HeatingForce is reader-supported. When you buy through links on our site, we may earn an affiliate commission. Learn more
Boiler Losing Pressure? Here’s Why and What You Need to Know
If your boiler is losing pressure, there’s a good chance you’ve been told to adjust this pressure using the filling loop to “top up the boiler”. My advice would be. Don’t.
Hopefully, the boiler installer dosed your new system with inhibitor as well as fitting a scale reducer (in hard water areas) and a magnetic system filter.
Inhibitor breaks down any bad particles in the system, allowing the scale reducer to catch them. Every time you top up the boiler using the filling loop, you are diluting the inhibitor that’s in the system.
Here are some of the reasons that result in a 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 is losing pressure, but first, let’s look at the DIY fix…
Table of Contents
- The [DIY] Fix For A Boiler That’s Losing Pressure
- Is Your Boiler Losing Pressure But No Leak?
- Causes of a Pressure Loss in Your Boiler
- #1 – Pressure Relief Valve on the Boiler is Faulty
- #2 – Auto Air Vent Leak in Heating System
- #3 – Leaking Radiators, Towel Rails and Radiator Valves are Leaking
- #4 – Radiators Are Full of Air
- #5 – Dodgy Expansion Vessel
- #6 – Soldered Joints on the Boiler Have a Leak
- #7 – The Pressure Gauge on Your Boiler is Faulty
- What Your Boiler Losing Pressure Means — And What You Need to do to Get It Fixed
- What’s Next When Your Boiler Suffers Pressure Loss?
The [DIY] Fix For A Boiler That’s Losing Pressure
99% of the time, a boiler loses pressure due to boiler leaks in the boiler itself, or a leak somewhere in the heating system.
Before spending 100s of pounds on a repair engineer, there is a DIY fix that will sort out your boiler’s pressure. And, it comes in the form of the Fernox F4 leak sealant.
This sealant travels around your heating 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 fixing your boiler’s pressure loss in the process!
Need a new boiler?
Get a FREE fixed quote with finance options.
Is Your Boiler Losing Pressure But No Leak?
99% of the time, a boiler losing pressure is 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 overall heating system.
Causes of a Pressure Loss in Your Boiler
Most boilers operate at 1.0bar-1.5bar. But they can approach 3bar if there is a problem like a 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.
Central heating systems can collect air pockets over time (and they certainly do when they are refilled).
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.
This is the most common cause when looking at boiler pressure loss. The smallest of leaks can cause your system to lose 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 in the system will need to be repaired or replaced.
For the time it takes to drain down a system, a new boiler replacement rather than a fixed boiler is usually the better option for your home or business property.
What kind of fuel does your boiler use?
Grab your fixed price online, ask for next-day boiler installation.
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, simply open the vent with the key slightly. You’ll start to hear air hissing out. Once the air stops water will start to drop out (or spray out if you’ve opened the vent too much). Close the vent off. Do this to every single radiator and towel rail in the property or home.
It’s worth taking a small container to collect any water and a towel to protect carpets.
Remember if you are constantly removing air, the boiler pressure will drop each time you bleed the radiators.
Every now and then expansion vessels will need to be repressurised. If they are 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 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, the vessel will need to be replaced rather than fixed in order to stop the boiler losing pressure.
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 get the boiler pressure back to where it should be and see if there is any pressure loss.
This is unlikely, but it’s happened before with boiler pressure. There’s a chance the pressure gauge on the front of the boiler is not reading correctly.
This is quite dangerous, especially if you are constantly topping up the pressure. It’s likely that this is the case if you top up the boiler pressure and nothing happens (or the pressure changes very little).
If the pressure doesn’t change and it’s not the pressure gauge, that’s a sign of a leak that you’d almost certainly notice!
Need a new boiler?
Get a FREE fixed quote with finance options.
What Your Boiler Losing Pressure Means — And What You Need to do to Get It Fixed
Boilers That Lose 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, a boiler could lose pressure when it’s on.
Boilers That Loses 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.
What’s Next When Your Boiler Suffers Pressure Loss?
Struggling to understand why your boiler is losing pressure?