I keep finding air in the tops of the same two radiators. This made sense in the old vented system, because air could dissolve into the water at the cold surface in the cistern, then come out of solution later in the hot parts. But we have a sealed system (with a combination boiler) now.
I'd also expect to get a bit of dissolved air (to come out later) in the mains water when I add more to the system. But usually I bleed the two radiators, check the pressure gauge on the boiler, & find I don't need to add more water, but still find some air a week or two later. Where is it coming from in that case? Just more air from the previous top-up coming out a while later?
I've assumed it gets drawn in from a tiny leak somewhere when the system cools down. Locally, there are going to be bits under negative pressure, even when the system as a whole is slightly pressurised.
The whole system is under pressure either from gravity or from a filling loop. There is not really a "suck" side to the pump - it is a "circulator rather than a positive displacement pump
Water is strange stuff when its hot it expands and when its cold it expands. However there is always air in it, just more air in some bits and less in others. So I think the answer is that being lighter than water it takes some time to get to a high point in the system where it gets trapped. Brian
The plumbers (who I think are very good) drained & power-flushed the whole system in spring 2014 when they installed the new boiler, & added corrosion inhibitor. They may have added inhibitor in spring
2015 when they serviced the boiler, & definitely added it when they moved & replaced a radiator & serviced the boiler in spring 2016.
With open vented with even a little bit of pump over it will. Sealed system is trickier to get air in but the chances are it's not air but hydrogen released when the iron grabs the oxygen from a water molecule.
Most probably hydrogen from electolytic/bi-metallic corrosion. That produ ces H2 & O2. The oxygen combines with the inside surface of your radiators to form magnetite sludge. Igniting it at the air vent is most unlikely to c ause an explosion (I've never heard of that happening), but the recommended procedure is to bleed the gas into an upturned glass/bottle (H2 floats upw ards) and then ignite that. You may otherwise have difficulty in closing th e air vent after it has turned into a blow-torch (see Youtube).
If hydrogen, the gas in the upturned glass will light with a pop and burn upwards.
The important thing is that you'll usually only get the electrolytic/bi-m etallic corrosion if the water is acidic. The acid usually gets in to the s ystem as the residues from active flux (contains ammonium chloride, forms H Cl on heating ISTR) that hasn't been thoroughly flushed out.
The French braze copper heating pipes (no flux needed); they have virtuall y no black sludge problems, there is no market for power-flushing in France . Go figure.
It's been power-flushed. They will have used an acidic cleaner (HCl); tha t has to be flushed, neutralized (with caustic soda), flushed, checked with litmus paper (did you see that done?) and flushed again for good measure.
It might be air (nitrogen); that can get drawn into the system on the suc tion side of the pump, as mentioned elsewhere. The oxygen forms magnetite, as above. The expansion vessel connection (neutral point, PONPC) should be on the pump inlet side.
Most good domestic inhibitors (Sentinel, Fernox, etc) contain sodium mol ybdate; ISTR that the mixture is slightly acidic. I like to run a flushed s ystem with clean water for a few hours, check it's not acidic, then add inh ibitor.
The system contains pressurised water, but partial pressures of oxygen and nitrogen in the water will be well below air pressure when the system cools, so it will suck these gases in through any small leaks, to dissolve in the cold water. When the water is heated, the solubility of oxygen and nitrogen reduces and they'll come out as gas bubbles and collect at high points. When temperature drops again, the partial pressure of oxygen and nitrogen in the water will drop, and it will suck in more to dissolve. It may seem strange that air is passing through a leak in the opposite direction to the water, but that's what happens.
In a heating system, the water leak may well not be noticable, as the heat may dry the water as fast as it's leaking. To find it, wait until you aren't using the heating and it stays cold, and then feel around the pipework and radiator plugs for leaks. If it's a sealed system, you might also increase the pressure to the normal hot running pressure, to speed up any leak. (Remember to let it back down to normal afterwards, and don't blow off the excess via the over- pressure valve, as you might end up with dirt on the seat, which makes it leak. Dried crystals on pipework/radiators can also be a sign of a leak.
It does ignite. (Someone else mentioned hydrogen sulphide, but there's no eggy smell. Once the gas is all out & water starts to squirt, there's a non-eggy "chemical" smell, which I assume is the inhibitor.)
How much do I need to worry about this, & what should I ask the plumbers to do? (The boiler is due for an annual service in 2 months anyway.)
The power-flushing was several years ago. The air (not really air, as we've established) seems to more common now than it was.
If it's acidic water that causes/allows the problem, why are good inhibitors slightly acidic?
BTW, I just drew a small sample from the drain at the bottom of the heating system: the water looked clean, had the same non-eggy chemical smell, & had a pH of 7 to 8.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.