Boiler Filling Loop Not Working

First, which kind of filling loop do you have?

There are three common types, and the fault finding is different for each. Work out which one you have before deciding it has failed.

The external braided loop is a silver flexible hose, usually under the boiler, with a small valve at each end. One end connects to the cold mains, the other to the heating return. Each valve is turned with a lever or with a flat screwdriver in a slot. When the slot or lever runs along the pipe, the valve is open. Across the pipe, it is shut.

Integrated loops sit inside or under the boiler. Many Vaillant ecoTEC plus combis have an integral filling loop with a small tap or lever underneath, near the front pipes, which you open to let water in. Worcester Bosch Greenstar boilers use either a keyed filling link, where a small removable key slots into the underside, or a keyless filling link with a blue lever you pull down to fill and release to stop.

System and regular boilers often have the loop well away from the boiler, near the cylinder in an airing cupboard. If you cannot find anything under the boiler, look there before deciding the loop is missing. Our guide to the boiler water refill point shows where to look on the main layouts.

Check it in this order before calling anyone

Everything below is outside the boiler casing and safe to try. Work down the list and stop as soon as the pressure starts to move.

• Open the mains-side valve fully first, then the boiler-side valve, slowly. Both must be open, and it is common to open one and forget the other
• Run the cold kitchen tap. If it is weak or dry, the stopcock is off or there is a supply problem, and no filling loop will work until that is fixed
• On a braided hose, check both nuts are tight and the rubber washers are in place. A hose that has been taken off and put back can be cross-threaded
• On a Worcester keyed link, check the key is fully seated and turned. Without the key, the link does nothing
• Tap the gauge glass lightly. An old needle can stick at zero or at one reading
• Watch for 30 seconds. A healthy loop moves the needle steadily, not in one jump

Never use pliers or grips on the valves. If one will not turn by hand or with a screwdriver, leave it. Forcing a seized valve can snap the spindle and leave you with water running from the mains that you cannot stop.

Why a filling loop stops letting water through

If the valves are open, the mains is fine and the gauge still does not move, the blockage is inside the loop. Every legal filling loop has a backflow device, usually a double check valve, that stops heating water flowing back into your drinking water. It is required because the Water Supply (Water Fittings) Regulations 1999 treat heating water as a contamination risk to the mains.

That check valve has a light spring and a rubber seal. Grit from the mains, limescale in hard-water areas and debris from a recent mains repair can jam it shut. London water is hard, and a loop that has sat unused for a year or two is more likely to stick.

On integrated loops the same thing happens to the filling valve inside the boiler. None of these can be cleared from outside. An engineer either strips and cleans the valve or, more often, replaces the loop or the internal filling valve.

When the loop works but the pressure will not hold

A different complaint gets reported as a broken filling loop. You open the valves, the needle climbs to 1.5 bar, and within minutes or hours it drops back. The loop has done its job. The water is leaving somewhere else.

Look first at the copper pipe that runs from the boiler through the outside wall. If it is dripping after you top up, the pressure relief valve is letting by. Then check under radiators, around valves and below the boiler. If there is nothing visible, the leak may be under a floor or inside the boiler. Our page on losing pressure with no visible leak covers how to track it down.

The opposite problem also happens. You close the loop and the pressure keeps creeping up for days. That usually means a filling valve is passing water even when shut, or on a combi, a leaking plate heat exchanger. On a braided hose, disconnect one end once both valves are closed. The regulations expect a temporary filling connection to be disconnected after use anyway, and doing so proves whether the loop is the culprit.

Can I use the heating while the loop is broken?

If the pressure is above about 1 bar and holding, yes. The filling loop only matters when you need to add water. A system that holds its pressure can run for months without it.

If the pressure is at or near zero, the boiler will usually refuse to fire and show a low-pressure fault. Resetting it will not help, because it has no water to heat safely. Our guide to boiler pressure at zero explains what that means and what not to try. There is no safe way to top up through a radiator valve or bleed screw, so do not try.

What the engineer does and what it typically costs

On a boiler pressure repair visit the engineer first confirms the mains pressure at the loop, then isolates it and checks whether water passes the check valve. If the gauge is suspect, they compare it against a test gauge on a drain point. Once the loop is replaced they refill the system to the cold pressure the manufacturer gives, usually 1 to 1.5 bar, bleed the radiators and look for the reason you needed to top up in the first place.

A new braided filling loop is an inexpensive part and a quick job on accessible pipework. An integrated filling valve inside a combi takes longer because the boiler has to be drained. Worcester sells its keyless filling link as a separate accessory, and it is a common upgrade when a key has been lost. Gas work is carried out by Gas Safe registered engineers.

The £60 call-out gets someone to you at any hour, and the repair price is agreed with you on site before any work starts. See our prices for how that works.