Hot water goes cold too quickly: how to find out why

Measure it before you phone anyone

"It goes cold too quickly" is not something anyone can act on. Ten minutes with a bucket and a clock turns it into something that is.

1. Leave the hot water alone for two hours so the cylinder is fully charged.
2. Measure the flow rate at the bath tap. Run it fully hot into a bucket of known size and time the fill. Ten litres in forty seconds is fifteen a minute.
3. Run the same tap continuously, fully hot, and note the minute it stops feeling properly hot.
4. Multiply. Flow rate times minutes is the usable hot volume you got.
5. Note whether the fall is gradual or sudden. This decides more than the total does.

Then read the capacity off the data badge near the top of the cylinder. A 210 litre cylinder giving 60 litres of usable hot water is not too small. Most of the water in it is cold.

Turning litres into a verdict

Usable volume is always larger than the cylinder, because stored water is hotter than the water you wash in. Stored at 60°C and blended with mains cold at roughly 10°C for a 40°C bath, each stored litre makes (60 − 10) ÷ (40 − 10), about 1.7 litres. Store at 50°C and the sum gives 1.3 — a fifth less bath for a ten degree change you would not notice by hand. That is why a drifted thermostat produces a complaint out of proportion to the fault.

What the pattern rules out

What you observed	What it points at
First draw-off as long as ever, but the next one is hours away	A recovery problem, not a storage one. Scaled coil, or too short a heating schedule.
First draw-off short, temperature sliding down through it	The cylinder is mixing. Broken cold-feed baffle, or a secondary return stirring it.
First draw-off short, then the temperature falls off a cliff	A small volume of hot sitting on a lot of cold. Classic failed lower immersion.
Volume is fine, but the tundish or overflow drips	Hot water leaving the cylinder on its own. Expansion vessel or pressure relief valve.
Volume short and the boiler pressure keeps rising	The coil. Mains-pressure stored water pushed into the heating circuit.

The five causes, in the order they turn up

A dead lower immersion element

Many cylinders carry two immersion heaters, or one long one and a short boost. Heatrae Sadia list domestic immersions from 280mm to 914mm long, and the length is the point: heated water rises, it does not sink. A 280mm element near the top of a 1500mm cylinder heats what is above it and leaves the rest cold indefinitely. So when the lower element or its thermostat fails, nothing announces itself. The water is still hot. There is simply a third as much of it.

The test takes thirty seconds. An hour after a reheat, with nothing drawn, run your hand down the cylinder. Properly charged, it is hot to within a hand's width of the base. Cold from halfway down is a plumber, not a leak detection visit.

A scaled coil

In an indirect cylinder the boiler water runs through a coil and never mixes with the water you wash in. London water is hard, and hardness deposits on the outside of that coil exactly where the heat has to cross. Scale conducts poorly, so the coil goes on transferring heat, just more slowly each year.

This is why the timing pattern matters. A scaled coil does not shrink your first bath; it lengthens the gap before the second. If the volume is unchanged but the cylinder now needs most of an afternoon to be ready again, look at recovery rather than at leaks: coil condition, whether the zone or diverter valve fully opens, and how long the hot water programme actually runs.

A cylinder that has lost its stratification

Cylinders work because hot water floats. Cold mains enters at the base through a baffle that spreads it sideways and slowly, so it slides in underneath the hot rather than churning through it. When that baffle corrodes away or breaks off, a fast draw-off turns the cylinder into a mixing vessel. You do not run out of hot water; you run out of water hot enough, part-way through the bath.

The tell is that gradual slide, and that it is worse at high flow than low: fill at half the flow rate and you get noticeably more usable water. A failed element does not do that.

A secondary return that never stops

Larger houses and converted flats sometimes have a secondary circulation loop: a small pump pushes hot water in a circle past the distant bathrooms so nobody waits at the tap. On a timer, tied to the cylinder thermostat, it is a good thing. Running around the clock, or circulating by gravity because the non-return valve has failed, it bleeds heat into the pipework and feeds warm-but-not-hot water back into the cylinder, where it mixes what is stored.

Check it at six in the morning, before any tap has been used. If a pipe at the cylinder other than the boiler flow is warm after eight hours of nothing being drawn, something is circulating that should not be.

A cylinder that is simply too small now

Worth naming, because it is common and it is not a fault. Nothing broke; the household changed. A pumped shower, a replaced shower head or a second person washing in the same half hour all draw more stored water per use. If your measured volume roughly matches the badge, the cylinder is working, and the answer is scheduling or a larger one.

The two that are leak-shaped

A failed coil. A pinhole in the coil of an indirect unvented cylinder lets stored water at mains pressure, typically two to three bar, into a sealed heating circuit sitting at one to one and a half. Water moves from high to low, so the boiler gauge climbs rather than falls and the discharge pipe drips after every cycle. On an older vented cylinder it shows differently: the feed and expansion tank overflows, or the hot taps run discoloured because heating water is reaching them. If your gauge is moving the other way, that is a different problem — start at a boiler that keeps losing pressure.

A leaking secondary return. A secondary loop is a long run of pipe, often buried in screed, holding hot water continuously. More pressurised hot water passes through it than any other pipe in the building, and when it fails it produces exactly this complaint plus a warm patch and higher water use. That is a trace job: thermal imaging along the run, then tracer gas on the drained loop where thermal cannot separate the leak from the pipe's own heat. Where these loops fail and how each point is tested is set out in leaks on a secondary hot water circulation loop.

One more to rule out on an unvented cylinder, though it is a fault rather than a leak. If the expansion vessel has lost its air charge, every reheat pushes hot water out through the tundish and cold mains replaces it. A wet tundish is the symptom. Unvented cylinders are notifiable work under Part G, so that check belongs to someone qualified to open the system.

Four safe checks, and one thing not to do

• Measure a hot tap with a cooking thermometer after a full minute, and compare it to the thermostat setting.
• Feel the cylinder top to bottom an hour after a reheat.
• Look at the tundish and the outside discharge pipe. Both should be bone dry.
• Photograph the boiler pressure gauge now and again in a week. Rising and falling mean opposite things.

Do not turn the cylinder thermostat down to save energy. HSE guidance is that hot water is stored at least at 60°C and reaches 50°C at the outlets within a minute, with scald risk controlled at the tap by a blending valve rather than by storing cooler. Storing cooler also makes this problem worse, for the reason the arithmetic gives.

Have five things written down before anyone attends: cylinder size and whether it is vented or unvented; flow rate, minutes of hot, and the two multiplied; gradual fade or sudden drop; when it changed and what changed around then; whether the tundish is dry and which way the gauge is moving.

When this is not a leak, and not us

If your measured volume roughly matches the badge, nothing is leaking and nothing is faulty. If the cylinder is cold from halfway down, that is an immersion heater or its thermostat. If recovery is slow but volume is normal, that is a service question. We will say so on the phone rather than attend and charge you for it.

Call us when water is going somewhere you cannot see: a tundish that will not stop, a gauge that climbs on its own, a warm patch on a floor near a hot run, or a water bill that has moved with no change in how you live.

What a visit costs

£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 the leak, you do not pay for the time spent looking. A loss on a heating circuit or a cylinder coil sits under boiler and heating leak repair; a buried secondary return or another hot run is water leak repair. The visit includes a written report set out to support a trace-and-access insurance claim. What your insurer then covers is their decision, not ours.