On 2017-03-08, Adam Funk wrote:

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.)

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.