How to Repressurise a Boiler (Any Make): Find Your Filling Loop First

Which filling loop do I have?

A combi or system boiler runs on a sealed circuit, and the filling loop is the one place where mains water can be let into it. There are four common kinds, and the steps for one do not work on another. Shine a torch under the boiler and match what you see.

• Braided hose: a short silver flexible pipe between two small valves on the pipes below the boiler, each with a lever or a screwdriver slot. The hose may have been taken off, leaving two capped valves
• Key-operated link: a plastic filling port on the underside with no handle of its own. It needs a separate small key, which Worcester stores in the removable bottom panel of its boilers
• Lever link: a coloured lever you pull down and hold. Worcester’s blue keyless filling link and Baxi’s green Easy-Fill link are the two you are most likely to meet
• Built-in valve: a small knob or tap among the pipes at the front underside of the boiler, fitted on some Vaillant ecoTEC combis

Nothing under the boiler? A system boiler’s loop may be on pipework elsewhere, often in the airing cupboard by the hot water cylinder. A regular boiler fed by a small tank in the loft has no filling loop and nothing to top up, because the tank refills it. Never take the boiler casing off to look.

What should the gauge read before and after?

Read the gauge with the heating off and the radiators cold. Worcester Bosch asks for 1 to 1.5 bar on a cold system and Baxi for about 1.5 bar, so 1 to 1.5 bar is a safe target on most makes; your user guide gives the exact number for your model. Below 1 bar, top up. Many boilers stop firing and show a low-pressure code somewhere below about 0.5 bar, as an Ideal boiler showing F1 does when the pressure is too low to run safely.

The target is a cold figure because water expands as it heats. A hot system reads a few tenths higher than it will in the morning, so topping up to 1.5 bar while it is hot can leave it back under 1 bar once it cools. Letting it cool first also keeps your hands off hot pipework.

Overfilling is the opposite mistake. Go well above 2 bar cold and a hot system can climb towards 3 bar, where the pressure relief valve opens and lets water out through a pipe to the outside wall. That leaves you short of water again, and a relief valve that has opened can start to drip.

Some newer boilers show pressure as a number on the display instead of, or as well as, a dial. The target is the same. If a dial and the display disagree by more than a few tenths, have the gauge checked rather than guessing which is right.

How to repressurise a combi or system boiler, loop by loop

For every type: switch the boiler off at the controls, wait about an hour for it to cool, and put a towel under the loop. A few drops as you open or close a valve are normal. Watch the gauge the whole time, because strong mains pressure can push the needle up fast.

Braided hose. If the hose is off, screw it onto both valves by hand and check the rubber washers are seated. Open one valve fully, then open the other slowly until you hear water hiss into the system. At 1 to 1.5 bar, close both. A valve is open when its lever or slot lines up with the pipe and shut when it sits across it.

Key-operated link. Push the key fully into the port, lined up as the markings show, and turn it to lock it in. Then turn the filling knob, or the key itself on some models, to start the flow. Stop at 1 to 1.5 bar, reverse the steps and pull the key out. Our Worcester low pressure guide walks through the keyed Greenstar link in detail.

Lever link. Pull the lever down and hold it; water flows only while you hold it. Let go when the gauge reaches the target and the lever springs back to shut off. The Baxi Easy-Fill steps and its pressure codes are in our Baxi boiler pressure page.

Built-in valve. Open the knob or tap slowly until you hear water, watch the gauge, then close it at 1 to 1.5 bar. Close it firmly but not hard, because a plastic head can crack. The Vaillant pressure page shows where the ecoTEC valve sits, and the Glow-worm pressure page covers the loops Glow-worm boilers use.

Finish the same way on all four. Check nothing is dripping from the loop, switch the boiler back on and press reset once if it is showing a fault. Run the heating and look at the gauge again when the radiators are hot. A rise of a few tenths is expected.

A braided hose loop is only allowed as a temporary connection. Under the Water Supply (Water Fittings) Regulations 1999 its double check valve is accepted for filling, but the hose should be taken off afterwards so heating water cannot find its way back into the drinking supply. Lever links and built-in valves are designed to stay connected. Leaving a hose on also hides a slow leak, because a valve that lets by keeps topping the system up.

I have overfilled it. How do I bring it down?

If the gauge went past 2 bar while you were filling, check every filling valve is fully shut first. Then, with the heating off and the system cool, bleed a downstairs radiator. Open the bleed screw a quarter turn with a cloth underneath; any air hisses out, then water follows. Close it after a few seconds and look at the gauge.

A sealed system holds little water, so the needle can drop quickly. Go in short bursts until it reads 1 to 1.5 bar, and stop if it goes under. Do not open the pressure relief valve or loosen any fitting under the boiler to speed it up. Our guide on how to reduce boiler pressure covers the other safe ways, and what it means if the reading keeps rising after the loop is shut.

How often should I need to top it up?

A sealed system in good order loses very little water. Topping up once or twice a year, often after bleeding radiators, is normal. Once a month is a warning. Once a week or more means water is getting out somewhere, and topping up only treats the symptom.

Every top-up also brings in fresh, oxygen-rich water and dilutes the corrosion inhibitor in the system. That speeds up rust inside steel radiators and the black sludge that blocks pumps and heat exchangers, so frequent topping up costs you in the long run as well as hiding a fault.

Before you book anyone, check three places. Look at the copper pipe that leaves the boiler through the outside wall: drips or a wet patch under it after a top-up point to the pressure relief valve. Wipe a dry piece of kitchen roll around every radiator valve and towel rail joint. Look under the boiler for fresh drips or a white crust on the pipework. If all three are dry, the leak may be under a floor or inside the boiler, or a flat expansion vessel may be making the relief valve open each time the heating runs. Our guide to pressure dropping every day shows how to tell those apart.

What an engineer does if topping up will not fix it

On a boiler pressure repair the engineer starts with why you are topping up, not the top-up itself. They check the boiler gauge against a test gauge, drain the boiler side to zero to test the expansion vessel charge, and see whether the relief valve is letting water by. If the filling loop itself is at fault, a new braided loop with a double check valve is a small part on accessible pipework. A built-in valve or lever link usually takes longer, because the boiler has to be isolated and drained first.

Anything behind the boiler casing is a job for a qualified engineer. Gas work is carried out by Gas Safe registered engineers. The £60 call-out applies at any hour, and the repair price is agreed with you on site before any work starts. See how our call-out costs work.

If you smell gas at any point, stop. Get the windows open, turn the gas off at the meter if that is safe, do not use switches, and ring the National Gas Emergency Service on 0800 111 999 from outside.