Why gravity systems airlock and mains-pressure ones rarely do
In a vented, gravity-fed system the hot water is pushed by height alone. A cold water cistern in the loft feeds the cylinder, and the only thing moving water out of a hot tap is the vertical distance between the water level in that cistern and the tap itself. Roughly speaking, every metre of height is worth about a tenth of a bar.
That is not much force. A top-floor bathroom sitting two or three metres below the cistern is running on around 0.2-0.3 bar. A pocket of air trapped in a horizontal pipe run has to be pushed uphill by that pressure, and often it simply is not. The water stops, the air stays, and the tap gives you a cough and nothing else.
The same air in an unvented cylinder or a combi would be gone in seconds, because mains pressure is measured in bar rather than fractions of one. That is why an airlock is nearly always a story about a vented system, a loft cistern and the taps furthest from the head.
Clearing a genuine airlock
The crossover method
This is what most plumbers do first, and it works because it borrows mains pressure to do what gravity cannot.
- Find a mains-fed cold tap. In almost every house that is the kitchen cold, because the drinking water outlet is fed directly from the rising main rather than from the loft.
- Connect a length of hose between that tap and the hot tap that is airlocked. A washing machine hose with both ends clamped on tightly is the usual tool.
- Open the hot tap first, then the mains cold. Mains pressure now pushes backwards through the hot pipework, driving the air pocket up the draw-off and out through the vent pipe over the loft cistern.
- Give it thirty seconds to a minute. You will usually hear the air arrive in the loft.
- Close the mains tap first, then the hot, and take the hose off straight away.
One warning that is worth taking seriously. While that hose is connected you have joined the mains directly to stored water, which is exactly the kind of cross-connection the Water Supply (Water Fittings) Regulations exist to prevent. It is a temporary measure that comes off the moment the air clears. A hose left permanently coupled between the two is a backflow risk and a reportable defect, not a repair.
The drain-down method
If you would rather not cross-connect anything, the slower route is to empty the pipework and let it refill in the right order. Turn off the mains at the stopcock, open every tap starting at the top of the house and working down, flush the toilets, and let the whole lot run dry. Then close the taps, open them a crack, restore the mains, and bring them up to half open before opening fully. Air is pushed ahead of the returning water instead of being trapped behind it.
Before either: look in the loft
A surprising share of "airlocks" are a cistern problem. If the ball valve is sticking, or the cistern is silted up, or the cold feed outlet is partly blocked, the water level drops far enough that the feed pipe starts gulping air on heavy draw-off. Clear the airlock all you like — the cistern will make you another one next week. Look at the water level with nothing running, watch the valve refill, and check the outlet is not furred over.
The tell that matters: how long does it stay clear?
This is the whole diagnosis, and it costs nothing to run. Clear the air once, then wait.
| What happens next | What it means |
|---|
| Cleared once, stays clear | A one-off. Air got in during a drain-down, a repair, a new cylinder, or a mains interruption in the street. Nothing further to do. |
| Back after a few months | Worth watching. Often a cistern or a pump issue rather than a leak, but note the date each time. |
| Back every few weeks | The system is ingesting air continuously. Something is open to atmosphere, or water is leaving and being replaced. |
| Back within days, or radiators need bleeding weekly | Treat as an active leak until proven otherwise. Air and water are going in and out of the same hole. |
Air does not manufacture itself. A closed body of water that keeps acquiring air is either being fed air deliberately by a design fault, or it has a path to the outside world that water is also using. On the jobs where that second thing turns out to be true, the recurring air was the first symptom by a long way — months before a ceiling stained or a floor went soft.
Where a system actually draws air in
The negative-pressure side of a pump
This one catches people out because there is nothing to see. On the suction side of a shower pump or a circulator, pressure can fall below atmospheric while the pump runs. A joint there does not drip — it inhales. You can wipe that fitting dry all day and never find a drop, and it will still be feeding air into the system every time the pump starts. A failed pump seal behaves the same way.
Pumping over
The classic vented-system fault. If the pump is fighting an undersized or partly blocked cold feed, it lifts water up the vent pipe and discharges it back into the loft cistern. On the way round, the circuit takes air with it. You can often hear it: a trickle into the cistern whenever the heating runs. This is not a leak and no amount of leak detection will find one. It is a pipework and pump-position fix.
A cistern that keeps refilling
Stand in the loft when nothing in the house is running. If the ball valve hisses open on its own, stored water is leaving somewhere. Check the warning pipe outside first, because an overflow running quietly into a gutter is the most common and the least interesting answer. If the overflow is dry and the valve still refills, the loss is in the pipework.
A sealed circuit being topped up
Where the cylinder is heated by a boiler on a sealed primary, the same logic applies in a tighter form. Every top-up at the filling loop puts fresh, oxygenated mains water into the circuit. That water gives up its gas into the high points, so you bleed radiators more often, and the oxygen accelerates internal corrosion, which makes more pinholes, which means more topping up. If you are repressurising the boiler more than once or twice a year, the air in the radiators and the falling gauge are one fault, not two.
Worth separating out: gas that keeps appearing in radiators is not always air drawn in from outside. Corrosion inside an untreated system generates hydrogen, and a circuit full of black water can produce gas indefinitely without any breach at all. The difference is that a corroding system loses no water. A leaking one does. That is why the pressure gauge, or the cistern's refill habit, is the honest witness and the bleed valve is not.
Two checks worth doing before you call anyone
First, the meter. Close every tap, turn the stopcock off at nothing else running, and read the meter over a couple of hours. There is a catch specific to gravity systems that is worth knowing: a leak on the stored hot side only registers at the meter while the cistern's ball valve is actually open, so a short test can pass cleanly even though water is disappearing. Read it overnight instead. The method is set out in full in our guide to reading your meter to find a leak.
Second, write down the dates. Every time you clear air or top up, note it. Three dates tell an engineer more than any description of the noise, and if it ever becomes an insurance claim that list is the first piece of evidence anyone asks for.
What we do with it
If the pattern says ingestion, the work is not guesswork. The circuit is isolated and pressure-tested to prove the loss is real and to size it, the suspect runs are traced with thermal imaging, and tracer gas goes into a drained circuit where thermal cannot see. Pumps, vents and cold feeds get ruled in or out on the same visit, because on this particular fault the answer is as often a pumping-over pipe as a pinhole, and you should not be paying to open a floor for the first one.
We charge £150 an hour, agreed before we attend, with no call-out fee — the full basis is on our pricing page. If we do not find a leak, you do not pay for the time we spent looking. Where a leak is found, the visit comes with a written report prepared to support a trace and access claim; what your insurer then covers is their decision, not ours. Our own handymen and builders put floors, plaster and tiling back afterwards, so the hole we make is not left for you to solve.
And if the diagnosis is a badly positioned pump lifting water over the vent, we will tell you that, and it is a plumbing job rather than a detection one.
