The row of pipes under a combi looks confusing, but each one has a single job. Once you know what they do and how to read a valve, you can check the positions in a minute. You only ever need to operate two things yourself: the filling loop, and a look at the gas valve to confirm it is open.
The pipes under a combi boiler, one by one
Most combis have five copper pipes in a row across the bottom, plus a plastic condensate pipe and a smaller copper discharge pipe. A common layout from left to right is heating flow, hot water out, gas, cold mains in and heating return, but the order varies between brands and models. The diagram in your installation manual shows the layout for your boiler.
| Pipe | What it does | Valve on it |
|---|---|---|
| Gas inlet | Brings gas from the meter to the boiler | Gas isolation valve, usually with a yellow or black lever |
| Heating flow | Sends hot water out to the radiators | Isolation valve |
| Heating return | Brings cooler water back from the radiators | Isolation valve, sometimes with a filter nearby |
| Cold mains in | Supplies cold water for the boiler to heat for your taps | Isolation valve |
| Hot water out | Carries hot water to your taps and shower | Usually none, or an isolation valve |
| Condensate | Plastic pipe carrying acidic waste water to a drain | None |
| Pressure relief discharge | Copper pipe that dumps water outside if pressure gets too high | None |
The filling loop is the link between the cold mains pipe and the heating circuit. It lets you add water when the pressure drops.
How to tell if a valve is open or closed
Valves under boilers turn a quarter of a turn between open and closed, so the rule is simple:
- Lever handle: in line with the pipe means open. Across the pipe, at a right angle, means closed.
- Screw slot: many water isolation valves have a slot for a flat screwdriver instead of a lever. Slot running along the pipe means open, slot across the pipe means closed.
- Filling loop taps: these may be small levers, knobs or slots, and follow the same rule.
Some valves are hidden behind a plastic cover or pipe shroud, which simply unclips. Do not remove the boiler's front casing to look.
Which way each valve should be left
| Valve | Normal position | Can you touch it? |
|---|---|---|
| Gas isolation valve | Open (in line) | Look only |
| Heating flow and return valves | Open | Leave for an engineer |
| Cold mains valve | Open | Leave for an engineer |
| Filling loop valves | Closed | Yes, only to top up pressure |
Engineers close the heating and water valves to work on the boiler without draining the whole house, and they should open them again when they finish. If one looks closed and you have no hot water or heating since a recent visit, ask the engineer to come back rather than turning it yourself.
If the gas valve lever sits across the pipe and you did not close it, do not simply open it: someone may have shut it for a safety reason. Call a Gas Safe registered engineer. If you smell gas, do not touch any boiler valve. Turn off the emergency control valve at the gas meter if it is safe to do so, open windows and call the National Gas Emergency Service on 0800 111 999.
The filling loop: built in or braided hose
There are two main types. An external filling loop is a silver braided hose with a tap at each end, joining the cold mains to the heating return under the boiler. An integral filling loop is built into the boiler and is worked with a lever, a knob or a removable key, depending on the brand. Our boiler filling loop guide covers both in detail.
To top up, open the filling loop valves slowly, watch the gauge rise to the level your manual gives (usually 1 to 1.5 bar cold), then close both valves fully. Many external hoses are meant to be disconnected after use.
A stiff filling loop valve is common because it is rarely used. Turn it gently by hand or with the correct key. Do not force it with pliers or a long lever, because a snapped valve can flood the floor. A seized loop is a simple job for a heating engineer or plumber.
What happens if a valve is in the wrong position
- Gas valve closed: the boiler tries to light, fails and shows an ignition lockout code.
- Heating valve closed: no heat reaches the radiators, and the boiler may overheat or show a circulation fault.
- Cold mains valve closed: the boiler has no water to heat, so the hot taps run dry or barely trickle while the cold taps work normally.
- Filling loop left open: mains water keeps pushing into the heating system, the pressure climbs past 3 bar and the boiler dumps water through the discharge pipe outside.
The discharge pipe and losing pressure
The small copper pipe running outside is there so the boiler can release water safely if the pressure gets too high. A filling loop left open, or one that lets water through when closed, is a common reason for it dripping. Our guide to the boiler pressure relief valve explains why it drips and what an engineer will check. Replacing that valve is an engineer job.
Pressure going the other way, falling over days or weeks, usually means a small leak on a radiator or pipe joint, water lost through that discharge pipe, or an expansion vessel that has lost its air charge. Recently bled radiators also drop the pressure. See our guide to a boiler losing pressure for how to narrow it down.
Combi, system and regular boilers, and brand differences
A system boiler has no cold mains in or hot water out, because it heats a cylinder instead. You will find heating flow and return pipes, a gas pipe, a filling loop, a discharge pipe and a condensate pipe, plus motorised valves nearby that direct water to the radiators or the cylinder. A regular (heat only) boiler fed from a tank in the loft often has no filling loop at all, since the tank tops the system up. Valve styles, colours and pipe order vary between Worcester Bosch, Vaillant, Ideal, Baxi and other makers, but open in line and closed across holds for all of them.
Anything beyond the filling loop and a look at the gas valve, including gas pressure checks, replacing any valve and working on the heating isolation valves, is a job for a Gas Safe registered engineer. For more on how combis work, see our combi boiler guide.