...then it comes out the way BOx and Jake have described; you open the little bleeder valve(*) and let the bubles out until you get pure water -- not necessarily "pure" in the sense of being particularly clean, but in the sense of being a constant stream, which shows that it's *just* (more or less dirty) water. As opposed to sporadic sploshings, a "stuttering" stream, which tells you it's interspersed with air.

Not to worry, though: Usually, you *don't have* any air trapped in the system to begin with -- why would you? It would have to *get* in there somehow; there would either have to be a leak somewhere, where water drips out and is replaced by air that slips in (to take up the space vacated by that water; "Nature abhors a vacuum", and all that), or you'd have to have opened up the system yourself, like if you exchanged a radiator for a newer one or something.

But, barring any major plumbing jobs on your part, some joint in the piping would have to be leaky -- and since most of the joints in this system are where the pipes connect to the radiators, they're usually handily near the radiators' bleeding valves. And leaking water doesn't vanish without a trace, but usually leaves tell-tale signs: the linoleum, parquet, or whatever other flooring material you have, will probably show stains. Look for discolorations or buckling below the bleeding valves and heat-regulation "taps" (which is usually around where the pipes go down through the floor, if you've got vertical ones) -- even on tiles or glazed concrete, you'd probably get calcium deposits, if the leaking's been going on for a while.

Also, air in the system is often recognizable by the sound it makes; an audible bubbling, gurgling, irregular kind of thing. Usually, you should have to press your ear against a radiator in order to be able to hear the gentle, *constant*, swishing sound of the water itself streaming through the system. (Sometimes, you can even hear the sound of the pump working, down by the boiler, in a radiator on the top floor. If you listen *really* carefully (or have a noisy old pump).)

The third sign you might have air in the system is if a radiator isn't evenly warm. When you fire up the furnace to max heat (normally, a thermostat down by the pump should do this for you as you open more radiators' "heat taps"), if you then notice that one or more regulators don't feel equally hot *all the way up*, then that might be because they're filled with air, not water, up at their tops. Air leads heat much less efficiently than water does, so if there's air behind the metal, less heat will be conducted to your hands than if there were water. (You might have to wait a while to be sure -- the metal itself does conduct, and if it's thick also stores, some heat.)

All in all, this may sound like a tremendous lot of pitfalls and potential problems -- but relax: It's really not all that bad; trust me. All you have to do is, some day before temperatures go below freezing (but it's all to the good if there's a definite nip in the air -- so, late September, early October, where you're at?) check out your heating. You do this by:

1) Make sure that the system's water reservoir (down by the pump, next to the boiler) is full or nearly full with water -- you don't want to *create* any air bubles, by making the system start to suck air.

2) Open the taps on all radiators to full heat. As you do this, examine the pipes and radiators around the joints where the pipes enter the radiators, and the floor below, for water stains and residual encrustations. Oh, and for actual wet water, of course!

3) Wait a while -- not as long as it takes to go to work and come back, but put them on before you do your weekend grocery shopping or whatever, and continue this check-list when you get back a few hours later.

4) When you come home again, the house might be unbearably hot -- this is why it's useful to do this on a cool day; just open some windows! :-) Now walk around and look at each radiator: Does it gurgle? Does it feel evenly hot all the way up? Do you see any water dripping from the pipe joints or the "tap" thingy you turned to max before you left?

If the answers, are no, yes, and no, respectively, for each and every radiator, then you have no worries. If you *want to*, you can of course bleed each one a little, just to see that there really is air-free water running through; but if the answers to the three questions above were the "correct" ones, then in all likelihood that *is* exactly what you'll see.

Ironically, though, the taps themselves are sometimes *conditionally* leaky: Turn them to a position they're not "used to", and they start to dribble a little! The irony is, the most unusual position is often maximum heat, because the system tends to be a tad over-dimensioned. Therefore, you might see some water dribbling from the tap itself -- *just because you're testing*! But if the other indicators I mentioned would suggest no leakage, then you're probably OK anyway... Turn it down to somewhere a little below max (you'll probably feel a range from fully closed to somewhere well below fully open where the tap moves easier), and see if the dribbling stops; it probably will.

This is why you made sure the system had water, checked for stains and water before leaving, and didn't stay away for the whole day: To avoid the *utter* irony of *introducing* air bubbles, or finding water leaks, from taps "unused" to being max open -- due to your very testing for air bubbles, you see?

But really, I expect *if* you run a thorough test like this, all you'll find is that the system works perfectly (apart from one or the other dribbling tap); they usually do.

So, relax.

---

(*): In the house where I grew up, in Sweden, the socket was a square hole, and the "key" or special wrench was a stub with a square cros section; dimension about 3/16 to 5/16 inch (? Around 5 mm, as I recall).