There are a few scenarios that lead to a boiler’s pressure being higher than it should be.
The heating system has been overfilled with water
Your boiler may have a faulty expansion vessel
External filling loop is letting-by water.
Your boiler may have a faulty plate heat exchanger
What Pressure Should Your Boiler Be?
But, before trying to fix the problem, you need to understand exactly what pressure your boiler needs to be at.
Typically, you’ll find that boiler manufacturers recommend a pressure of 1.3 bar (check your owner’s manual).
What Boiler Pressure Is “Too High”?
Anything above 1.8 bar isn’t ideal. 2.3 bar starts to become more serious and when most boilers hit 3 bar, they’re in the red zone on the pressure gauge.
So, the unit is likely to lock out and display a fault code.
If this happens, I’d suggest speaking to one of our Gas Safe boiler engineers.
Problems That Lead To High Boiler Pressure
Below are the top causes of high boiler pressure.
#1 – Heating System Has Been Overfilled
The first and most obvious problem that leads to high boiler pressure, is a heating system that has been overfilled.
By topping up water via the external filling loop, you can add pressure to your boiler. Your boiler needs sufficient water in it to operate correctly. This stops a “dry fire” scenario that might otherwise damage the pump, heat exchanger and other expensive parts.
However, if you overfill the system, it can put an unnecessary strain on the whole heating system. It’s going to mean copper joints are more likely to fail, the pump’s seals are more likely to blow, and it can even induce cracks in the heat exchanger.
So, what’s the fix for a boiler pressure that’s too high in this scenario? Simply drain water from the heating system.
You have two options here as a point for bleeding the system:
Radiator bleed valve
You can remove water from the system via the radiator bleed valve, using a specifically designed radiator key.
Bleeding water from radiators tends to be messier, so have towels ready to cover walls/flooring, and make sure you have a small container to catch the water you’re bleeding from the system. Again, shut off the valve once at 1.3 bar.
If you know where the drain cock in your property is, you can connect a small hose and open the valve slightly. This will allow water to escape. With someone helping you, they’ll be able to tell you when the pressure on your boiler is around 1.3 bar, so you can close the valve.
If you remove too much pressure from the system, don’t worry. You can refill the system using the external filling loop.
#2 – Boiler Pressure Rises (Less Than 0.3 Bar When Heating Is On)
Next, we find a lot of people trying to find faults when their boiler’s pressure rises by around 0.3 bar when hot water and heating is on.
Naturally, parts expand when they heat up. So, as your heating/hot water comes on, you’ll see the pressure slowly rise.
However, there should be pressure release devices in your boiler that stop pressure rising too far. So, you should expect to see your boiler’s rise by less than 0.3 bar.
Feel free to ask one of our engineers to check the boiler’s pressure during the next service. However, small rises in pressure are normal. Once the pressure rise becomes 0.5 bar or more, that usually signifies that there’s a problem.
#3 – Boiler Pressure Rises (More Than 0.3 Bar When Heating/Hot Water Is On)
As we’ve mentioned, a rise in pressure of under 0.3 bar is normal. But, if you notice that the higher the pressure goes, the quicker it increases (and it’s rising by well above 0.5 bar), there is likely to be a pressure release issue.
Different boilers have different parts to help release pressure when it’s at its highest. This includes expansion vessels, auto air-vents, pressure release valves and so on.
If these units fail, they might not be operating in the way they should. Instead of releasing pressure, they are containing it. And that means that the boiler’s excess pressure has nowhere to escape.
Not only does this mean you have a faulty pressure release unit, it means you’re putting excess strain on many boiler components. And eventually, the boiler will get to a point where the pressure is simply too high, and it will lock out.
You’ll need a boiler repair engineer to come and assess your boiler. They’ll be looking at anything that helps to relieve pressure from your appliance. It’s likely to be a faulty part that needs replacing
#4 – Boiler Pressure Rises (Even When Heating/Hot Water Isn’t on)
The final possibility is that the filling loop (the device used to increase boiler pressure) isn’t fully turned off.
If the valve is stuck partially open (or hasn’t been closed fully), that’s going to constantly leak small amounts of water into the system. When this is the case, the boiler pressure will rise constantly, even when hot water and heating aren’t currently on.
Ask our engineer to disconnect the external filling loop. With both valves closed, it shouldn’t be letting-by any water. If it is, that’s going to make your boiler pressure too high.
If our engineer notices that a valve is getting partially stuck open, a replacement filling loop is going to be needed.
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