Why Your Boiler is Losing Pressure and How to Fix It

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

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.

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

Great video! A couple of handy hints relating to pressure loss. First, a plastic bag tied over the end of the pipe from the pressure relief valve is an easy way of determining if water is escaping. The pipes are often angled back towards the wall to prevent scalding, so putting a tray underneath isn't always possible. And second, if the internal expansion vessel is at fault, rather than have the boiler dismantled and a branded replacement vessel put in, simply ignore the faulty one and have a cheap generic (Screwfix or whatever) external vessel added elsewhere in the system at a convenient point. Much cheaper and far easier to subsequently replace if it fails again.

I came home Saturday morning to discover my boiler off and displaying a low pressure fault code. True enough the gauge was almost at zero. Ten minutes doing a bit of refilling via the loop and bam, I'm up and running again. It's only happened twice in four years and then only when we have a sudden cold snap - I'm calling that 'normal'. Great vid sir.

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.

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!

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!

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

Thanks Stuart for some well explained and useful information. And thanks for the heads up last week on the Screwfix/DeWalt drill deal. I'd been after a couple of 5ah batteries for a while. Screwfix sell them for £80 each (I know that I can get them a little cheaper elsewhere) therefore two would be £160. So for £10 less not only did I get the two batteries, I also got another charger, a case and a combi-drill. Result!👍😁 Thanks again.

Very nice video as always! Interestingly my boiler lost a lot of water last year (had to fill it up every two weeks or so), but it seems completely fine now! Over the summer, the boiler has been on "summer mode" i.e. running only the water heater, without any loss of water. And now that I'veturned it back into winter mode for the central heating... everything is fine! The problem seems to somehow have cured itself over the summer. Very confusing.

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.

Brilliant and the timing could not be better, thank you

Thank you proper DIY. Very informative and just what I was looking for my sisters CH system loosing pressure.

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

These videos always leave vital snippets of info out.

I don't have a boiler, and doubt I ever will... but I still find it fascinating to watch you explain this (and many other subjects I don't have a need to know about). You are a great teacher and entertainer!

Great video. Thank you for the tips. This was me earlier this year. But I had a fualt in a weld on a part of the boiler, then water came gushing through my ceiling.

Roger B from the skill builder channel suggested a great way of finding a seeping leak. Put after shave in the system. Where the clean smell is the dirty problem is hiding