On 2017-02-25, Onetap wrote:

Most probably hydrogen from electolytic/bi-metallic corrosion. That
produces 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 cause 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 upwards) and then
ignite that. You may otherwise have difficulty in closing the 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.

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 important thing is that you'll usually only get the
electrolytic/bi-metallic corrosion if the water is acidic. The
acid usually gets in to the system as the residues from active
flux (contains ammonium chloride, forms HCl on heating ISTR) that
hasn't been thoroughly flushed out.

The French braze copper heating pipes (no flux needed); they have
virtually 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); that has to be flushed, neutralized (with caustic soda),
flushed, checked with litmus paper (did you see that done?) and
flushed again for good measure.

The power-flushing was several years ago. The air (not really air, as
we've established) seems to more common now than it was.

It might be air (nitrogen); that can get drawn into the system on
the suction 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
molybdate; ISTR that the mixture is slightly acidic. I like to
run a flushed system with clean water for a few hours, check it's
not acidic, then add inhibitor.

If it's acidic water that causes/allows the problem, why are good
inhibitors slightly acidic?