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How to Release Pressure From a Boiler
The gauge is higher than it should be, possibly near the red, and you want it down. The good news is that letting a little out is easier and safer than putting it in, and you almost certainly already own the tool. The better question, once it is down, is why it went up.
The Short Answer
Turn the heating off and let the system cool. Put a bowl and a towel under a radiator bleed valve, open the valve a quarter turn with a bleed key until water trickles out, and watch the gauge. Close it as soon as you approach 1.2 bar, because the needle keeps easing down for a moment afterwards. That is the whole procedure. If it climbs again on the next heating cycle, the pressure is a symptom rather than the fault.
Cool It Down First, and Read It Again
A large share of high pressure readings are not high at all. A sealed system reads 0.3 to 0.5 bar above its cold figure while the heating is running, and that is normal expansion doing exactly what it is supposed to. A gauge showing 1.7 in the middle of a January evening on a system that sits at 1.2 cold is fine.
So switch the heating off and give it a couple of hours, then look again. If the cold reading is inside 1.0 to 1.5, nothing needs doing and the job is finished before it started. Bleeding a system down to 1.2 while it is hot leaves you at around 0.8 in the morning, which is a lockout waiting to happen, and then people top it back up and conclude the boiler is unpredictable.
The reading that matters is always cold. Everything below assumes you have taken one.
The Method, With a Bleed Key
A radiator bleed valve is the right release point for this. It is designed to be opened by hand, it is at a known place, and unlike anything on the boiler itself it does not require taking a cover off.
- Turn the heating off and let it cool. Cold water is much easier to deal with when it lands on your hand.
- Pick a radiator, ideally not one standing on a carpet you care about, and preferably the lowest one in the property, since the water arrives with a little more push there.
- Bowl underneath, towel under the bowl, and a second cloth wrapped around the valve to catch the spray when it first opens.
- Open the bleed valve a quarter turn with the key. You will get air first if there is any, then water. Water is what you want here, since it is water you are removing.
- Watch the gauge, not the bowl. It moves slowly at first. Close the valve as you come down towards 1.2, because it continues to settle for several seconds after you shut it.
- Dry everything, and check the valve is properly closed by running a dry tissue around it a minute later. A bleed valve left a fraction open is a very slow leak that you have just created.
If It Goes Straight Back Up
This is the part worth reading, because a system that needs bleeding down more than once is telling you something specific and it is not going to stop on its own.
The commonest cause is the filling loop. If it has been left slightly open, or a valve on it is passing, mains water is feeding into the system continuously at a pressure far higher than the system runs at. It will climb until the safety valve lifts, every time. This is why a flexible loop is meant to be disconnected at one end after use rather than merely closed, and it is the first thing to check because it is free to check.
The second cause is the expansion vessel, and the signature is different: normal cold, very high hot, back to normal once it cools. There is nowhere for the expanding water to go, so all of the expansion turns into pressure. That is a real fault and bleeding it down every week is managing it rather than fixing it.
Third, and less common, a plate heat exchanger inside a combi can develop a fault that lets mains pressure across into the heating circuit. The tell is a system that climbs even with the loop disconnected and the heating off, particularly noticeable after somebody runs a hot tap.
You can distinguish the first two yourself in an evening. Disconnect or fully close the loop, note the cold pressure, run the heating for an hour, and note it again once it has cooled overnight. Back where it started means the loop was the problem. Higher again means the vessel.
If You Cannot Bleed a Radiator
Bleeding is the method above because it is the safest release point in the house, but there are real reasons it is not available: a bleed valve painted solid, a radiator behind furniture that will not move, a carpet you would rather not risk, or no radiator within reach of anywhere you can put a bowl.
The alternative is the drain-off cock, a small valve usually found at the lowest point of the system, often on a downstairs radiator tail or near the boiler, with a hose connection on it. Attach a short length of hose, run it to a drain or a bucket outside, and open the valve a fraction. It gives far more control than a bleed nipple and the water goes somewhere sensible rather than into a bowl you then have to carry.
Go in small bursts and watch the gauge, because a drain cock moves considerably more water than a bleed valve and overshooting is easy. Close it firmly, and check it is not weeping afterwards, since drain cocks that have not been used in years sometimes do not reseat cleanly.
What Not to Use
Do not open the pressure relief valve by hand to let pressure out. It is a safety device, its seat is machined to seal at 3 bar, and lifting it manually with debris in the system very often means it never seats cleanly again. You then have a system that will not hold pressure at all, and a genuinely simple problem has become a part and a visit.
Do not drain from the boiler itself unless you know what you are doing with it. There is usually a drain point, and there is usually also a lot of adjacent pipework carrying full mains pressure, and the margin for error is smaller than at a radiator two rooms away.
And do not leave a bleed valve open and go and do something else. A radiator bleed nipple passes more water than people expect once the air is out of the way, and coming back to a wet floor after five minutes is an entirely ordinary way for this to go wrong.
How Much to Take Out
Less than you think, and this is where people overshoot. On an average flat, moving the gauge from 2.5 bar to 1.2 is a matter of a small teacup, not a bowl. The bowl is there to catch drips and the initial spray rather than to hold the volume.
Take it out in stages if you are unsure. Open, count to three, close, look at the gauge. Two or three rounds of that gets you there with much more control than one continuous run, and the gauge lag is the whole reason: the needle is telling you where the system was a moment ago.
If you do overshoot into the low range, that is not a problem beyond being mildly annoying, and topping it back up is covered in the repressurising guide. Nothing is damaged by a system briefly sitting at 0.9 bar.
Doing This in a Flat, With Someone Living Underneath
The mechanics of releasing pressure are the same everywhere. The practical problem is not, and in a city that is mostly flats it is worth planning for, because the failure mode here is not a broken boiler, it is a ceiling.
There is often nowhere convenient to put the water. A tenement flat commonly has no outside tap, no downstairs utility, and a bathroom at the far end of a long hall from the radiator you have chosen. So the water you take out has to be carried through the flat in a bowl, and that is exactly the moment it gets tipped down a hallway. Bleed the radiator nearest the bathroom rather than the one nearest the boiler, even if that feels less logical.
Old floors make it worse. Original floorboards with gaps, and the sixty or eighty year old fill and levelling underneath them in a converted flat, will pass a spilled bowl of water to the plaster below quicker than a modern floor would. There is no waterproof layer in the way. A towel down before you start costs nothing and is the difference between a wipe and a conversation with a neighbour.
And it is worth knowing that in a lot of Glasgow flats the radiator valves and bleed nipples are original to a retrofit done decades ago, and the brass has been painted over more than once. If a bleed valve will not turn, do not lean on it. A snapped or rounded nipple on a filled system is a considerably larger problem at eleven at night than a slightly high gauge, and the gauge will wait until morning.
When to Call Someone Out
The bleeding itself is a customer job. What is worth a call is the pattern: pressure climbing back within a day or two, a gauge that reaches 3 bar every heating cycle, water discharging outside, or a bleed valve that will not move. The first three are a filling loop, an expansion vessel or a heat exchanger, and each has a different fix, so identifying which one matters more than getting the number down again. If you have already run the overnight test described above, say what it showed when you call, because it removes most of the diagnosis before anyone arrives.
Boiler Repair in GlasgowCommon Questions
Through a radiator bleed valve, with the system cold. Bowl and towel underneath, open a quarter turn until water runs, and close it as the gauge approaches 1.2 bar. There is no separate release control on the boiler intended for this, and the safety valve is not it.
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Read Next
Full guide
What Should the Pressure in a Boiler Be?
On a sealed system, which is nearly every combi and most modern system boilers, aim for 1.0 to 1.5 bar when the heating has been off for a few hours and the system is cold.
How to Repressurise a Boiler
Turn the boiler off and let it cool.
Is Low Boiler Pressure Dangerous?
Low pressure is not dangerous.
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