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.
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…
Table of Contents
The [DIY] Fix If Your Boiler Pressure Keeps Dropping
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.
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).
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.