théière Posted October 29, 2013 Share Posted October 29, 2013 Air must be getting in up stream of this rad or it's rad connection. Link to comment Share on other sites More sharing options...
HoneySuckleDreams Posted October 29, 2013 Author Share Posted October 29, 2013 The feed and return pipes run up the center of the house and the radiators spur offIf it's a connection then wouldn't I see water seeping out at some point as the pressure would be greater in the system than the surrounding environment ? It seems such a simple setup.I can try tightening everything up this weekend when I can purloin some wrenches. Link to comment Share on other sites More sharing options...
Théière Posted October 29, 2013 Share Posted October 29, 2013 Air molecules are much smaller than water molecules and water is more viscous, so air can be drawn in by a venturi effect without showing any water leak. Link to comment Share on other sites More sharing options...
Baz Posted October 29, 2013 Share Posted October 29, 2013 If there is not a leak you can cure the problem permanently by replacing the existing radiator air valve with an automatic bleed valve. Very cheap and easy to do and do not even have to drain the radiator. Look here for an example:http://www.amazon.co.uk/Pack-Radiator-Auto-Bleed-Valves/dp/B009OAIAEABaz Link to comment Share on other sites More sharing options...
Baz Posted October 30, 2013 Share Posted October 30, 2013 If there is not a leak you can cure the problem permanently by replacing the existing radiator air valve with an automatic bleed valve. Very cheap and easy to do and do not even have to drain the radiator. Look here for an example:http://www.amazon.co.uk/Pack-Radiator-Auto-Bleed-Valves/dp/B009OAIAEABaz Link to comment Share on other sites More sharing options...
Pierre ZFP Posted October 30, 2013 Share Posted October 30, 2013 I had this and it was traced to a pin hole leak in the radiator itself. Manufacturing fault on one of the welds. Had to replace the Rad but at least it was under guarantee Link to comment Share on other sites More sharing options...
Rabbie Posted October 30, 2013 Share Posted October 30, 2013 [quote user="Théière"]Air molecules are much smaller than water molecules and water is more viscous, so air can be drawn in by a venturi effect without showing any water leak.[/quote]Is that so? Nitrogen and oxygen, the main constituents of air, have a molecular weight of 28 and 32 respectively while water has a molecular weight of 18. Link to comment Share on other sites More sharing options...
Théière Posted October 30, 2013 Share Posted October 30, 2013 I was waiting for this, Water is water because of hydrogen bonding hence it forms larger chains of molecules than the actual sum of the molecular weight of each element. [geek] Link to comment Share on other sites More sharing options...
Rabbie Posted October 30, 2013 Share Posted October 30, 2013 Of course. Although the molecules are relatively small they clump together to form larger units. Link to comment Share on other sites More sharing options...
Théière Posted October 30, 2013 Share Posted October 30, 2013 It's jolly handy they decide to split apart at 100 C or we would have to remember an awkward number like 212 F Link to comment Share on other sites More sharing options...
nomoss Posted October 30, 2013 Share Posted October 30, 2013 [quote user="HoneySuckleDreams"]The feed and return pipes run up the center of the house and the radiators spur offIf it's a connection then wouldn't I see water seeping out at some point as the pressure would be greater in the system than the surrounding environment ? It seems such a simple setup.I can try tightening everything up this weekend when I can purloin some wrenches.[/quote]Only if the system pressure is kept higher then the outside pressure throughout the system.The pressure at the highest part will be lower than that at the lowest part - each metre of difference in height will reduce the pressure at the top by 1.4 psi (0.1 kgf/cm2), and the pressure will be lower again on the return side of the system because of the pump suction. Link to comment Share on other sites More sharing options...
Chancer Posted October 30, 2013 Share Posted October 30, 2013 A saucer of water at sea level will be at the same pressure as the air around it, 1 bar or atmospheric pressure.At a depth of 10m the water pressure will be at 2 bar, 1 bar static pressure + 1 bar hydrostatic.To get water up a pipe in a heating system to a height of 10m needs a mains or pumped pressure of at least one bar (2 bar if you want to talk absolute pressure)Ergo water in a system has to be at at least the same pressure as the outside air.I have never understood the physics of how air can enter a gravity fed CH system other than air that wasnt purged. Link to comment Share on other sites More sharing options...
Pickles Posted October 30, 2013 Share Posted October 30, 2013 Just thinking out loud (as it were)...In terms of sources of "air", if we start with a properly purged and properly pressurised system (taking into account that the pressure that matters is that at the highest point of the system), either:water must be getting out somewhere; orgas is being generated in the system.Assuming that for whatever reason, air is in the system, then once you start bleeding radiators, you are dropping the pressure (unless you have an auto-top-up), and I wonder if non-brazed joints are intended to be watertight when the external pressure is greater than the internal pressure (eg radiator fittings at the highest point). When you top-up, you will introduce air that is dissolved in the water which will probably come out of solution at some point, leading to a further (but small) need for re-bleeding.So, how can water be getting out?I would guess that if there is no evidence (damp patch, pool of water etc) then there could be a leak that is slow enough that the water evaporates away. Alternatively, depending on the system, a slow water/water heat exchanger leak or an air/water heat exchanger leak could be difficult to detect.How can gas get in?Other than from being dissolved in top-up water, I suppose the only other routes are from places where the exterior pressure is higher than the interior pressure, and from chemical reactions (dissimilar metal joints) within the pipework.Does that sound reasonable? Link to comment Share on other sites More sharing options...
Chancer Posted October 30, 2013 Share Posted October 30, 2013 Reasonable? Yes, there has to be a reason for the phenomenon, but the internal pressure being lower than the external pressure? - Thats the bit I just dont see as possible other than perhaps cavitation around the pump impellor, A french system is pressurised to a greater pressure than the static head and then is also pumped (correct me if I am wrong) a UK system will have a header tank giving a positive hydrostatic head pressure even at the highest point in the system.On a UK system water doesnt have to get out, it could be pushed back up to the header tank but in that scenario what could be pushing the air in with sufficient force?My UK system collected air in one upstairs rad for decades but I think that stopped when I changed the pump and reduced its speed, it definitely stopped a rattling sound like a coin shimmying down inside a vertical pipe whenever the pump stopped, I drained it down and refilled/purged after decorating this summer, it will be interesting to see if any air gathers.The house is rented so I dont get to see/hear of little things like air in rads. Link to comment Share on other sites More sharing options...
Pickles Posted October 30, 2013 Share Posted October 30, 2013 [quote user="Chancer"]Reasonable? Yes, there has to be a reason for the phenomenon, but the internal pressure being lower than the external pressure? - Thats the bit I just dont see as possible other than perhaps cavitation around the pump impellor, A french system is pressurised to a greater pressure than the static head and then is also pumped (correct me if I am wrong) a UK system will have a header tank giving a positive hydrostatic head pressure even at the highest point in the system.On a UK system water doesnt have to get out, it could be pushed back up to the header tank but in that scenario what could be pushing the air in with sufficient force?My UK system collected air in one upstairs rad for decades but I think that stopped when I changed the pump and reduced its speed, it definitely stopped a rattling sound like a coin shimmying down inside a vertical pipe whenever the pump stopped, I drained it down and refilled/purged after decorating this summer, it will be interesting to see if any air gathers.The house is rented so I dont get to see/hear of little things like air in rads.[/quote]As I wrote, I'm just thinking out loud, but I think in many cases the need to bleed is down to air locks not being fully purged and/or bad design preventing full purging, such that air is always in the system. However in a UK sealed system the pressures are not normally very high: we had a 3-storey "town house" with a sealed system, no auto-top-up (you had to do a manual connection and fill by turning on an in-line tap) and no header tank (I have a feeling that header tank systems are no longer fitted in the UK): it was possible for the system pressure to drop such that if there was air at the highest part of the system, and you opened up the purge valve, the water pressure was such that the water level did not rise. Under such circumstances (ie insufficient pressure), I suspect that the condition could exist that air could get in through the highest joints. Re cavitation, other than as a mechanism by which some of the dissolved air could be "encouraged" to come out of solution or in some way drawing air in through seals by the pump, I'm not sure how it could contribute to air gathering in the system.Thinking a little wider: one of the things that could reduce the pressure in a sealed system I suppose is an over-enthusiastic pressure relief valve: if it is set to go off at too low a pressure on a "tall" system, you could get pressure drop problems particularly as the system cools. Link to comment Share on other sites More sharing options...
dwmcn Posted October 30, 2013 Share Posted October 30, 2013 Honey,Buy an old VW Beetle. No radiator involved.David Link to comment Share on other sites More sharing options...
Rabbie Posted October 30, 2013 Share Posted October 30, 2013 [quote user="dwmcn"]Honey,Buy an old VW Beetle. No radiator involved.David[/quote]Really helpful comment. Why bother to post if you have nothing of value to say Link to comment Share on other sites More sharing options...
Alan Zoff Posted October 30, 2013 Share Posted October 30, 2013 When we had a similar problem, I was told that our system was running so hot that the valves were venting steam and then drawing in air when the rads cooled. This apparently explained why we couldn't find a damp patch but were always having to bleed the top radiators. Seemed to stop when burner pressure was reduced. (Well I think that was what they said and did but plumbing remains one of life's many mysteries.) Link to comment Share on other sites More sharing options...
Chancer Posted October 31, 2013 Share Posted October 31, 2013 I can see a little merit in that scenario, it would have to be a leaky valve, not difficult if the gland nut wasnt tightened, one which only leaked at a higher than normal temperature and pressure, one would rise with the other.So I can see a leak not being visible, at a stretch I can see air entering when it had cooled and re-sealed, its less dense than water, but only in a completely sealed system without an expansion tank which i dont think exists, in that scenario as the water cooled a partial vaccum would occur and air would enter via the leaking valve.Like I say the idea has merit and is probably the correct one, it just needs a little more explanation as to how the air could enter. Link to comment Share on other sites More sharing options...
dwmcn Posted October 31, 2013 Share Posted October 31, 2013 Rabbie,I've been using forums long enough to know that everybody is an expert on everything and posters like to prove it by throwing words like molecules and Venturi around. I've seen about a dozen solutions here so far. Why not call a plumbing/heating repairman to fix the problem?Is that valuable enough for you?David Link to comment Share on other sites More sharing options...
nomoss Posted October 31, 2013 Share Posted October 31, 2013 [quote user="dwmcn"] Why not call a plumbing/heating repairman to fix the problem?[/quote]x2 - so long as it's not the one who installed the system I ripped out in our house [:D] Link to comment Share on other sites More sharing options...
Théière Posted October 31, 2013 Share Posted October 31, 2013 [quote user="dwmcn"]Rabbie,I've been using forums long enough to know that everybody is an expert on everything and posters like to prove it by throwing words like molecules and Venturi around. I've seen about a dozen solutions here so far. Why not call a plumbing/heating repairman to fix the problem?Is that valuable enough for you?David [/quote]pachappa, I thought you had been banned from this forum? Link to comment Share on other sites More sharing options...
powerdesal Posted November 1, 2013 Share Posted November 1, 2013 '' I've been using forums long enough to know that everybody is an expert on everything ''I guess that 'everybody' includes you, therefore you also think you are an expert on everything - correct ???????? Link to comment Share on other sites More sharing options...
Théière Posted November 1, 2013 Share Posted November 1, 2013 [quote user="dwmcn"] I've seen about a dozen solutions here so far. David [/quote]Mathematics is't their strong point either as there have only been two solutions so far if you don't count the stupid contribution of "buy a beetle".The other posters have mainly discussed how this may have occured. Link to comment Share on other sites More sharing options...
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