Why your boiler keeps losing pressure overnight (and the filling loop mistake plumbers see)

The first time you notice the boiler pressure needle has dropped into the red overnight, the reaction is usually universal: you stare at it, exhale through your nose, and give the white case a small, optimistic tap-as though that could possibly solve it.

By then the radiators are barely warm, the shower is only lukewarm, and the whole place feels just that bit colder than you’ll admit to anyone else in the house. Between putting the kettle on and padding around looking for your slippers, you open the cupboard, spot the gauge hovering miserably near zero, and feel that low-level panic: “Great. How much is this going to cost?” Maybe you’ve already topped it up once, turned a valve, closed the door and tried to forget about it. Then you wake up the next morning… and the pressure has dropped again. That’s when people call a plumber-and it’s also when the same mistake appears, again and again.

The 7am phone call plumbers know by heart

Ask any British plumber what winter sounds like and they’ll tell you: the ringtone at 7am, followed by a tired voice saying, “Hi mate, my boiler keeps losing pressure overnight.” It’s almost always overnight. Nothing obvious is happening-lower thermostat setting, no taps running, no radiators audibly venting. Just a sealed system quietly losing pressure while you’re asleep. By the time you clock it, the school run is imminent, you’re already running late, and everyone is shivering in dressing gowns.

Every plumber I spoke to said a version of the same thing: they can often guess what’s wrong before they’ve even stepped out of the van. Not because your boiler is uniquely troublesome, but because so many households are making the same easy-to-make error without realising it. When I asked what the “most common” cause was, a couple of them actually laughed. “Honestly?” Dan told me. “It’s people fiddling with the filling loop and not shutting it off properly. That’s the big one.”

We tend to think of boilers as delicate, mysterious machines that require specialist knowledge and obscure tools. Plumbers see a much more ordinary explanation: rushed mornings, skim-read instructions, and that very British confidence in acting like we understand something-right up until it stops working. Your pressure disappearing sits neatly in the space between those two perspectives.

The innocent little lever everyone gets wrong

The filling loop that won’t stop filling

If you’ve got a combi boiler or any sealed system, there’s usually a small run of pipework nearby called the filling loop. It’s typically two small valves-often with black or blue handles-and sometimes a braided silver flexible hose connecting them. Its purpose is simple: let mains water into the heating circuit so you can raise the pressure when it drops. You open it, watch the gauge rise to about 1–1.5 bar, then shut it again. Done. In theory.

In practice, this is where it frequently goes wrong. Dan described what he walks into week after week: “Gauge at nearly three bar, PRV (pressure relief valve) pipe dripping outside, customer insisting they definitely turned it off.” In most cases, the filling loop hasn’t been closed properly-it’s only slightly open. Not enough to look obvious, just a millimetre of movement that allows a steady trickle to keep feeding the system day and night. While you sleep, the pressure climbs, and the boiler does exactly what it’s designed to do: it dumps the excess through the safety valve. By morning, water has been lost from the system and the needle is back near the bottom.

That’s the nasty twist: plenty of “low pressure” call-outs are caused by accidentally creating “too high pressure”. The boiler vents to protect itself, and what remains isn’t sufficient for stable operation. You see the low reading the next day, assume there must be a leak hidden somewhere behind walls or under floors, and start searching online for worst-case scenarios about new pipework-all because a tiny valve wasn’t fully closed after the last top-up.

The lever that lies to you

The awkward part is that the handles can be deceptive. On many installations, the lever can feel as if it has reached its stop before it has actually shut off completely. There can be resistance, a little slack, or it may look like it’s at a right angle when it’s still ever-so-slightly open. As plumber Sarah put it: “People hate forcing anything on a boiler, they’re scared of snapping something. So they give the lever a gentle turn, feel a bit of resistance, and think, ‘That’ll do.’ It doesn’t.”

And, realistically, hardly anyone checks the pressure every day, regardless of what the manual recommends. Most people only look when something’s already wrong. That’s why a partially open filling loop can sit there for weeks, quietly throwing the system out of balance while you get on with life. You might notice the pressure needle wandering up and down and blame the weather, or chalk it up to an “old boiler” being moody. Meanwhile, the safety valve may be venting every night, the expansion vessel is taking a battering, and that harmless-looking lever is at the centre of it.

One engineer compared it to “like leaving a bathroom tap slightly running all the time, but behind a panel so you can’t see it.” Even if there is a sound, it’s usually too subtle to pick out from normal household noise-just a faint hiss or a quiet murmur through pipework. By the time you step outside on a cold morning and notice a steady drip from the copper pipe protruding through the wall, the system may already have been stressed.

Why the pressure always seems to vanish overnight

There’s a reason this error shows itself in the morning: your heating behaves differently as it cools. In the daytime-when radiators are hot and you’re using hot water for cooking and showers-the water in the system expands. The pressure rises, sometimes close to the top of the green zone. When the heating switches off and everything gradually cools, the water contracts, pressure falls, and any weak point in the setup can start to show.

That’s when a poorly closed filling loop, a worn seal, or a tired expansion vessel tends to reveal itself. If the pressure has been creeping too high because the filling loop was left slightly open, the safety valve may have been releasing water each evening while the system was at its hottest. By morning there’s simply less water circulating. The gauge might read 0.5 bar, the boiler won’t fire, and you’re standing there in your socks wondering what you did to deserve it.

When it happens, the lack of heating and hot water feels major-cold tiles, cold air, and an immediate sense of looming expense-so it’s easy to assume the cause must be major too: hidden leaks behind plasterboard, corroded pipes under the floor, the cost of replacing the whole system. Yet in a surprising number of homes, the culprit is no more dramatic than a 90-degree turn on a small lever you barely remember touching.

The one thing plumbers wish you’d stop doing

The top-up trap

With a slight wince, every plumber I spoke to mentioned the same pattern: people topping the boiler up every single day. Pressure low in the morning? Open the filling loop, push it back up to 1.5 bar, carry on. Next morning it’s low again. Repeat. After a week, you’ve put several litres of fresh water into what’s meant to be a sealed, treated circuit.

It feels logical on the surface-like adding oil to a car engine. But heating systems aren’t designed to be refilled continually. Mains water brings oxygen and minerals that encourage corrosion inside radiators and boiler parts. Sludge builds, corrosion spreads, seals degrade and split. So you’re not only papering over the real issue-you’re also helping create another one. One plumber summed it up without fuss: “If you’re filling it every day, don’t. Call someone. That’s not normal.”

This is where the human side matters, because daily top-ups are usually driven by worry, not laziness. Plenty of people feel awkward calling an engineer “just for low pressure”, so they keep adjusting things, hoping the system will settle down. They don’t want to seem clueless when someone turns up, or feel as if they’re wasting a professional’s time if it turns out to be minor. But nearly every engineer told me they would much rather visit for a filling-loop problem than return months later to find the expansion vessel and PRV have been pushed to their limits.

DIY pride vs quiet reality

There’s also the familiar British impulse to “have a go”. Many of us inherit homes-and with them, half-remembered instructions: “If the heating plays up, just twist that silver thing until the needle moves.” No explanation. No reminder to close both valves firmly afterwards. Just a habit that gets passed along from one owner to the next. Then the house changes hands, and the new person is suddenly responsible for a system they never consciously chose to learn.

The truth is, most boiler disasters start as tiny misunderstandings, not dramatic explosions. A valve that wasn’t fully shut. A gauge reading taken the wrong way. A drip ignored because it’s outside, out of sight, and only seems to appear on cold mornings. When plumbers talk about the “mistake that causes boilers to lose pressure overnight”, they aren’t judging you. They’re recognising a repeating pattern across hundreds of hallways and airing cupboards-people doing their best to keep the house warm.

Small checks that feel like control, not homework

If there’s a practical takeaway, it’s that you don’t need to become a heating engineer to reduce the chances of this happening again. When you top up the pressure, do it slowly. Keep your eye on the needle. Aim for roughly 1.2–1.5 bar when the system is cold, then close both sides of the filling loop fully back to “off”. Don’t rely only on how it feels-actually look at the lever positions. Are they truly at a right angle to the pipe? If you’re uncertain, gently check each one again.

Instead of forgetting about it for a week, take a quick look at the gauge later that day, and again the following morning. Does it climb to two or three bar when the heating is running? Does it drop back down once everything cools? And outside, does the copper pipe near the boiler-pointing down through the wall-show a constant drip? Those small details can tell a plumber a great deal, and they can also help you work out whether you’re dealing with the classic “filling loop left open” scenario or something more serious, such as a genuine internal leak.

Plumbers aren’t expecting spreadsheets or daily logbooks. They simply wish more people saw their boiler as less of a mysterious white box and more like a slightly grumpy pet: mostly fine on its own, but fairly clear about warning signs once you know where to look. A hiss that wasn’t there before, a permanently damp patch beneath a pipe, a pressure needle that swings wildly whenever the heating comes on-those are the sights and sounds worth noticing.

Why this one small fix can save a winter of stress

When a boiler loses pressure overnight, it can feel like the beginning of an expensive saga. You start thinking about call-out charges, bracing for bad news, remembering the friend who had to replace an entire system last year. Yet in a surprising number of houses, the story ends with an engineer kneeling under the boiler, giving a dry half-smile and saying, “Here you go – your filling loop was just slightly open.” They shut it properly, set the pressure correctly, perhaps swap a relief valve that’s been put under strain, and the whole situation melts away into normal, quiet warmth.

That doesn’t mean every pressure drop is your fault, or that it’s always fixable without parts. Seals wear out, expansion vessels lose their charge, and concealed leaks under concrete floors do happen. But the recurring pattern plumbers keep running into-the mistake they wish people would stop repeating-is that tiny act of not quite closing a valve, then living with the knock-on effects for weeks. A millimetre of movement on a lever you last touched in a hurry, half-asleep, in your pyjamas.

Next time you open the cupboard and see the needle sitting in the red, pause. Check the filling loop properly-really check it. Make sure both valves are as fully shut as they can be, not merely “close enough”. And if you’re no longer topping up occasionally but doing it daily, ring for help before you reach for the lever again. Because behind most overnight pressure drops there’s a reason-and more often than you’d think, it’s simply the one small mistake nobody ever took the time to show you how to avoid.