I’m a heating engineer and I often watch videos like this behind my hands like watching a horror movie 🫣 but I’ve got to say, fair play my mate, your explanation was spot on, to the point and made a lot of sense 👍🏻 although opening your PRV to show it coming out outside was a risky move, they have a habit of never fully closing once being opened.

Also another reason for pressure drop can be people (DIYers) not understanding the difference between single and double barrier  push fit pipe. I’ve seen it before where single barrier pipe has been used and what this does, is allow the oxygen and hydrogen in the water escape through microscopic holes in the pipe wall, without any signs at all.

Great video pal

Excellent advice, and I'm the son of a retired Plumber and Gas Engineer

Great video Stuart. This will possibly save some folk a bit of cash.
Love the references to various laws:
Charles's Law: When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be in direct proportion
Boyle's Law: The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.
Coles Law: Thinly sliced cabbage

I've had exactly this problem in the bungalow that we converted for my disabled brother. It had a new heating system and new combi boiler installed, and all was well for a year or so, then it kept losing pressure when the central heating was used. It went from needing re-pressurising every few months to every couple of days. The problems were essentially all down to the installation. Most importantly, the plumber had not installed automatic air vents in the system. Being a bungalow with a solid floor, the plumbing went through the loft, and was one set of inverted loops. Any gas would accumulate in the pipes at the top, and gas expands more than water when heated, that pushed the pressure over 3 bar when the central heating worked hard, pushed coolant out the pressure relief valve and the problem got worse. This was made worse by insufficient inhibitor. Further, the system contains a lot of water, and the expansion vessels in modern combis are relatively small and the pressure goes up markedly on big systems when the central heating works hard.

So, having had a fruitless chase for leaks, I ended up installing a couple of automatic vent valves in high points on the system, a supplementary expansion vessel to give a bit more stability to the pressure when the system is heated up, installed a magnetic filter, which is also a good place to put in additives on a combi system, and dosed it well with inhibitor.

Now it seems to be very stable, and the pressure fluctuates less than half a bar between when it's cold and when the central heating is up to full temperature.

Now in my spare time I'm working on a little project to use a microprocessor and pressure sensor which I can monitor remotely over the Internet with an email warning system if pressure gets low. I'd rather have a log of what the system is doing and deal with it at the right time than an emergency call when the heating has broken and a 70 mile round trip for a 2 minute job. Nobody seems to sell such a system, and I would have thought it of great benefit to anybody who has something like holiday properties that they rent out. Perhaps there are some "smart" boilers that can do this sort of stuff.

Another lesson is don't assume plumbers and heating engineers always do their jobs properly.

nb. what is coming out of your boiler vent is most certainly not smoke (if it is, then it is time to call the fire brigade). What you are seeing is condensing water vapour, something entirely different and what what clouds are made of. To maximise boiler efficiency, then you actually want as little of that to appear as possible; you want the condensing to happen inside the boiler as that is where the maximum heat is extracted. Condensing boilers are at their most efficient with a hot water temperature of about 55 Celsius, so to save a bit of gas, try turning down your boiler temperature if your radiators are large enough. If you install new radiators, make then big ones as the heat pumps we are told we will have to use in the future work at around 50C, not the 80C that many boilers are set to.

I would also add, if you are checking for water coming out of the pressure relief valve, don't mistake that for the condensation outlet pipe. The latter should always be producing water when the boiler is working, although it is generally in plastic, not copper.

Thanks for this video, my boiler has been losing pressure for a while now, after I watched your vid, I suspected it was the expansion vessel. Had to get a plumber out to recharge it, but was good to have a bit of information prior to the plumber coming out. He was nice enough to show me exactly what had gone wrong and explained it pretty much like you have in your video. thanks again!

Lots of useful comments below.  Its like our local plumber's merchant (dedicated store),  if you have a problem, chew it over with the staff & whoever's in the shop, and someone will have an answer.  Nice vid, thanks.

I learnt some of this from my plumber recently. I was having to top up regularly and noticed when I did, the pressure would rise very high when it switched on. It then started to leak from inside the boiler (not to a drain), so I was just getting water coming out of the bottom of the boiler.

It turned out this was the pressure release valve. I'm my case, the membrane hadn't failed, but the vessel was still being fully taken up by water (I guess the membrane in my boiler is very flexible). The plumber pumped up the air pressure behind the membrane and set the water pressure to 1bar. He said if there was an air leak on the air side of the membrane it would happen again - then he can take another more costly look (in the case it could be a damaged membrane or a faulty valve), but I think in this case the air just very slowly escaped from the air side over time, as it has held the same 1bar pressure for 3 months now.

Part of the issue was possibly due to when I was topping it up over the last 10 years, I was putting more pressure in than I should have been, which would have put more pressure on the membrane long term. This video helped me more fully understand what was going on - so thanks for that.

You saved my life this is the first time I've been in this situation living without a man and I did it myself only with your help thankyou you saved my life now I have hot water xxxxxxx

Nice well explained video.  Just to say though that the 'plume' that can be seen coming out of the flue of a gas boiler, especially in cold weather, is normally just steam. If for any reason it appears to be (and might smell) like actual smoke or something burning then switch the boiler off immediately and call a heating engineer.

Love your videos . I’m a plumbing enthusiast and love to know how the main components I depend on work in my house . Thank you , you are one of my favourite YouTuber with detailed information even a layman like me can understand.

I want to thank you for this beautifully clear explication of one of the most complex mysteries in any homeowner’s life: that marvel of the modern world,the condenser boiler. I now almost think I understand it, and certainly there is nothing in the manual itself to help in that department. You are a great asset to homeowners everywhere - and I would happily stand you a pint. Cheers mate!

Thank god i still have my bog stnd boiler fitted in 1968 and still works 100%.
These modern boilers ftighten me.
Mine has only 3 parts and basically no electrics.

Sometimes with the slow release of water problem, just stick a balloon over the prv outlet outside, this works specially if the pressure s over say a week or two and will pick up small drips as opposed to a tray.

One tip you missed is check for a leaky schrader air valve on the PV - Have encountered just such a case where the insert refused to seat properly, was jammed solid and so could not be extracted and replaced - The solution was a cheap valve extender with it's own schrader valve (usually sold for bikes etc) screwed over the original, still working a treat 5 years later...

I am going to wait a bit longer,maybe you are wrong and it fixes itself 😂

Condenser boilers don't emit smoke, it's CO2 and water vapour.

Coming from someone like yourself I found this video really easy to understand because I was thinking the same way as you & not like the expert who we can't do without but doesn't like to give us DIY people to much information so well done again on your explanation to things around the home.

So helpful--I'm new to the UK and I have a new combi boiler which I didn't understand at all. Now I feel more comfortable with it and a lot smarter! Thank you.

Just replaced our old expansion vessel. Been leaking for too long. Finally got round to it. Great video!

Years ago, I had a Potterton boiler, great & economical when it worked, but the most unreliable piece of equipment ever (and after sales service non existent).  Always slowly losing pressure, & after YEARS of trying everything, I found that the body of the alloy heat exchanger was seeping water when it got hot.  As it slowly leaked, being hot, the water evaporated, hence I could NOT find the leak.  In desperation, I fired it up with the covers off (!!), and sure enough, after 30 seconds or so, I could see it weeping.  Vaillant from now on.