How to repressurise a boiler

Why the pressure matters

A modern boiler runs on a sealed heating system, which means the water in your radiators is held at a slight positive pressure rather than being fed from a tank in the loft. That pressure keeps water in contact with every surface of the heat exchanger. Below about 0.5 bar the boiler can no longer guarantee that, so it locks out rather than risk running the exchanger dry.

Almost every manufacturer has a code for this. Vaillant calls it F.22, Worcester Bosch calls it CE 207, Ideal calls it F1, Baxi calls it E119. They all mean the same thing and they all clear the same way.

First check you have a sealed system at all

This guide does not apply to every house, and it is worth thirty seconds to be sure it applies to yours. If your boiler has no pressure gauge and there is a small plastic tank in the loft with a ballcock in it, you have an open-vented system. It maintains its own water level from that tank, there is no filling loop to find, and a system like that losing water shows up as noise, cold radiators or an overflowing tank rather than as a pressure reading.

Two other cases are worth ruling out before you touch anything. If you have a heat pump rather than a boiler, do not treat low pressure as a job for the owner: at least one manufacturer explicitly instructs the householder to inform their installer instead, because these circuits are much larger, often contain antifreeze that must not be diluted with plain water, and losing pressure means a leak in a great deal more pipework. And if the boiler is visibly leaking, if there is water inside the case, or if you can smell gas, stop reading and ring someone.

Read the gauge properly

There are three kinds of gauge and they mislead in different ways. A traditional dial usually has a green band from roughly 1 to 1.5 bar and a red zone above about 2.5. Some boilers show pressure as a number on the digital display, sometimes only when you press a particular button, which is why plenty of people believe their boiler has no gauge. And some systems have the gauge on the pipework below the boiler rather than on the appliance, occasionally inside a cupboard alongside it.

What the reading should be depends on temperature, and this catches out more people than anything else in this guide. Cold, aim for 1.2 bar. Once the heating has been running for half an hour, expect it to have climbed by a few tenths — 1.5 to 1.8 is perfectly normal, because water expands as it warms and the expansion vessel absorbs that increase. So a gauge at 1.8 with the radiators hot is not overfilled, and the same system at 1.8 stone cold is.

That relationship is also a free diagnostic. Note the reading cold and again after half an hour of heating. A rise of two or three tenths of a bar is healthy. No movement at all suggests the gauge or its sensor is stuck rather than that the system is unusually stable. A rise to 2.5 bar or beyond means the expansion vessel is not doing its job, and topping the system up will make that worse rather than better.

Find the filling loop

The filling loop connects your cold mains to the heating system so you can add water. It takes one of three forms.

The common one is an external loop: a braided silver hose, usually about 30cm long, with a small valve at each end, sitting on the pipework below the boiler. The valves are often small levers or slotted heads that need a flat screwdriver.

The second is an internal filling link, which is increasingly common on newer boilers. Instead of a hose there is a plastic key, often blue or white, clipped somewhere under the boiler. It slots into a fitting at the base and turns to open. Worcester Bosch, Ideal and Vaillant all use variations on this, and the key is small enough that it frequently goes missing — the manufacturer will sell you another, and it is worth taping it inside the boiler cupboard door once you have it.

The third is a keyless arrangement built into the underside of the boiler, worked by a lever or a knurled ring that you turn rather than a separate part you insert. These are the easiest to use and the easiest to leave slightly open.

If you genuinely cannot find any of them, stop and ask an engineer to show you once. Do not start opening valves you cannot identify — there are valves under a boiler that drain a system rather than fill it, and there are gas valves down there too.

Bleed first, if you are going to bleed

Order matters here and getting it wrong means doing the job twice. Bleeding a radiator releases water as well as air, so it lowers system pressure — sometimes by a surprising amount on a small system. If radiators are cold at the top as well as low on pressure, bleed them first, working from the ground floor upwards and finishing with the highest radiator in the house, and then repressurise once at the end. How to bleed a radiator sets that out step by step, including how to tell trapped air from the settled sludge that bleeding will not shift.

Fill it

1. Turn the boiler off and let it cool. Filling a hot system leaves you underpressurised once it cools down.
2. Find the pressure gauge — either a dial on the boiler front, a reading on the display, or a small gauge on the pipework beneath it.
3. Open both filling loop valves slowly. You will hear water flowing. Slowly matters: a fast fill pushes air around the system and stirs up debris that then finds its way into the pump and the heat exchanger.
4. Watch the gauge rather than the clock, and expect it to move faster than you think near the end. When it reaches 1.2 bar, close both valves.
5. Reset the boiler if it is locked out, and give it a few minutes to run.

That is the whole job. It takes five minutes.

If nothing happens when you open the valves, the most likely reason is unglamorous: the mains supply to the loop is off. Someone may have closed a small isolating valve on the pipe feeding it after previous work, or the house stopcock may be part closed. Check a cold tap runs strongly before assuming the loop has failed.

Do not overfill

If you overshoot past about 2.5 bar cold, the pressure relief valve will discharge the excess through a copper pipe outside the wall. That is not dangerous, but it means the system will end up low again once it settles, and a valve that has discharged once sometimes fails to reseat properly afterwards and drips from then on.

If you do overshoot, bleed a small amount out of a radiator bleed valve with a cloth underneath until the gauge comes back to 1.2.

Close the loop properly

This is the step people miss. If a filling loop valve is left even slightly open, mains pressure slowly pushes into the heating system, and you will find the gauge creeping up over days until the safety valve starts discharging. Close both valves firmly and check the gauge again the following day.

There is a regulatory reason for this beyond tidiness. A filling loop is a direct connection between drinking water and the contents of a heating system, which is treated as a contamination risk under the water fittings regulations, and the standard arrangement is a temporary connection with a double check valve that is broken after use. That is why manufacturers' instructions say to disconnect a braided hose at one end when you have finished. If yours is permanently connected, it is worth mentioning at your next service rather than losing sleep over.

Losing pressure again: where the water actually goes

Repressurising is a fix for gradual loss, which is normal. It is not a fix for a leak, and the second time you do it in a season is the point to start looking rather than topping up again.

The single most missed answer is that the water is leaving outside. Every sealed system has a pressure relief valve, and its discharge pipe runs through the wall and points at the ground. If that valve has been disturbed — usually by a system that was overfilled once — it can weep continuously afterwards, and because the water lands outdoors nobody ever sees it. Go and look at that copper pipe, ideally when the heating has been running. A dribble, a green or white streak of limescale down the brickwork, or a damp patch on the ground below it is the answer.

Then the ordinary suspects, in the order they are worth checking:

• Radiator valves. Look at the spindle under the plastic head and at the nut where the pipe enters, on every radiator including the towel rail. A slow weep evaporates on a hot radiator and leaves a rusty tide mark or a small stain on the carpet rather than a puddle.
• The towel rail in the bathroom. It is the youngest joint in most systems and the one most often fitted last, and it leaks at the blanking plug more than anything else.
• The automatic air vent inside the boiler. These fail by seeping, and the water then evaporates off warm metal, so the sign is corrosion or staining inside the case rather than water on the floor. That is behind the casing and not something to look for yourself.
• Buried pipework. Microbore under a solid floor, or pipes chased into a wall, produce a system that loses pressure with nothing visible anywhere. This is what a leak trace exists for, and the clue is often a warm patch of floor.
• The expansion vessel. If pressure swings widely — low when cold, up towards the red when hot — then the vessel has lost its air charge and the safety valve is dumping the excess every time the heating runs. That is why the system needs filling again next week, and adding more water makes the swing worse. Recharging or replacing a vessel is engineer work; it is done through a small valve like a bicycle tyre valve, with the system drained down.

Two things are worth knowing about the cost of ignoring it. Fresh mains water carries dissolved oxygen, and oxygen in a sealed heating system produces magnetite — the black sludge that blocks radiators, wears out pumps and eventually restricts the heat exchanger. And every top-up dilutes the corrosion inhibitor that was dosed into the system when it was installed, so a system topped up repeatedly needs re-dosing as well as fixing. A leak trace is £90 to £160 and costs a great deal less than either.

How often is too often

A sealed system in good order should hold its pressure for a year or more, and many hold it for several. Once at the start of the heating season, after work has been done, or after bleeding radiators is unremarkable. Twice in a winter is a system telling you something. Monthly is a leak, and weekly is a leak worth finding urgently, because the water is going somewhere and it is either into your building or out through a valve that is now part of the problem.

If the pressure is too high instead

The same gauge sends people the other way, and the remedy is different. Let water out at a radiator bleed valve with a cloth and a small container underneath, a little at a time, until the gauge reads 1.2 cold.

If it climbs again over days, there are two likely reasons and they are easy to tell apart. A filling loop not fully closed lets mains pressure creep in continuously, so close both valves properly and, on a braided loop, disconnect it. If the loop is definitely shut and the pressure still rises, watch what happens when somebody runs a hot tap: pressure climbing while hot water is being drawn on a combi points at the plate heat exchanger letting mains water cross into the heating circuit, which is a repair rather than an adjustment.

What it costs to have done

An engineer's visit to repressurise a system and check why it went down is typically £80 to £150, and most will look for the cause while they are there. Replacing an expansion vessel is usually £150 to £350 depending on access, and a pressure relief valve is a similar job. A leak trace on buried pipework runs from £90 to £250 depending on the method. Those figures are the reason this guide exists: the five-minute version is free, and knowing when the five-minute version is the wrong answer is what saves the rest.

What you should not do

Do not remove the boiler casing. On a gas boiler that is legally restricted to Gas Safe registered engineers, and on an oil boiler it is an OFTEC registered technician's work. Everything described here is done from outside the case.

Do not keep resetting a boiler that will not hold pressure. The lockout is protecting the most expensive component in the appliance, and overriding it repeatedly is how a £150 repair becomes a new boiler.

Do not add anything to the system through the filling loop. Inhibitor and leak sealer are dosed properly through a radiator or a dosing point by someone who knows how much the system holds, and leak sealer in particular is a decision to make deliberately rather than as a first response, because it can block the things you did not want blocked.

If your boiler is locked out on a code and topping up has not cleared it, the code is worth reading on its own terms — our boiler fault codes explained guide covers how the same conditions are numbered differently by each manufacturer. And if this happened in a cold snap with no water loss to explain it, read what to do when your condensate pipe freezes first, because a frozen drain locks a boiler out with the pressure perfectly normal.