This will be air in the system. You probably already know how to remove air. You might not realise it yet though. Here’s how I would try to explain it to a lay person. 
Imagine that your heating system is a bottle of Coca Cola. It looks innocent enough when it is in the bottle but you just know that it is full of gas. How would you remove it all?
1) Replace the air with liquid. (fill the bottle with coke)
The gas at the top of a bottle is easy to remove. You can open the lid with the bottle standing upright, pour coke inside the bottle, and any trapped gas will escape out of the top.
For a heating system you fill up the system, you open the manual bleed valves / automatic air vents, then you close them when you see liquid coming out.
You have now pushed out most of the “trapped air” (pockets of air that rise to the top of the system) by doing this.
Keep the fill loop connected but switched off. We will need it later for refilling the system and don’t want to introduce extra air when we do.
2) Close the lid, shake it up, then open the lid again. (coke becomes Guinness becomes coke)
If you shake up a coke bottle all of the “trapped gas” becomes “entrained gas” - small micro bubbles that move along with the liquid - even if you are trying to pour that liquid downhill. (watch a pint of Guinness settle)
If you let the coke bottle sit then “entrained gas” will slowly rise to the top again and become “trapped gas” again.
We can use this trick to “move” any “trapped air” that was in places we couldn’t let it out into places that we can let it out.
Bring the system fill pressure up as high as you dare. (e.g. 2.5 bar) This will squash these air pockets into as small a space as possible.
With the system still cold flick the circulator pump on and off (maximum or zero speed) to move stir this air up then let it settle out in the high spots. This is trying to drag the air from trapped places (e.g. a pipe in the attic, where the flow moves quickly like a Mentos dropped into a bottle of coke) to somewhere that you can vent it out (e.g. a radiator, where the flow moves slowly like a pint of Guinness on the bar).
All the radiator valves should be fully open* to maximise the flowrate, and the diverter valve should be switched from heating to hot water and back for this process. Many heat pumps have a “purge cycle” or “venting cycle” that will do this pump on/off and diverter to heating / hot water for you automatically.
Keep running around all our high spots venting any trapped air until the cycle is over.
*If you know the layout of the system you can try to close valves and flush the air out of it “one part at a time” - this will increase the speed of the water in the pipes - but shouldn’t be needed unless you are working with “ceiling drops” or similarly awful pipe layouts that really don’t want to move the air downhill.
3) Boil it off (the cook off)
Even though you have removed the “accessible / trapped” air, and you have also moved the “inaccessible / trapped” air to somewhere that it becomes “accessible / trapped” air so that you can remove it, there is STILL going to be some air in the system.
That glass of coke, bubbles slowly rising to the top, for a very long time.
You can make the process faster by (1) REDUCING the pressure (liquids under lower pressure cannot dissolve as much gas - bubbles only come out of the coke when you release the pressure by opening the lid) and (2) HEATING it up (hot liquids cannot dissolve as much gas as cold liquids - this is why everybody sees the meters warning you about air in the system during last part of the hot water cycle)
You need to reduce the system pressure as far as you can (this varies depending on the height of your building, but something like 0.7 bar should be safe) and then “cook off” all the dissolved gas by running the system as hot as it will possibly go (for an hour or so) and then release this form all the usual places.
Top up the system again when you are done. Now (and only now) remove the fill loop.
4) Microbubble deaeration (optionally - add mentos to the coke)
There might still be some gas dissovled in the liquid. If it comes out anywhere, it will be where the temperature is the highest and the pressure is the lowest. You can help it come out by creating places for the bubbles to “nucleate”, collect into a pocket of “trapped gas”, and then release it.
If this were a bottle of coke you’d throw in a mentos. Here’s a Mythbusters link to break up all this text:
Glycol was pure water is a little like diet coke vs coke. The glycol is gloopier (for google translate users: stickier) and more likely to retain the microbubbles than the regular coke.
If this is a heating system you can add a microbubble deaerator. (sprivovent etc) This should be placed at the hottest, lowest pressure, point in the system.
This is probably one for the plumber rather than the eight year old! They might not be keen on leaving the cap off the deaerator in case it dribbles / leaks later. Best to leave the cap open when setting up the system, and to open it periodically, but otherwise leave it shut.
5) Check water quality
The air NEEDS to be out to avoid mixing with iron and forming rust sludge etc; not just for the heat meter to work correctly.
The water quality also NEEDS to be decent to avoid fouling/corroding your expensive new installation that is expected to last at least a decade.
Professional installers will check water quality; particularly after the “cook off” when the water might have picked up not just dirt but also any salts / nasties that might have been in an old system.
Professional heating system checks (e.g. if you are paying for an annual service package) should also include water quality checks.
6) Check the homework
If you install your OpenEnergyMonitor heat meter / monitor BEFORE the system is commissioned, then you will be able to see exactly when the installer did step 1 (first filled it), step 2 (ran the purge / venting cycle), and step 3 (ran it as hot as it would go for an hour)
I would DEFINITELY recommend this when working with a “high street” installer where the site teams are under time pressure from management and cutting corners won’t be noticed until years down the line. At this point it is too late to conclusively say whether or not it was the initial installation at fault and it will be down to you to foot the bill.
Credits
Thanks to @glyn.hudson and Adam Chapman for the conversation about getting air out of systems last week.
I said I’d have a go at explaining it if they didn’t have time for a video. I’m kinda hoping for something ludicrous involving clear pipes, radiators, and coke…
In the meantime here’s the slightly meaner version of mentos in coke along with some youtube science.
