Why Is My Boiler Losing Pressure

A boiler that keeps losing pressure is telling you that water is leaving its sealed heating circuit, or that the circuit can no longer cope with the water expanding as it heats up. The usual causes are a small leak somewhere on the pipework or radiators, a pressure relief valve letting water out through the pipe on the outside wall, or an expansion vessel that has lost its charge. Bleeding the radiators drops the pressure as well, and that one is expected rather than a fault.

An occasional top-up is normal. Needing one every few weeks is not, and it is worth working out which of the three causes you have before topping up turns into a habit. This is the gauge on the boiler, which belongs to the heating, and it has nothing to do with the pressure at your taps. We are a Gas Safe registered and OFTEC certified business based in Bearsted, and below is what the gauge should read, what you can safely check yourself, and what needs an engineer.

What the gauge should read

Read it with the heating off and the system cold, because the needle rises as the water warms and expands. Vaillant's guidance on boiler pressure puts a cold system at between 1 and 1.5 bar, and anywhere between 1 and 2 bar is fine once the heating is running. It also gives 3 bar as the upper limit the system should stay below.

Most boilers show the pressure on a dial on the front, and newer ones show it as a figure on the display. Below 1 bar cold, it needs topping up. Let it fall far enough and the boiler will stop and show a fault code, which is one of the reasons a boiler locks out. A reading that is high when cold is a different problem, and it is covered further down.

Why a sealed system loses water at all

Your boiler and radiators share one closed loop of water that goes round and round. Nothing is meant to leave it, so the pressure only falls when water or air does. Air is the harmless version. Automatic air vents let a little out over time, and bleeding a radiator lets out more, which is why the gauge drops after you have bled them and needs bringing back up afterwards.

Water is the version worth chasing. A system that holds steady for months and only drops after the radiators have been bled is behaving normally. One that drifts down week after week with nobody touching it has a way out, and there are three usual places to find it.

A small leak somewhere on the system

Start with what you can see. Radiator valves, the joints on a heated towel rail and any exposed pipework are the obvious places, then the pipework you cannot see, under floors and boxed in behind kitchen units. Baxi's advice on low pressure makes a point worth knowing. On hot pipework a slow leak can evaporate before it ever forms a drip, so look for staining, rust or corrosion rather than waiting for a puddle.

A leak on the heating circuit is a plumbing repair rather than a boiler fault, and finding it is most of the job. Alastair R put the order well in a Google review of a leak we traced and fixed, writing that we “took the time to identify the real problem of a leak and then ensured that this was fixed before dealing all back up.” That order matters, because topping a system back up over a leak only feeds it.

The relief valve and the pipe on the outside wall

Every sealed system has a pressure relief valve, a safety device that lets water out if the pressure climbs too far. Its discharge pipe usually runs from the boiler to an outside wall and ends as a short length of copper pointing down towards the ground.

Walk round and look at it. A wet pipe, a drip or a pale stain on the brickwork underneath means the valve has been letting water out. Sometimes that is because the pressure really did climb too high, which points at the next cause. Sometimes the valve itself is the fault. Baxi's guidance is that a valve dripping while the gauge reads low can have debris in it, and that cleaning or replacing it is work for a Gas Safe registered engineer.

An expansion vessel that has lost its charge

Water takes up more room as it heats, and on a sealed system it needs somewhere to go. The expansion vessel is that somewhere. It is a small tank, often inside the boiler casing, split by a rubber diaphragm with air on one side, and the air squashes up to make room for the extra water.

Over time that air charge leaks away. With no cushion left, the pressure shoots up every time the heating runs, the relief valve opens to protect the system, water goes out of the pipe on the wall, and the gauge reads low once everything has cooled down. That is why a flat vessel and a dripping relief valve can turn up together, and why changing the valve on its own can bring the same problem straight back.

Recharging or replacing the vessel is a job for a Gas Safe registered engineer, and Baxi says the same. It is one of the things an annual boiler service should check, which is the cheapest moment to find it.

Topping it up yourself, and when to stop

Most boilers are topped up through a filling loop, a short braided hose underneath with a valve at each end, or through a built-in lever or key. The exact method is in your boiler's user manual, and it is worth following that rather than a general guide, because the fittings differ between makes.

The shape of it is the same on most. Let the system cool, open the valves slowly while watching the gauge, close them once the needle reaches the cold pressure your manual gives, and make sure both are fully shut afterwards. If the pressure falls again as soon as you close them, stop there and get it looked at.

In our experience, finding out where the water is going works better than topping the system up every few weeks, because each top-up treats the reading rather than the cause. Ideal's guidance on topping up is plain about it. A system can need repressurising once or twice a year without anything being wrong, but topping up once a month or more points to a small leak or a faulty pressure relief valve.

A top-up once or twice a year is normal. Needing one every few weeks means water is leaving the system, and the top-up only hides where.

When the pressure is too high instead

The opposite problem has its own causes. Vaillant lists three, a system that has been overfilled, a faulty filling device, and a fault in the boiler itself, and advises bringing a cold reading above 2.5 bar back down before the boiler runs. A filling loop that lets water by when it is supposed to be closed is the easy one to miss, because it pushes the pressure up slowly while nobody touches anything.

A pressure that climbs high when the heating is on and then drops away when it cools is the expansion vessel again, so the same check applies. If a high reading will not settle once the system is cold, Vaillant's advice is to have a Gas Safe engineer diagnose it.

What to check yourself, and what to leave to an engineer

Reading the gauge cold, bleeding radiators, topping up through the filling loop and walking round to look at the discharge pipe are all yours to do. So is looking for staining under radiators and along any pipework you can see. Anything inside the boiler casing is not, and neither is the relief valve or the expansion vessel. A boiler is a gas appliance, and work on it belongs to a registered engineer for good reason.

If the pressure keeps dropping, a boiler repair visit finds the cause rather than resetting the symptom, and an annual service at £90 is the natural time to have the vessel checked before winter. A leaking radiator or a tired valve is radiator and heating work. And if the heating has gone off in a house with somebody vulnerable in it, treat it as an emergency call-out rather than waiting, with call-outs from £80.

So, read the gauge cold, expect a top-up once or twice a year, and treat anything more often as a sign that water is leaving the system. Look at the pipe on the outside wall first, then the radiators and the pipework you can see, and leave the valve and the vessel to an engineer. CTL Plumbing & Heating covers Maidstone and the rest of Kent from Bearsted, with free home visits and free quotes across Kent. If your boiler keeps losing pressure, get in touch and we will find where it is going.