Air in a rad.

[Dave Plowman (News)]

About Easter last year I replaced my boiler. At the same time fitted a ladder type towel rail. And it keeps on collecting air - enough to stop the top two rungs or so getting to full temperature. Bleeding it cures things for a short while. But the pressure remains where it was after I filled the system some 9 months ago. No other rad is effected - although I do sometimes hear what appears to be an air bubble circulating. Fernox inhibitor in use.

How can air get into a system that doesn't appear to be leaking? I've heard of 'gas' forming in a system that's corroding badly - but can this happen with inhibitor?

[D.M. Procida]

We had a new boiler, and the system flushed. 18 months later (in October, when the heating when back on after the summer) I bled several radiators, and then the pressure kept dropping to the point that the system cut out.

This went on for a couple of weeks, during which I hunted for leaks and signs of discharge through the overpressure valve, then stopped.

I can only assume that vast quantities of air had collected in the system, then making their way to the radiators. But it was strange that the pressure drops didn't seem to be directly related to the bleeding.

It could be that for some reason you new radiator is now the place where all the air in the system decides to collect, and that once you have it all out things will return to normal.

Daniele

[ransley]

=A0 London SW

If its to low on water, or a leak air will re enter, does highest rad have air when cold and running. Air can take alot of work to remove on old systems that pipes settle

[The Natural Philosopher]

Yup.

New installations seem to take YEARS to settle.

I bleed mine every year - always get a half radiator of gas, in one of the highest radiators!

[Dave Liquorice]

Air is compressable. On a sealed system the expansion vessel will take up the loss of volume until it runs out of puff.

Most systems have one rad like that, frequently the first one from the boiler. One test Mr Plowman could do would be to try and ignite the "air" when he bleeds the system, corrosion produces Hydrogen... But as it has Fernox in (strong enough concentration?) I doubt it is corrosion.

[BigGirlsBlouse]

I had exactly the same problem except in my case I didnt have a new boiler fitted. It was regularly collecting air (not surprised I suppose with it being the highest point), but I also noticed that I could hear bubbles coming through my lounge rads now and again, especially when the system was hot. My system is a close circulation open vent type not a combi like yours. I thought that maybe this is because I have the pump running flat out to give fastest heat pickup through all radiators, however the expansion pipe for the system is within 6 inches of the pump inlet..and I live in a bingalow, so maybe when its sucking at max its drawing air in through the expansion pipe? Anyway...to cut a long story short... recently the said Myson 1800mm high towel warmer had a leak on the bottom threaded adaptor (failed weld by the look of it) and when I bought a new one it had a 10 year guarantee that isn't there on the original (2000 manufacture and failed after 8 years). When I came to change it over xmas the amount of black sludge in the bottom of the radiator was considerable indicating corrosion despite the system being filled with corrosion inhibitor. Its still a mystery to me as its only happened to this rad and some of my older rads are at least twice that age and show no sign of developing a leak (yet).

[Timothy Murphy]

May I jump in this thread to ask a somewhat similar question?

I am in a house in Italy, and one radiator only gets hot on the left. It's what I would call a standard radiator, and 3 or 4 of the 20 or so divisions get hot - the ones on the left, where the hot water supply comes in at the top, and goes out at the bottom - but the divisions on the right remain cold. I've tried bleeding it on the right, but no air or water comes out.

It is as though the radiator is blocked in the middle.

This radiator is on the top floor. I think it is probably at "the end of the line". The rest of the central heating system works fine.

[John]

.

"Every year"! so for several months the radiator must be inefficient. Why not do it weekly?

[John]

On this type of system I believe it was considered better to have the flow and return at opposite ends to encourage the flow diagonally into all the 'divisions'.

[Cicero]

========================================= If the radiator has a fully removable bleed valve it could be kept fully functional with a minor (and reversible) modification. Remove the bleed valve and replace with an elbow like this:

to this a vertical length of 15mm tube and one of these:

added length of tube should be longer than the height of the cold sections of the towel rail to ensure that water can reach the top of the radiator leaving the air to collect in the tube extension.

I haven't actually tried this but the idea occurred to me as a result of fitting remote TRVs to some of my radiators last week.

Cic.

[Bob Minchin]

Ideally the flow should be from bottom to top not as you have stated and ideally from one corner to the diagonal or bottom to bottom either way round but even so I suspect the system needs balancing essentially too much flow in the other radiators. Make sure they are all bled and set the thermostats or main valves to maximum opening. Measure the input output temperature difference at each radiator and adjust flow with the lockshield valve until you get about 20 centigrade degrees difference on each rad. You should also get a similar differential across the boiler.

Bob

[John]

I would have thought that if going to that amount of trouble, an automatic air bleed valve (of the sort fitted to boilers ) would be more worthwhile. They operate with a float.

However, why not just leave the bleed key on the vent and give it a bleed very few days. The only 'chore' of bleeding is usually finding the key!

[Cicero]

========================================== I agree that it might need a bit of effort but I was thinking in terms of monitoring the situation since there's some doubt about whether the problem is air or gas. It might also be possible to get a fairly accurate measurement of how much air/gas is being introduced/ produced because the single 15mm vertical pipe could be checked to see how much is cold.

It really depends on whether you like a bit of tinkering with things.

Cic.

[The Natural Philosopher]

I only run the rads in winter.

Off all summer, and only occasionally used in winter.. The UFH is main heating: thats low down under floor. No air stays there..;-)