Hot Water Coming Out of the Cold Tap? Causes and Fixes
Why a cold tap runs warm or hot: hot water crossing through a mixer tap or shower valve, a cold pipe sharing space with hot pipes, or a warm loft tank. How to find which one you have and how each is fixed.
Hot water coming out of a cold tap is usually caused by one of three things. Most often, hot water is crossing into the cold pipework through a mixer tap or mixer shower whose check valves (small non-return valves) have worn out, so whenever the hot side has more pressure it pushes through into the cold. Second, the cold pipe may run alongside hot water or heating pipes, so the water sitting in it warms up and the tap runs warm for a few seconds before going cold. Third, in older houses the bathroom cold taps may be fed from a loft tank that gets warm in summer. Timing tells you which: warm for a few seconds points to the pipe run, warm that keeps coming back while someone uses a mixer points to crossflow, and warm bathroom taps with a cold kitchen tap points to the tank.
Key Takeaways
- Warm for a few seconds, then cold: the cold pipe runs next to something hot.
- Warm or hot on and off, especially with a mixer in use: hot is crossing into cold through a failed check valve.
- Only the tank-fed bathroom taps warm: the loft tank or its pipework is heating up.
- Crossflow is most common with combi boilers and unvented cylinders, where the hot water is at mains pressure.
- Cold water pipes should be installed so the water isn't warmed above 25°C where reasonably practicable.
Hot water coming out of the cold tap is usually caused by hot water crossing into the cold pipework through a mixer tap or mixer shower whose check valves have worn out. The other common causes are a cold pipe running next to hot water or heating pipes, which warms the water standing in it, and a loft tank that warms up in summer and feeds the bathroom cold taps. The quickest clue is timing. If the tap runs warm for a few seconds and then goes cold, it's the pipe run. If it runs warm on and off, especially while someone is using a mixer somewhere else, it's crossflow. If only the bathroom taps are warm and the kitchen cold is fine, look at the tank.
Which cause do you have?
Run each cold tap in the house in turn and note what happens. Start with the kitchen cold tap, which is normally fed straight from the mains.
| What you notice | Most likely cause |
|---|---|
| Warm for a few seconds, then properly cold | The cold pipe runs close to hot pipes, heating pipes, the boiler or an airing cupboard |
| Warm or hot on and off, sometimes very hot, often after a mixer or shower has been used | Crossflow through a mixer tap or mixer shower with failed check valves |
| Stays warm however long it runs | Crossflow that's happening constantly, or a cold supply that's picking up heat along a long run |
| Bathroom cold taps warm, kitchen cold tap fine, worse in summer | A cold water storage tank in a warm loft, or its pipes running through a warm space |
| Only one tap or one shower affected | That tap or shower valve, or the pipe that feeds only it |
Crossflow through a mixer tap or shower
A single-lever mixer tap or a mixer shower joins the hot and cold supplies inside its body. If the two supplies are at different pressures, the stronger one can push through into the weaker one whenever the mixer is open or left in a middle position. To stop this, mixers have check valves, small one-way valves on each inlet, either built into the tap's cartridge, the shower valve's inlet elbows or the flexible hoses. The Water Fittings Regulations require water systems to have adequate protection against backflow, and these valves are part of that.
Check valves wear out, and they get stuck open with limescale, which London's hard water supplies plenty of. Once they stop sealing, hot water can travel back along the cold pipe to other cold taps in the house.
Crossflow into the cold is most common where the hot water is at mains pressure, because the hot side can then match or beat the cold side as pressures change around the house. That means homes with a combi boiler or an unvented cylinder. Our guide to combi vs system boilers explains how each one supplies hot water. In older homes with a tank-fed hot supply and mains cold, crossflow usually goes the other way: cold pushes into hot, and the hot tap or shower runs lukewarm instead. If that's your problem, see no hot water in the shower.
How to find the mixer that's doing it
- Run a cold tap that has been running warm until you can tell whether it's warm now.
- Under each mixer tap, find the small isolating valve on the hot hose. Turn the screwdriver slot a quarter turn so it sits across the pipe to shut it.
- Close one hot isolating valve at a time and use the house normally for a while. If the cold taps stay cold with one particular mixer isolated, you've found it.
- Don't forget mixer showers, bath shower mixers and any thermostatic bar valve. Many bar valves have isolating screws in their inlet elbows.
- Reopen every valve you closed when you've finished.
The fix is to replace the faulty check valves, replace the cartridge that contains them, or fit single check valves on that mixer's hot and cold supplies. On a kitchen or basin tap, a new tap may be the sensible option if the cartridge isn't available. Our guide on how to replace a kitchen tap covers the hoses and valves involved, and our shower valve service covers bar and concealed valves. If instead one tap gives no hot water at all, see our guide to a hot water tap not working.
Cold pipes running next to heat
If the tap is warm for a few seconds and then runs cold, the water standing in the pipe has been warmed on its way. Common culprits are:
- A cold pipe running in the same boxing, floor void or wall chase as hot water or central heating pipes.
- A cold pipe passing through an airing cupboard, a boiler cupboard or next to a hot water cylinder.
- Underfloor heating or heating pipes under a kitchen or bathroom floor, with the cold supply running between them.
- A long cold run in a warm flat, where the water sits in the pipe for hours between uses.
The Water Fittings Regulations say a pipe supplying cold water to any tap should be installed so that, as far as reasonably practicable, the water isn't warmed above 25°C. Where a cold pipe has been run tight against a heating pipe, the fix is to insulate the cold pipe, separate it from the heat source or reroute it. Access is usually the bigger part of the job than the plumbing, especially under floors.
A warm loft tank
In many older London houses the kitchen cold tap is on the mains, but the bathroom cold taps, the toilet and sometimes the shower are fed from a cold water storage tank in the loft. In summer a loft can get very hot, and so can an uninsulated tank. Its pipes can also pick up heat where they run through the loft or an airing cupboard.
The regulations require storage cisterns to be insulated against freezing and undue warming, and to have a cover that keeps out light and insects. HSE guidance on legionella control aims to keep stored cold water below 20°C. A tank that's regularly warm, uncovered or has sediment in the bottom needs looking at. Our guide to cold water storage tank problems covers what to check, and our cold water tank service covers repairs and replacements.
Less often, a warm cold supply near a hot water cylinder can come from the cylinder's own fittings. On an unvented cylinder, a faulty valve in the inlet group can let expanded warm water back into the cold pipework. That's a job for an engineer qualified to work on unvented cylinders, not a DIY check.
Is the water safe to drink?
Run the cold tap until the water is properly cold before drinking it or using it for cooking. Don't drink water that has crossed over from the hot side, or water from a tank-fed bathroom tap. The regulations require every home to have at least one drinking water tap, and in most homes that's the kitchen cold tap on the mains. If the kitchen cold tap itself is running warm for more than a few seconds, get it looked at rather than living with it.
When to call a plumber
Call a plumber if you can't find which mixer is causing the crossflow, if the cold taps are running hot rather than just warm, if the kitchen cold tap is affected, or if the cold pipe needs insulating or moving under a floor or behind a wall. It's also worth calling if you think the loft tank is warm, dirty or uncovered.
We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our tap repair service covers mixer cartridges and check valves across London, and the pricing page explains how jobs are charged.
Cold tap running warm?
Tell us which taps are affected and what heats your water. We'll confirm the price before starting.
📞 Call 020 7870 3200Or see our tap repair service and pricing.
Frequently Asked Questions
Why is hot water coming out of my cold tap?
Usually because hot water is crossing into the cold pipework through a mixer tap or mixer shower with worn check valves. It can also be a cold pipe running next to hot or heating pipes, which warms the water standing in it, or a loft tank that gets warm in summer and feeds the bathroom cold taps.
How do I stop hot water crossing into the cold?
Find the mixer that's causing it by turning off the hot isolating valve under each mixer tap, or the hot supply to each mixer shower, one at a time and checking whether the cold taps stay cold. The fix is new check valves, a new cartridge, or single check valves fitted on that tap's hot and cold supplies.
Why does my cold tap run warm for a few seconds?
The water sitting in the cold pipe has been warmed by hot water or heating pipes running next to it, or by a warm space such as an airing cupboard or a boiler cupboard. Once that water has run through, the tap goes cold. Insulating or rerouting the cold pipe fixes it.
Is it safe to drink water from a cold tap that runs warm?
Run the tap until it is properly cold before drinking. Don't drink water that has crossed over from the hot side or come from a warm loft tank. Water should normally be taken for drinking from the kitchen cold tap, which is fed directly from the mains.
Can a combi boiler cause hot water in the cold tap?
The boiler itself rarely does, but with a combi the hot water is at mains pressure, close to the cold. Pressure changes as taps open and close mean a mixer tap or shower with a failed check valve can let hot water push into the cold side. Fixing the check valve on that mixer usually cures it.
Sources and further reading
Source links checked 28 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.