Boiler Losing Pressure? Here’s Why and What You Need to Know

Is your boiler losing pressure, and you’re stuck looking for answers?

The good news is that only one fault usually causes low boiler pressure. The bad news is that adding water without fixing the issue won’t solve anything. At any rate — you’re in the right place, as today we explain how to fix low boiler pressure and restore the correct pressure reading.

Boiler losing pressure

If your boiler pressure keeps dropping, there’s a good chance you’ve been told to use the filling loop to “top up the boiler”. 

My advice would be: don’t.

Hopefully, the installer dosed your new boiler system with inhibitor and fitted it with a scale reducer (in hard water areas) and a magnetic system filter.

An inhibitor breaks down any bad particles in the system, allowing the scale reducer to catch them. Every time you top up the boiler with cold water from the mains using the filling loop, you are diluting the inhibitor that’s in the system.

Here are some of the reasons that result in a gas boiler losing pressure. And of course, if you need a boiler repair specialist to visit you home or business, get in touch.

We’ll jump through the possible reasons your boiler keeps losing pressure, but first, let’s look at the DIY fix…

The [DIY] Fix If Your Boiler Pressure Keeps Dropping

Boiler Losing Pressure

99% of the time, modern combi boilers will keep losing pressure due to leaks, whether in the boiler itself or elsewhere in the heating system.

Before spending 100s of pounds on a repair engineer, there is a DIY fix that may sort out your boiler’s pressure — it comes in the form of the Fernox F4 leak sealant.

This sealant travels around your hot water system (including the boiler) and works its way into small gaps (i.e. the area that’s leaking).

Fernox is a well known brand in the boiler protection/repair space, and the F4 leak sealant will get to work within 1-24 hours, hopefully stopping your boiler pressure dropping.

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Boiler Losing Pressure, No Leak

In 99% of cases, a boiler losing pressure means a leak — it’s as simple as that. Unless the dial sits at 0 even when topped up (which could be a sign of a faulty pressure gauge), there’s a leak somewhere – even if you can’t see it in the boiler or the heating system.

Whether you have an oil, system, or combi boiler, pressure loss is usually directly related to a leak in boilers. We’ve already created a guide on why boilers leak and how to find a boiler leak here.

Causes Behind Your Boiler Losing Pressure

#1 – Faulty Pressure Relief Valve on the Boiler

Most modern boilers operate at 1.0-1.5bar. But their pressure can approach 3bar if there is a problem, like a boiler leak. A pressure release valve notices this abnormal change in system pressure and releases it. If the PRV has failed, it will cause the boiler to lose pressure.

#2 – Auto Air Vent Leak in Heating System

Central heating systems can collect air pockets over time (and they certainly do when you refill them).

Bleeding the radiators in your home or office is the best solution, but the auto air vent (which is normally inside the boiler casing on newer combis) can also help to combat this problem that ends up affecting boilers.

However, if the auto air vent is faulty, it could be losing pressure via a leak.

#3 – Leaking Radiators, Towel Rails, and Radiator Valves

This is the most common cause behind a combi boiler losing pressure. The smallest of leaks can lead your system to low pressure.

Our advice is to do a visual check of all radiators, radiator valves, and towel rails. If there is any sign of water escaping, then this is the likely culprit. A small weep on a radiator valve is enough to cause pressure loss on your boiler.

If the connections are extremely loose, tightening them up (carefully) will usually solve the boiler pressure issue. If it doesn’t, the culprit will need to be repaired — or replaced.

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#4 – Radiators Are Full of Air

If you’ve had work done to your central heating system, it may have collected some air in the radiators and towel rails. You can release this system pressure by using a bleed key.

To bleed radiators and towel rails, slightly open the vent with the key. You’ll hear trapped air hissing out. Once the air stops, water will start to drip (or spray out, if you’ve opened the vent too much); this is your signal to close the vent off.

Repeat this procedure on every single radiator and towel rail on your property. It’s worth taking a small container to collect dripping water and a towel to protect carpets.

Also, remember if you are constantly removing air, the boiler pressure drops each time you bleed the radiators.

#5 – Dodgy Expansion Vessel

Every now and then, expansion vessels will need to be repressurised. If they’re not, they can affect the pressure of the boiler in your home. In some cases, the boiler losing pressure could be for this very reason.

A more likely cause, though, is that the Schraeder valve is leaking. This is the valve on the vessel that will look the same as your car inner tube valve. A faulty valve can cause pressure loss in a boiler.

Another possibility is that the diaphragm on the vessel has degraded over time, and the boiler pressure is being lost there.

If repressurising the expansion vessel doesn’t work, or the suspect is a diaphragm or Schraeder valve, you’ll need to replace the vessel altogether to stop the boiler losing pressure.

#6 – Soldered Joints on the Boiler Have a Leak

As we’ve already mentioned, leaks are the most common cause of a boiler losing pressure. If your system is particularly old, there’s a good chance soldered joints have become weak, which can cause a new leak. You only need a tiny leak for a system to start losing pressure.

If you can find the leak, call a Gas Safe engineer to come and re-solder the joint to stop the leak. Use the filling loop to top up the boiler, and you should see the pressure rise back to where it needs be; then, monitor the pressure gauge for any pressure loss.

#7 – The Pressure Gauge on Your Boiler is Faulty

It’s unlikely, but it happens — there’s a chance the gauge on the front of the boiler is not reading the pressure correctly. This condition is quite dangerous, especially if the incorrect reading leads you to top up your system all the time.

If you top up the boiler and the pressure gauge doesn’t respond, chances are it’s busted — otherwise you’d certainly notice a leak of that magnitude.

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What Your Boiler Losing Pressure Means — And What You Need to Do to Get It Fixed

Boiler Constantly Losing Pressure When the Heating Is On

When you switch on your heating, pipes, fittings, and radiators expand, and boiler pressure increases. So, a heating system might not leak when it’s off, but may start to do so when you switch it on.

Boilers Losing Pressure Slowly/Quickly

The speed at which your boiler loses pressure determines how bad the new leak is. So, if your boiler is losing pressure slowly (i.e. you can’t visibly see the dial going down, but pressure goes down overnight), the leak is small.

If you can visibly see the pressure gauge going down, your boiler is losing pressure quickly, and the leak is big. You’ll need to get the boiler fixed as soon as possible, as it could be causing water damage in your home or business.

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Is Low Boiler Pressure Dangerous?

It depends on the cause of the boiler pressure loss. Chances are that the root problem is a leak, and leaking water can damage electronics, your home’s structure, and your possessions.

If the pressure loss you’re seeing is caused by a faulty pressure gauge, then it’s equally dangerous because then you don’t know what the true boiler pressure is.

Boiler Losing Pressure Even AFTER Repairs

If you’ve addressed the issues above but you’re seeing low boiler pressure anyway, and the appliance is not under warranty, you’ll need to consider replacing your boiler altogether.

And whilst installing a new boiler is by no means cheap, it may cost you less than replacing one faulty part after another to no avail (or worse, getting a persistent leak that damages your property).

To get an idea of how much replacing your faulty appliance may cost, you can read our in-depth boiler replacement cost guide.

On average, however, you can expect to pay at least £2,000, including installation. Keep in mind, though, that this figure hinges on a tonne of variables, such as:

  • Your existing and new boiler types — Are you swapping a combi for a combi? Or do you have an older, conventional boiler, but would like a combi instead? In the latter case, you may end up paying more.
  • Whether you must move your boiler to a new location — Relocating your boiler will incur additional expenses.
  • Where you live — If you’re a Londoner, you’ll pay more, sorry.
  • The list goes on.

To get an accurate quote, you can ring up a local installer and provide them this information. You can also get a fixed price on your screen now by filling in Heatable’s anonymous, 90-second questionnaire (10-year warranty included).

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What’s Next If You Experience Boiler Pressure Loss?

Struggling to understand why your boiler is losing pressure? Or, need some further advice on how to fix your pressure dropping on your boiler every day?

Leave a note in the comments about your boiler and we’ll get back to you.

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  1. Hello there
    I’m wondering whether a leaking combi-care inhibitor could be the leak that’s causing pressure to be lost? Is that even possible? Every time I top up the valves, it stays at 1’5 for a bit, six hours later it’s at 1 and a day after that, it’ll be back at 0. The combi boiler people have been here twice now – they say it’s nothing to do with the boiler (under warranty), but that scale inhibitor is leaking more and more. What does it sound like to you?
    Thanks for your help!

    1. Hey Michaela,

      I assume they are talking about a scale reducer, and not scale inhibitor. Inhibitor is fluid, mixed with water to protect the heating system.

      Yes, a leak of any kind will cause boiler pressure to drop. Leaks can come from anywhere (scale reducers, magnetic system filters, pin holes in rads/pipework, weak soldered joints, PRV valves, boiler connections etc).

      It’s a simple case of finding the leak, and fixing it. Without looking, if the reducer is actually leaking, I’d assume the olive in the scale reducer has been compressed (so the reducer isn’t sealed). So, either a new olive (inexpensive) or a new scale reducer.

      Either way, the system will need to be drained, problem fixed, and then refilled.

      Not really sure why they didn’t just do this while they were there.

  2. Hi, my husband bled radiators with the heating switched on, and now our 9 year old combi is gradually losing pressure over a few hours. We repressurise to 1.1, and it might go up to 1.3, but no higher. It goes down to 0.4 or 0.3 a few hours later…although it lasted 24 hours yesterday! We have checked for leaks and tightened radiator valve connections. The pipe outside the house is very slightly dripping. Thanks so much.

    1. Did it lose pressure before? Sounds like it didn’t. If it didn’t, could be one of the bleed valves wasn’t tightened up afterwards. But, that’s a lot of pressure loss in a fairly short period of time, so I’d imagine you’d notice it pooling below the side of the rad that has the bleed valve on it.

  3. Hi there, we had a new boiler installed in October and pressure was set at 1.5. Since then the pressure has slowly dropped with it now at 0.9 (2months later). So hasn’t dropped dramatically or quickly. No visable leaks and heating/hot water coming on fine. We did also have a new filling fitted in the airing cupboard too. The only thing that we think it might be is my husband bled rads when we had the old boiler and when closing the valve on one of them the nob snapped off. We thought we got away with it has no air being released, no sound or water. Could this be the cause of loss of pressure? Please Help! Much appreciated 🙂

    1. The smallest leak over time, could show that amount of pressure loss. Dab it with a tissue next time the heating is on and see if there’s any moisture.

      It sounds like a small leak, so I wouldn’t expect to find pools of water anywhere.

  4. Hi there, I’d like to ask what I suspect will be a daft question, however, here goes – my parents combi boiler is losing 1.0 bar of pressure every 3 hours or so when the heating is on and it also loses pressure overnight when off, the pressure relief outlet remains dry (did the balloon on the pipe to check) and various boiler parts have been replaced by a professional engineer to no avail, so on the basis that they keep topping up with water via the filling loop, must there be a leak, or could the top up water be diverted elsewhere so that it might not be seen draining off? I’ve seen one post indicating similar frequency pressure drop which was apparently found to be caused by an air lock in a radiator and this person said that they couldn’t fathom where all the water was going that they kept putting in the system! Sorry that’s a bit long winded and thanks

    1. Hey Jack,

      That’s a lot of boiler pressure to be losing. Even 1 bar over a week, where the leak is visible, you’d notice small pools of water wherever the leak originates from.

      Airlocks can cause slight pressure loss, but they usually make their way into the system via leaks, or unsealed units.

      Here’s more on boiler leaks: https://heatingforce.co.uk/blog/boiler-leaking-water-from-bottom/

      There’s a chance the leaks just hidden somewhere and it’s hard to advise without looking. So, I’d get someone in to check it ASAP.

      Hope that helps

  5. My Worcester boiler never locks even though the pressure gauge goes to almost zero when cool. The gauge just makes the green 1 bar every time it’s on. If I fill it to green when cool, that only last a day and then sinks slowly to red and happily starts up. It is weird. It seems to lose no more pressure when in red. What you make of it please? Thanks

    1. Hi Eugeune,

      Higher pressure is higher force, so water escapes via small leaks easily. As pressure drops, it gets harder and harder for it to escape. So the pressure loss will reduce, as pressure drops.

      There’s a leak somewhere or a dodgy expansion vessel/PRV somewhere in the system.

  6. Hello – I would really appreciate your thoughts. I had a new boiler installed just over a week ago, and the very next day, when the heating was put on, the pressure rose rapidly to over 3,2, so I turned it off.
    Eventually after much hassle manufacturer’s engineer arrived some 3 days later, and discovered the expansion vessel was completely flat. This was charged and all settled down and holding steady at 1,5bar – for 3 days – then dropped to under 1, so I turned it off again.
    I was fully aware that I could re-fill the system to achieve the correct pressure, but didn’t because I wanted to know why it had happened.
    Installer came and did just that, and I was unhappy and called the manufacturer out again to check the expansion vessel.
    Vessel all fine, and everything was checked and declared nothing wrong with the boiler. When I asked why the pressure had dropped, and how could I have confidence that it wouldn’t happen again, as I do not think I should be re-filling the system every 3 days, the explanation given was that it was possibly air in the system.
    As radiators had been bled following the top up, I wondered how air got in. I was told there is air in water and can be released over time into the system.
    Is this true?
    I rarely had to re-fill the system with my old boiler unless some work done, and bled rads when necessary, and am not happy that I would continually be checking the pressure and re-filling and bleeding.
    Thank you

    1. Hey Anne,

      Is this consistently dropping pressure, or did it drop to 1 bar and stay there?

      You’re correct, you shouldn’t have to keep checking/refilling.

  7. Thanks for responding.

    I am not sure if the pressure would have stayed steady, because when it dropped below 1 bar and the display was flashing, I turned it off.

    The explanation about air in water being released over time was what I wondered about.

    It may be ok now, and I am checking the pressure only because i am worried the pressure will hold for only three days again.

    After 2 visits from Vaillant and 2 remedial visits from the installer, I am not full of confidence and the boiler is only 2 weeks old.

    Fingers crossed!

  8. I have recently had a new pump fitted () recently I ave noticed when it switches off its sounds like uts either its dry or like chains rattling. Also pressure on my tank drops every couple of days.

  9. Hi,

    I can’t repressurise my 24i junior. When I do it in the usual way the water goes straight through it and out the overflow / discharge pipe outside with no change in boiler pressure at all.

    I was on the phone earlier and when I finished the call the boiler was making an unusual noise and at zero pressure. Am gonna have quite the little ice drink in the morning with the water its thrown out because I’ve tried it a few times.

    Last attempt I powered off, reset when turned on, left for 3-4 minutes before adding water in usual way and no change.

    Thank you for any help.

    Craig

    1. Are you seeing fluctuations in pressure at all? When heating is on or off? If not, could be the gauge.

      If not, it could be the PRV.

  10. Hi, grateful for your views. I have a Vaillant combi boiler, just under 3 years old, and am having trouble with the pressure.
    I had a new pressure sensor fitted 6 weeks ago (F75 warning appeared.) I kept an eye on pressure which was fine for next couple of weeks. End week 3 this had dropped to 0.5bar. Refilled to increase pressure to 1.3 bar (cold.) 3 days later checked again and pressure once again down to 0.5. Refilled, pressure to 1.3, over next 10 days dropped to 1.0, following day down to 0.6. Topped up to increase pressure back to 1.3, next day pressure had fallen to 0.4. Repeated topping up to 1.3 pressure this time remained at 1.3 for 3 days but then fell next day to 0.1.
    There have been no changes in number radiators in use or settings. There are no signs of any leakage
    (radiators, walls or ceilings,)and no underfloor pipes.
    When operating after warming up the pressure is a consistent 2.5, latterly I’ve been monitoring this and it doesn’t appear to fluctuate, temperature showing around 72-74 when on.
    Grateful for any views.

    1. Hi Jim,

      Have you have someone inspect other parts besides easy to spot visible leaks? The PRV for instance?

      1. I’ve now booked a Vaillant engineer to inspect it. I suspect with the combined drop in pressure over few weeks this will amount to a fair amount of water and I would have expected to see some visible signs of this.

  11. Hi I have system boiler vaillant eco tech plus about 7/8 years old had recently changed pump on the boiler keep getting f.22 fault which is referring to drop in pressure pressure drops every 4/5 hours if the heating stays on for 24 hour than there is no drop in pressure only when it’s turned off can’t see any leaks PRV also changed it slowed the drop but it still dropping I have noticed air In few the radiator there is air in the system so I keep bleeding the radiators plumber who changed the pump checked expansion vessel it was dead so he charged it up any advise !!

  12. Hi, engineer inspected and fitted couple of parts, expansion chamber found to be in order. Everything else appeared to be working ok.
    Following a sudden pressure drop in couple hours following (1st time happened during day) engineer suggested I monitor the position by isolating the boiler over night. Following 2 mornings pressure had dropped by 0.2 bar in boiler, but possibly due to boiler not being fully cooled down when isolated, on opening valves again pressure dropped further 0.1 bar both occasions. Morning 3 pressure had dropped in boiler by 0.5 bar but dropped no further when valves opened.
    Still no sign of any leaks, all pipes either in ceiling, down walls ( plasterboard) or along walls, downstairs
    floors are concrete. Neither any signs of water from ‘overflow ‘ pipe.
    I understand that the loss on opening the valves points to system but loss when boiler isolated suggests boiler! The point that keeps coming back to me is that these significant losses of pressure followed the fitting of a new pressure sensor.
    Grateful for any views!

    Jim

  13. Hi,
    We have a Grant Vortex oil boiler, approx 3 years old feeding into a thermal store. We noticed last night that we had no heating/hot water and on checking the boiler, we found the pressure is at zero. We’ve checked all the rads and there’s no air in them. Connecting the pipe on the boiler to run water through did nothing to the pressure gauge. Any ideas on what we should try next or would you just suggest calling a plumber?!
    Many thanks

    1. Yeah, sounds like a reasonable sized leak so wait for a plumber. Filling the boiler up is just going to mean it dumps all the water out again and could cause damage depending on where the heating system is leaking.

  14. Hi there,

    We have an external Grant oil boiler on an unvented system with an Albion hot water cylinder. The whole house has underfloor heating though there are also three towel rails on the upstairs circuit.
    We suffered a burst pipe during the cold spell (ran out of oil, idiots). A plumber came out, sorted the pipe and refilled the system. Everything seemed fine for a week but now we are losing pressure on a daily basis. I.e. last three days we have refilled system to 0.6bar and it has dropped to 0.2bar while UFH is on during the day. When system is off it just about retains the pressure but I imagine we would notice it dropping over the course of a week.
    We can’t find any leaks internally and as the UFH was fine prior to the burst pipe we are assuming (possibly incorrectly) that this wouldn’t suddenly have failed.
    Do you have any ideas? I am waiting for the plumber who came out originally to get back to me, and also a boiler engineer, but wondered if there were any specific questions to ask them? Will we be killing the boiler and/or system pumps by running at such low pressure?
    Any advice or suggestions very gratefully received!
    Thanks in advance, Sanchia

    1. There’s another quick guide here https://heatingforce.co.uk/blog/boiler-leaking-water-from-bottom/

      But I’d get someone out as soon as possible & just note down anything that you’ve noticed/that’s been done.

      The pressure will drop quicker when the heating on as everything is expanding. If there is a leak, the extra pressure is going to push water out faster.

      If it was cold enough to burst a pipe, good chance it caused issues elsewhere. The cold could of weakened it almost to failure point.

  15. Hi. Had a new combo boiler fitted about 18 months ago. Everything fine until around 6 weeks ago when the maintenance of our apartment block did “something” to the water pressure in the building. Since then, a quiet alarm/whistling sound started. This has been getting louder and more frequent over the last 6 weeks until now when as soon as the burner kicks in, a constant high pitched sound comes on and stays on pretty much until we give in and turn off the heating. We’ve also noticed a slight drop in the pressure although it’s still within limits (1 when cool rising to 1.5 when turned on). Last night we heard loud banging in the pipe work running through our apartment – this was around 10 loud bangs every 30 minutes or so. This evening our water pressure in the taps and shower had completely dropped to just a trickle. The boiler pressure remains 1 when cold and 1.5 when switched on and the alarm/whistling sound is still there. Can anyone help?

    1. Hi Hollie. It sounds like there’s a few different issues. Tap pressure & boiler pressure aren’t related. Your landlord/water supplier would be responsible for mains water pressure. They have a legal obligation to do so. Once you top up your boiler’s pressure (with water), that water stays in the system (unless there’s a leak). There’s a few bits on noisy boilers, pipes etc here for your other problems https://heatingforce.co.uk/blog/boiler-fault-finding/

  16. Hi. I’ve been reading your information above and it is an excellent resource and such a help. However I’m none the wiser and still looking to find the fault. Obviously my boiler is losing pressure, Has been for nearly two months now. and I keep filling it up. I have had a plumber come and service it and he could provide no answers. so
    1] The boiler still works even if the pressure is at 0, which would mean there is no loss of actual pressure
    2] Initially, the pressure dropped at a much slower rate over the period of a week and now it does so almost on a daily basis.
    3] At the rate the pressure is dropping, if there was a leak, it should have become fairly visible by now but there are no visible leaks, under or around radiators or damp patches on ceilings from hidden pipes.
    4] There is no excess water coming out from outlet pipe either – so really puzzling where the additional water is going considering how many times it has been topped up.
    I would be very grateful for any insights as have already paid a plumber a call out fee once.

    1. I forgot to add to above comment

      that yesterday morning the pressure was down to zero [when not on] and the boiler still came on, so turned it off and topped it up to 1.5, when turned on again and while on the pressure climbed to 2.5, then reduced to 1.5 and this morning it had come down to under 1, back up again to 1.5 while on and now at 0.5 while off.

      Please advise if you can.

  17. Hello,
    Is it unusual for unvented gas central heating boiler pressure to decrease gradually over time? My Greenstar Bosch Worcester boiler pressure tends to drop from normal 1.5bar to 1 within two months. Can you help?

    1. Hi Derek. Your boiler shouldn’t lose pressure. Compared with most boilers that loses pressure, that’s fairly minor, but it doesn’t mean it won’t contribute to other faults (boiler leaks onto PCB for instance).

  18. I’ve lived in my flat since June and the pressure was fine until aroeubd Christmas time. Since then I got an F22 fault and had to fill the boiler. After that I noticed that the boiler was losing on average 0.2b per day. So I was refilling on average once a week. I had some work done on my bathroom a few weeks ago and now that the warmer weather is here I’ve noticed that for a week or so my boiler has stayed at 1.2-1.3b. Great! I wasn’t sure if it was now fixed as the towel rail was removed and put back on it if it was because I haven’t put the heating on at all? Last night I put the heating on as it was cold and today I’ve checked the pressure and it’s dropped right down to 0.8b overnight. So I assume it must be the central heating? I filled it up about 5 hours ago to 1.5b I haven’t put the heating on and now it’s sitting at 1.3b. I’ve checked for leaks around the radiators and in the airing cupboard where the tank used to be big can’t find any leaks. I’ve bled the rads and there was hardly any air. I had a plumber come round who couldn’t find a leak and told me to isolate the boiler for 24 hours which I did some weeks ago, whilst isolated there was no pressure drop and when I deisolated the boiler the pressure dropped slightly but not much at all. He told me I “probably” haven’t got a leak… I’m stuck now

  19. boiler has been serviced, radiators bled, inhibitor added and has magnaclean unit. its constantly losing pressure and having to top up every few hours. have checked for leaks nothing visable. Engineer opened up boiler to notice water on heat exchanger and reckons we will need to replace this. Been quoted £600 excl of labour. boiler is 10 years old – Worcester bosch greenstar 30CDI combi. Is this a common fault? Is it worthwhile considering just replacing boiler considering the price to replace and fit new part.

    1. If I were to make an honest decision, I’d go for a new Worcester Bosch from BOXT. Gets you 10 years warranty and a quick install. I don’t think I would pay £600 excl of labour to have it fixed.

  20. Hi,
    Recently balanced radiators in order to get the one furthest away from the boiler to get hot. Seems to have worked but I am now having to top up my boiler every morning. Have checked for leaks, bled radiators, checked Exp Vessel pressure. Can’t find anything obvious. Can balancing radiators have an affect on boiler pressure?

  21. Been looking for some tips to find a leak. Not sure how trapped air in radiators could be a cause of pressure loss though. The air trapped in a radiator would presumably make the radiator act like an expansion vessel slowing the rate of pressure drop due to a water leak. So bleeding a radiator of course won’t fix a leak, but it may cause your pressure to drop quicker afterwards. Liquids are incompressible unlike gases, so a system with zero gas in it will drop in pressure at the slightest of leaks.

  22. Hi, I have a potterton puma 80e. It runs really smooth great hot water and rads hot. Pressure vessel replaced last year, still holding pressure. Absolutely no leaks on the system including releaf valve and air vent. No air in the system. But, I am losing pressure very slowly. Just can’t figure it out, any help would be appreciated.

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