Warm Water From the Cold Tap? What Causes It and the Fix

The one minute test that sorts it

Run the affected cold tap into a jug and keep a hand under the flow. Then run it until the water is as cold as it is going to get, counting the seconds. Do the same at every other cold tap in the house, including the outside tap if you have one.

Warm for a few seconds and then properly cold is water that has been sitting in a pipe near a heat source. Warm for a minute or more, or never really cold at all, means warm water is being fed into the cold pipework rather than just picking up heat on the way. Which taps are affected is the second clue. Only the upstairs bathroom cold taps points at the loft tank on a tank fed system. The kitchen cold tap on its own points at a pipe run or at the kitchen mixer. Every cold tap in the house, worse after the hot water has been heating, points at the cylinder.

• Warm for a few seconds, then cold: heat pick up in the pipe run. Usually harmless.
• Warm for a long time at one tap or shower: crossflow through that mixer or shower valve.
• Warm at every cold tap, worst after the cylinder has heated: expansion on an unvented cylinder.
• Warm only at bathroom cold taps fed from the loft: the cold water tank is being heated.

Pipe runs that pick up heat

In a lot of London flats the cold main shares a boxing, a floor void or a service riser with heating flow and return pipes, hot water pipes and sometimes the boiler itself. Cold water that has sat still in that pipe for an hour takes on the temperature of its surroundings, so the first glass is warm. Heated floors, a pipe run through the airing cupboard and a cold feed routed past the back of a boiler are the common versions.

This is a pipe routing problem rather than a fault. The water fittings rules expect cold pipes to be kept away from heat or insulated where they cannot be, partly because warm drinking water is where bacteria grow. If the warm spell is short and the water then runs cold, lagging the cold pipe where it runs beside hot pipework is usually all it needs.

Hot crossing into cold through a mixer or shower valve

A mixer tap or a mixer shower connects the hot and cold supplies together inside its body. Whichever side has more pressure pushes water into the other one when the outlet is closed, unless there are check valves to stop it. Most modern mixers and thermostatic shower valves have them built into the inlets, and they wear, stick with scale, or were never fitted on older fittings.

On a combi boiler or an unvented cylinder, the hot side can sit at a higher pressure than the cold, especially at night when mains pressure in the street rises and falls. Hot water then creeps backwards into the cold pipe through that one fitting, and the cold tap nearest to it runs warm. The test is simple. Close the isolating valve on the hot supply to the suspect mixer, the small slotted valve under the sink or behind the shower panel, leave it an hour, then run the cold. If it now runs cold straight away, you have found it. The fix is new check valves or a new cartridge, and it matters for more than comfort, because the same fault lets stored hot water back into drinking water pipework.

Cold water coming out of the hot tap is the same fault in reverse

On a tank fed system the hot water is at low gravity pressure and the cold at the kitchen is at mains pressure. When a mixer or shower valve without working check valves joins the two, mains cold pushes into the hot pipework instead. The symptom is a hot tap that runs lukewarm or cold even though the cylinder is hot, often starting soon after a new mixer tap, a new shower or a kitchen refit.

The same isolation test finds it: close the cold supply to the new fitting and see whether the hot comes back at the other taps. If the whole cylinder is cold rather than just the hot taps, it is a different problem, covered in our guide to water not getting hot.

An unvented cylinder pushing warm water back along the cold

Water expands as it heats. An unvented cylinder cannot let that expansion go up a vent pipe, so it has an expansion vessel, a small tank with an air cushion, to absorb it. A check valve on the inlet group stops the expanded water going back into the mains. On most installations the cold for the house is also taken from the inlet group, after that check valve, so the pressure at the taps matches the hot.

When the air charge in the expansion vessel has gone, the expanding water has nowhere to go except back along that balanced cold pipework. Warm water from the bottom of the cylinder is pushed into the cold pipes a little further each heat up, and every cold tap on that supply runs warm for a while afterwards. The same flat vessel usually shows itself in other ways too: the pressure at the taps spikes after heating, and the expansion relief valve starts to drip through the tundish. If you have seen water in the tundish as well, our guide to a tundish dripping walks through what it means.

Recharging or replacing the vessel and checking the inlet group is unvented work, so it belongs with an engineer who holds the qualification for it. Fix it promptly: a relief valve that opens every day wears out.

If the warm water at the cold taps is actually hot, or the tundish is running with hot water or steam, turn off the immersion and the boiler feed to the cylinder and call an engineer. That is overheating, not expansion, and it should not be left.

A loft tank being heated on a tank fed system

Older houses with a cylinder in the airing cupboard often have a cold water storage tank in the loft feeding the bathroom cold taps, and a vent pipe from the cylinder that turns over the top of that tank. If the cylinder is overheating, or the heating pump is pushing water up the vent, hot water discharges into the cold tank. Over a day the tank warms, and the bath and basin cold taps run warm while the kitchen, which is on the mains, stays cold.

Take the lid off the loft tank and put a hand in the water, and look at the end of the vent pipe. Warm water, or water dribbling out of the vent, confirms it. The causes are a failed cylinder thermostat or immersion stat, which is overheating and needs sorting quickly, or a heating system problem where the pump pushes water over the open vent, which usually traces back to a partly blocked cold feed or the pump position.

When to call us

A cold tap that is warm for a few seconds is not an emergency. A cold tap that runs warm for a minute or more, warm cold at every tap after the cylinder heats, or a loft tank with hot water coming into it are all faults with a clear cause, and on an unvented cylinder they usually sit alongside a valve that is starting to wear. Hot water cylinders are our core work and we have 20 engineers across London, so we can trace which of these you have on one visit and put it right.

When you call, it helps to say whether you have a cylinder with loft tanks, an unvented cylinder with a tundish, or a combi, which taps are affected, and whether it is worse after the hot water has been heating. Our rate is on the pricing page and agreed before we attend.