I installed them at the top of a vertical riser pipe, so that they would be out of the flow stream and so air could settle out.

I think that's a key point.

An air-release valve should not only be at a high-point in the pipework, but also positioned clear from any flow.
You want the air to rise up through a bit of pipe before it meets the valve.

I have automatic air-release valves at the far ends of my UFH manifolds.

In my experience they do still purge air into the atmosphere with the cap still in place, provided it isn't screwed down hard.
The plastic cap isn't meant to be 'air-tight'.
Air escaping under pressure is still possible.

Yes, the automatic type can potentially jam.
I've had that happen one in the last 20-years.
And I now keep a spare on-the-shelf.

FWIW I'm in a soft-water area, with no limescale issues.

@editor 

Are you saying that Brendon delivered a bodge up? Is that the "recent bodged fix" you refer to, or am I misreading this?

So even if you do fit autovents you are not immune to air collecting in the system.

My view remains that, for pressurised heating systems, anything that permits pressure to escape automatically is not a good idea. I'm welcome to suggestions from this forum that my view is incorrect and being challenged on that one. We're all learning!

PS: If you're going to use an automatic air vent manually, and close it off, you might as well fit .... you guessed it .... a manual air vent! ;)

Top tip to fit an isolation valve!

All good points. I guess that at least with an automatic air vent valve you have the option to operate it automatically or manually, should you so wish. For most people, who may not even know what it is, it should just work and help keep air in the system to a minimum, and for those like me that worry they may start to leak over time, I can close them off and not worry. So from an installer's perspective, they are a no-brainer (other than the £10 of additional cost). I'm perplexed why @editor's were removed.

My view remains that, for pressurised heating systems, anything that permits pressure to escape automatically is not a good idea. I'm welcome to suggestions from this forum that my view is incorrect and being challenged on that one. We're all learning!

PS: If you're going to use an automatic air vent manually, and close it off, you might as well fit .... you guessed it .... a manual air vent!

I confess to the same misgivings!  My installer fitted one so I have one, but would I fit one again, possibly not, or if I do as you say fit an isolation valve immediately prior!

My view remains that, for pressurised heating systems, anything that permits pressure to escape automatically is not a good idea. I'm welcome to suggestions from this forum that my view is incorrect and being challenged on that one. We're all learning!

PS: If you're going to use an automatic air vent manually, and close it off, you might as well fit .... you guessed it .... a manual air vent! ;)

I'm wary of automatic air vents on pressurised sealed systems, as over time their operation slowly reduces the system pressure. If the heat emitter has a pressure sensor and minimum static pressure requirement, that can then cause a fault.

A manual vent next to the filling loop and system pressure gauge is ideal, fitted to a part of the circuit where air can accumulate to be released. That way, the system pressure can be topped up after manual venting. Automatic air vents should only be used with open heating circuit systems in my view.

When I had my system serviced recently, I asked the guy about the automatic air vents as I didn't really understand how they worked.

He advised once the system has been bled, to keep them closed by tightening the little screw cap and to manually open occasionally to manually bleed air from the system. He said the automatic valves can sometimes fail and then obviously you'll get a leak from the pressurised system if they are left open, so I just manually open mine occasionally now to check for air.

I remember talking to the installer when they were filling the system and them explaining that the automatic air vent on the top of the volumiser would just let all the air out (~50L worth) as the volumiser slowly filled with water from the bottom and then automatically seal once full. I think this automatic air vent may still be 'open'.

Does anyone know, if there is trapped air, do you hear a hissing sound as it's released with these automatic air vents, like the old fashioned ones or when bleeding a radiator, or are they silent?

I'm wary of automatic air vents on pressurised sealed systems, as over time their operation slowly reduces the system pressure. If the heat emitter has a pressure sensor and minimum static pressure requirement, that can then cause a fault.

A manual vent next to the filling loop and system pressure gauge is ideal, fitted to a part of the circuit where air can accumulate to be released. That way, the system pressure can be topped up after manual venting. Automatic air vents should only be used with open heating circuit systems in my view.

I have one installed when my heat pump was done, connected to the cylinder feed (or it may be the cylinder return).  The cylinder is on the first floor of my two storey house so this is pretty much the high point.  The installer did this without being asked, so its obviously standard procedure for him.

The heat pump has a 'purge building circuit' function which the installer ran for 10-20mins when he had finished the plumbing, to release most of the trapped air.  I think that this function just runs the water pump at maximum but Im not absolutely sure, it may play games also with the diverter valve to ensure that the whole system is swept.

Nevertheless there was one tall radiator which needed manual bleeding every day for about a month as dissolved air, which could not have been purged by the installer, came out of solution.  Im guessing most of the dissolved air ended up being vented by the air release valve, but the local 'high' represented by the radiator remained a trap for small bubbles which the flow through the radiator didn't sweep away.

There was a de-aerator on a public splash pool I was once responsible for, which certainly worked, but not on my heating system.