A hot tap that splutters while the cold runs normally usually means air is travelling through the hot pipework, and that points to an airlock if the bursts started after the water was off, after plumbing work or after a loft cistern was refilled. A hot tap that runs dry while the cold is fine points instead to air sitting at a high point and holding the water back, if the cylinder is heating and the valves feeding it are open. Both are plumbing faults rather than something you put right at the tap itself.
Hot tap splutters or hot tap runs dry: which symptom points where
The two symptoms sound alike, but they behave differently at the tap and send a plumber to different parts of the system. Compare what your own hot tap does before you decide which one you have.
| Criterion | Hot tap splutters and gurgles | Hot tap runs dry while the cold is fine |
|---|---|---|
| What the tap does | Water arrives in bursts with air between them, and the flow surges then drops away again | Little or nothing comes out, while the cold tap on the same basin runs as normal |
| What the air is doing | Air is moving along with the water and passing out through the tap | Air is sitting at a high point of the pipe run and holding the water back behind it |
| Noise you hear | Gurgling or knocking in the pipework, and sometimes a gurgle at the boiler | Often silence, because nothing is flowing through the hot run at all |
| System most often involved | A vented cylinder fed from a loft cistern, where hot water runs at low head pressure | The same gravity fed layout, and most often the taps highest in the house |
| Other causes that look the same | Low flow on a combi, or scale and debris at the tap outlet | A stuck diverter valve, a closed valve, or a cylinder that is not heating at all |
| What putting it right involves | Plumbing work on the affected hot run once the cause has been confirmed | Plumbing work, plus checks on the cylinder, the controls and the valves feeding it |
Air gets into hot water pipework in ordinary ways. A loft cistern drawn down low can let air into the feed. Plumbing work leaves air behind in the pipes when the supply is turned back on. A pump that moves water faster than the tank can supply it will also draw air in.
The cold tap is your control in this test. If the cold splutters as well, the problem sits in the incoming supply and not on the hot side at all.
When a spluttering hot tap points to an airlock in the hot water pipes
A spluttering hot tap points to an airlock when the bursts of air began after the water was off, after work on the plumbing, or after a loft cistern was refilled. Air pushed into the system rises to the highest point it can reach, because it is less dense than water. From there it moves in slugs each time a tap is opened, which is what produces the coughing flow and the gurgle.
Upstairs taps usually show it first. They sit closest to the top of the pipework, where the air has collected. In a house fed from a loft cistern, hot water arrives with only the height of that cistern behind it, so there is little force available to sweep a pocket of air along.
Gurgling is not proof on its own. Boiler manufacturers list trapped air moving through the system as one cause of a gurgling sound, alongside low water pressure, too little water in the header tank and a frozen condensate pipe. Showers on the same gravity fed layout often swing in temperature while this is going on, and our guide on mixer shower low pressure on a gravity-fed system explains why that happens.
When the hot tap runs dry while the cold is fine
A hot tap that gives nothing while the cold runs normally means the flow is being held back, and air at a high point is one explanation, provided the cylinder is heating and the valves feeding it are open. At low pressure, a pocket of air filling the top of a pipe loop can stop the flow completely. There is usually no noise either, because nothing is moving.
Work out which taps are affected before anything else. If only the highest tap in the house is dry while the ground floor still runs hot, air at a high point fits the picture. If every hot tap is dry, look past the pipework, because a cylinder that is not heating or a closed valve gives the same result.
On a combi boiler, a dead hot tap is rarely air. The hot water comes straight off the incoming mains at mains pressure, so trapped air tends to be carried out with the flow. A stuck diverter valve is the more common reason there, and that is engineer work. If you are not sure which type of hot water system you have, our guide on how to tell whether your hot water cylinder is vented or unvented sorts that out first.
Why an airlock affects the hot tap while the cold tap still runs normally
Cold taps keep running because they are fed at mains pressure, while hot water in a vented system is driven only by the height of the cistern above the tap. A vented cylinder is open to the atmosphere through its vent pipe and is gravity fed from a cold water storage cistern, normally in the loft. The pressure at the tap comes from that head of water alone, and some homes need a pump to get a workable flow from it.
An unvented cylinder behaves differently because it is sealed and connected to the pressurised mains. Hot water leaves it at mains pressure rather than being limited by the head of an open system. Air is far more likely to be carried away with that flow, which is why airlocks are a vented system complaint more than an unvented one. Manufacturers still allow for trapped air in their guidance, by asking for an air release point near the unit on the primary return.
The vent pipe itself is the safety feature of an open vented hot water system. It must rise continuously and discharge into the cold water cistern, taking up expansion and balancing the pressure. If you are weighing up losing the loft cistern, our guide on whether you still need the cold water tank in the loft sets out what changes in the house.
What else stops the hot water when the cold supply is fine
Trapped air is only one reason for no hot water when the cold runs normally. On a combi, hot water failing while the radiators still heat often points to a diverter valve that has stuck, and replacing or freeing it is work for a Gas Safe registered engineer. Separate controls for heating and hot water are another frequent cause, and a hot water programme left off is more common than people expect. Our guide on no hot water but the heating still works goes through that split in more detail.
System pressure matters too. Pressure that has dropped too low can stop a boiler producing hot water, and the gauge on the boiler or in the airing cupboard normally reads around 1 to 1.5 bar when the system is cold. Where a cylinder has an immersion heater, the element warms the water around it up to the temperature set on its thermostat, so a failed element leaves you with cold water and no air anywhere. Water that runs hot and then cold again is a different pattern, covered in our guide on the combi boiler hot water going hot then cold.
There are four things a householder checks: the boiler pressure gauge, the thermostat and programmer settings, the reset button, and whether radiators need bleeding. Everything else on the hot water side is work for a qualified engineer, including the cylinder, the valves, the expansion vessel and the pipework runs themselves.
Who may work on a hot water system in England
In England, the Gas Safety (Installation and Use) Regulations 1998 as amended make it unlawful to carry out work on gas fittings unless you are registered with the Gas Safe Register. HSE states that work on gas fittings in a domestic property must be done by a registered engineer who holds the right qualification for that work. Anything that involves the boiler, its gas supply or its flue sits inside that rule.
Unvented cylinders carry their own rule in England. Requirement G3 of the Building Regulations 2010, explained in Approved Document G, says a hot water storage system with no vent pipe to the atmosphere may only be installed by a competent person. It also requires provision so the stored water cannot exceed 100 degrees Celsius, and so that any hot water discharged by a safety device is visible but safely carried away.
The water fittings rules add a third layer. In England, Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires every water system to contain adequate backflow prevention. It also requires an unvented water heater to have temperature control together with a temperature relief valve or a combined temperature and pressure relief valve. That is why clearing air from a hot water system is treated as plumbing work rather than a household task.
Spluttering hot tap or dry hot tap: which one to act on first
Treat it as a spluttering hot tap if water still arrives in bursts with air between them, the cold tap is normal and the water is hot. That combination is an air and flow fault in the hot pipework, and a plumber traces the run to find where the air is held.
Treat it as a hot tap that has run dry if nothing comes out at all. Start with whether the cylinder is heating and the hot water programme is on, because a dead hot tap turns out to be heat, a valve or a diverter far more often than it turns out to be air. Tell the engineer which taps are affected and which floor they are on, since that narrows the search considerably.
Where the same airlock keeps returning on an ageing gravity fed system, the question stops being the airlock and becomes the system. A free home survey and a written fixed price let you compare another repair against a change of system. If that conversation turns towards mains pressure hot water, our page on Megaflo and unvented cylinders in London explains what the change involves.
Sources
- Heatrae Sadia – A-Z glossary of heating terminology (vented, unvented, vent pipe, head)
- Heatrae Sadia – Megaflo Eco SystemFit installation manual (air release point on primary return)
- Worcester Bosch – My boiler is producing a gurgling sound: what could be the cause
- Viessmann – My boiler is working but I have no hot water
- Glow-worm – Why don't I have hot water?
- Baxi – Can a hot water cylinder work with an immersion heater?
- HSE – Domestic gas: frequently asked questions
- HSE – Gas safety for employers
- GOV.UK – Approved Document G: sanitation, hot water safety and water efficiency
- legislation.gov.uk – Water Supply (Water Fittings) Regulations 1999, Schedule 2