Why is no water coming out of my hot tap but cold works?
Hot and cold water often reach your taps by different routes, so one can stop while the other carries on. No flow at all from the hot tap means water is not reaching it, which is a supply problem, not a heating problem. If water does flow but is only lukewarm, that is a different fault, covered in our guide to hot water not hot enough.
Start by working out which type of hot water system you have, because the causes are different for each.
| Your system | Where hot water comes from | Common causes of no hot flow |
|---|---|---|
| Combi boiler, no cylinder | Cold mains, heated as it passes through the boiler | Frozen pipe, closed inlet valve, blocked inlet filter, heavy scale |
| Cylinder with a cold water tank in the loft (open vented) | Cylinder fed by gravity from the loft tank | Empty loft tank, stuck float valve, airlock, frozen feed pipe |
| Unvented cylinder, no loft tank | Cylinder fed from the mains at mains pressure | Closed valve, blocked strainer or pressure reducing valve |
| Any system, one tap only | As above | Closed isolating valve, faulty tap cartridge, blocked aerator |
If you are not sure which you have, our guides to the combi boiler and the system boiler explain how to tell them apart.
One hot tap or all of them?
Try every hot tap in the house before doing anything else. The answer points you to the right section.
- One tap dry, the rest fine: the fault is at or near that tap. Go to the section on isolating valves and taps.
- All hot taps dry, kitchen cold fine: the hot supply as a whole has stopped. Check the loft tank if you have one, or the combi inlet if you do not.
- Hot taps dry and bathroom cold taps weak or dry too: on an open vented system, bathroom cold taps are often fed from the same loft tank, so an empty tank is very likely.
- Hot taps and all cold taps dry: the mains supply to the house is off. Check the stopcock and contact your water supplier about outages in your area.
Empty loft tank on an open vented system
In a home with a hot water cylinder and a cold water storage tank in the loft, the tank feeds the cylinder, and the weight of water from the tank pushes hot water out to the taps. If that tank empties, the hot taps run dry, while the kitchen cold tap, which normally comes straight from the mains, keeps working.
The tank refills through a float operated valve, often called a ball valve. If the float sticks, the valve wears out or the mains pipe feeding the tank freezes, the tank drains as you use water and does not refill. Take a torch and look in the tank if you can reach it safely. If it is empty or very low:
- Gently press the float arm down. If water starts to flow in, the float was stuck.
- If nothing flows in freezing weather, the feed pipe to the tank may be frozen. See the frozen pipes section below.
- If the valve still does not work, a plumber can replace it. It is a quick, inexpensive job.
If the cylinder may have drained, switch off any electric immersion heater until the supply is back. An immersion element heating in a partly empty cylinder can fail. Take care in the loft: stand only on boards or joists, and never lean on the tank.
Airlocks in the hot water pipes
An airlock is a bubble of air trapped in a pipe that stops water flowing. It is most common in open vented systems, where the pressure from the loft tank is too low to push the air out. Airlocks often follow a drained loft tank, work on the pipes or a pumped shower that pulled water faster than the tank could supply.
Signs include a hot tap that spits, splutters or hisses, then stops, or a tap that gave a trickle before going dry. Once the loft tank is full again, open each hot tap in turn, starting with the lowest, and leave it running for a minute. That often clears a small airlock on its own.
For a stubborn airlock, a common method described by DIY Doctor and other plumbing guides is to join a mains fed cold tap to the dry hot tap with a short hose, then open both briefly so mains pressure pushes the air back up the pipe. DIY Doctor warns that hot water cylinders cannot always cope with mains pressure and could rupture, so only do this for a few seconds at a time. If you do not know if your cylinder can take it, ask a plumber.
Frozen pipes
In cold weather, a frozen pipe is a likely cause of no hot water flow, particularly if the dry tap is on an outside wall or the pipes run through a loft, garage or under a suspended floor. On an open vented system, the pipe from the loft tank to the cylinder and the mains pipe feeding the tank are both exposed. With a combi, the cold mains pipe into the boiler is the one to check if the boiler sits on an outside wall or in a garage.
To thaw a frozen pipe, open the affected tap so meltwater can escape, then warm the pipe gently, working from the tap end back towards the frozen section. A hairdryer on a low setting, warm cloths or a hot water bottle all work. Never use a blowtorch or any naked flame.
Find your internal stopcock before you start thawing. Frozen water can split a pipe, and the leak only shows once it thaws. If water starts spraying, turn the stopcock off straight away and call a plumber.
Combi boiler: no water from the hot tap
With a combi, every drop of hot water passes through the boiler, so anything that blocks the water path through it can stop the hot taps completely while the cold taps, fed direct from the mains, carry on. The usual causes are:
- A closed cold water inlet valve. Most combis have an isolating valve on the cold mains pipe underneath. If someone has turned it off, or it was left part closed after a service, hot flow stops or slows to a trickle.
- A blocked inlet filter. Many combis have a small filter on the cold water inlet to catch debris. After mains work in the street, grit can clog it. Cleaning it is a job for a Gas Safe registered engineer.
- Heavy limescale. In hard water areas, scale can build up in the plate heat exchanger until very little water gets through.
- A frozen pipe into or out of the boiler.
The flow sensor is often blamed, but a sticking flow sensor usually gives you cold water at normal flow, because the boiler does not know a tap is open and never fires. That is a no hot water fault, covered in our guide to no hot water but heating works. If there is no flow at all, look for a blockage first. The boiler pressure gauge will not help here either: it shows the pressure in the heating circuit, not the water supply to your taps, as our boiler pressure guide explains.
Unvented cylinders
An unvented cylinder is fed straight from the mains and has no loft tank. The cold inlet passes through a group of controls, usually including a line strainer and a pressure reducing valve. If the strainer clogs or the valve fails, hot flow can drop to almost nothing. Building Regulations Part G requires unvented systems to be installed by a competent person, and cylinder manufacturers' manuals ask for the inlet controls to be maintained by a qualified installer, so call a plumber or heating engineer with an unvented hot water qualification rather than taking them apart.
One tap only: isolating valves and cartridges
If only one hot tap is dry, check the small isolating valve on the hot pipe under the basin, sink or bath. It usually has a screwdriver slot. When the slot is in line with the pipe the valve is open, and when it is across the pipe it is closed. These are often closed during plumbing work and not fully reopened.
If the valve is open, the tap itself may be the problem. Modern mixer taps use ceramic cartridges, and a failed hot side cartridge can block flow completely. Unscrew the aerator on the spout to check for grit. A replacement cartridge must match the tap make and model.
When to call a plumber or engineer
Call a plumber if the loft tank will not refill, an airlock will not clear, a frozen pipe has split or the fault is in an unvented cylinder's controls. Call a Gas Safe registered engineer if you have a combi, the valves under it are open, there is no frozen pipe and the hot taps are still dry, because the blockage is inside the boiler. Tell them whether the cold taps in the bathroom still work and when the problem started, as that narrows the cause straight away.