Topping up a combi or system boiler takes five minutes once you know where the filling loop is. This is the hands-on version: what to turn, in what order, and what to watch for afterwards.
Gas Safe registered 25 years in the trade 12 years running the business
Written by Chris Gill, C Gill & Son Plumbing & Heating ·
Before you touch anything
Repressurising applies to sealed systems: combi boilers and system boilers with a pressure gauge on the front or underneath. If your heating has a small tank in the loft that feeds it, the system tops itself up and there is no filling loop to use.
Switch the boiler off and let the system cool. Pressure reads higher when the water is hot, so topping up a warm system can leave it too high once it cools and heats again. Have your boiler manual to hand if you can; the exact layout differs between makes.
Finding the filling loop
The filling loop connects the cold mains to the heating circuit, usually underneath the boiler. It comes in a few forms:
A silver braided hose between two valves, each with a lever or a slotted screw
A single tap or lever built into the underside of the boiler, common on many combis
A blue key or plastic lever that slots into the base of the boiler
A loop that has been unscrewed and left somewhere nearby, which needs reconnecting first
Step by step
Check the gauge and note where the needle sits. Open the first valve on the filling loop, then slowly open the second. You will hear water hissing into the system and see the needle climb.
Watch the gauge the whole time. Stop when it reaches the level your manual gives for a cold system, which on many boilers is around 1 to 1.5 bar. Close both valves fully, one after the other, then turn the boiler back on and reset it if the display shows a fault.
If the needle overshoots, you can bring it down by letting a little water out of a radiator bleed valve. Our guide to boiler pressure too high explains what to do if it will not come down.
Leaks around the filling loop
A drip from the filling loop after topping up usually means a valve was not closed fully or the hose washers have perished. Close both valves firmly and dry the joints. If it still drips, the washers or the loop itself need replacing.
If water appears somewhere else after you have topped up, such as under the boiler or from the small pipe through the outside wall, the extra pressure may have pushed water past a weak seal or opened the relief valve. Turn the boiler off and get it checked rather than topping up again.
How often is too often?
Needing to top up once or twice a year, perhaps after bleeding radiators, is normal. Needing to do it every week, or every day, means water is escaping from the system. Every refill also brings fresh oxygen into the pipes, which speeds up corrosion and sludge.
The reasons pressure falls, from leaking valves to a tired expansion vessel, are covered in our guide to a boiler losing pressure.
When to stop and call an engineer
Stop if the filling loop valves are seized, if the gauge does not move when water is going in, if the needle climbs on its own after you have closed the loop, or if you are unsure which parts you are turning. Never take the boiler casing off. Internal parts are work for a Gas Safe registered engineer.
Need it sorted? Book a boiler repair
If the pressure will not hold, call or WhatsApp 07429 632804 with a photo of the gauge and filling loop, read about our boiler repairs, or use the quote form.
Covering Neath, Port Talbot, Swansea and the surrounding area.
Common questions
Repressurising FAQs
Clear answers before you arrange a visit.
What is a boiler filling loop?+
A short connection, often a braided hose with a valve at each end, that lets mains water into a sealed heating system so you can raise the pressure.
How do I top up boiler pressure?+
With the system cool, open the filling loop valves slowly, watch the gauge rise to the level in your manual, then close both valves fully.
How do I increase boiler pressure if there is no filling loop?+
Look for a built-in lever or key under the boiler. If there is genuinely no loop, or a tank in the loft, call an engineer rather than improvising.
How do I repressurise a combi boiler?+
The same way as any sealed system: switch it off, let it cool, use the filling loop or built-in lever to reach the right pressure, close it, then restart the boiler.
