Read this part before you call anyone
We are a leak company, and the honest answer to "no hot water" is that it usually has nothing to do with a leak.
The common causes are electrical or mechanical, and no water leaves the system at all: a programmer on the wrong schedule, a failed cylinder thermostat, a motorised valve that no longer swings across. On a combi, a stuck diverter valve, a scaled plate heat exchanger or a flow sensor that never tells the boiler you opened a tap. On an electric cylinder, a burnt-out immersion element or a tripped thermal cut-out. None of those need a detection visit.
This page is about the smaller group where losing water is why the hot water stopped. Those cases are real, expensive if left, and easy to tell apart if you know what to look at.
The two-minute check
Work down this in order and stop at the first line that matches. One question: has the system lost water, or has it just stopped heating what it still has?
| What you find | What it points to |
|---|
| Heating works normally, hot water does not | Almost never a leak. A diverter valve, a motorised valve, a cylinder stat or a programmer. A heating engineer, not us. |
| Pressure gauge below about 0.5 bar, or at zero | The boiler has locked out on low pressure, so nothing gets heated. Leak-shaped. |
| Boiler fires but the water stays lukewarm | Usually a coil, a crossover or a secondary return. Sometimes a leak. |
| Dripping into the tundish, or a wet outside discharge pipe | Water is being lost, continuously. Not a pipe leak, but it needs attention today. |
| Cold taps weak, or the loft tank is empty | A stuck float valve or a leak upstream. The cylinder has no cold water to heat. |
| Hot water runs rusty and heating pressure keeps moving | A failed cylinder coil letting the two sides mix. |
| Everything off and the water meter still turning | Something is running somewhere. A leak until proven otherwise. |
If the first row matched, you are done here and you have saved yourself a call-out. Any other row, and the rest of this page is about your fault.
Case one: the system dropped pressure and locked the boiler out
A system boiler heats a cylinder by pumping hot water through a coil inside it; a combi heats on demand through an exchanger. Either way, if the sealed heating circuit falls below its minimum pressure the boiler will not fire, and hot water and heating go together.
Most boilers cut out somewhere below 0.5 bar and post a low-pressure fault code. A healthy sealed system sits at 1.0 to 1.5 bar cold and should not need topping up more than about twice a year. If you have been refilling weekly, the lockout is not the fault. It is the symptom you finally noticed.
Topping it up brings the hot water back for a few days. That is the trap. Every refill puts fresh, oxygenated mains water into the pipework, which corrodes it from the inside, and the water you keep replacing is going somewhere. Repressurising is a five-minute job; working out why it keeps happening is not.
Where these losses turn up, in rough order: radiator and towel rail valve spindles, pipework inside the boiler case, a relief valve dumping quietly to the outside discharge pipe, and pinholes under a screed floor. The last shows no damp for months, because the screed absorbs it and the heating dries it between cycles. See pressure dropping with no visible leak for how that gets traced.
One pattern looks identical and is not a leak. If the pressure climbs high while the heating runs and falls below normal once it cools, the expansion vessel has probably lost its air charge. Nothing is escaping, and that is worth ruling out before anybody lifts a floorboard.
Case two: the cylinder coil has failed inside
An indirect cylinder keeps two bodies of water apart. Heating water circulates through a sealed coil; the water you wash in surrounds it and never touches it. When that coil corrodes through, the two mix, and the direction depends on which side is at higher pressure.
On an unvented cylinder
The stored water is at mains pressure, higher than the heating circuit, so mains pushes into the heating side. The signs run backwards from what people expect: boiler pressure that rises rather than falls, a relief valve discharging outside every time the system heats, radiators going noisy or airlocked. The boiler locks out on high pressure, not low.
On a vented cylinder with a loft tank
The heating side is at low static head, so it runs the other way. The feed and expansion tank in the loft fills and overflows, sometimes continuously, and the hot water can run discoloured because heating water carries rust and inhibitor with it. Discoloured hot water plus a permanently overflowing loft tank is close to diagnostic.
A failed coil is not repairable — the cylinder is replaced. What matters is proving it first, because both pressure symptoms get blamed on other things. Isolating the cylinder and pressure-testing each side separately settles it in under an hour, and that is worth having on paper before you buy a cylinder.
Case three: the cold water side has emptied
A cylinder can only give you as much hot water as it is fed. On a vented system that feed is a cold water storage tank in the loft, and it empties two ways: the float valve sticks shut, or the tank or its feed leaks.
Loft tank leaks do the most damage before anyone notices, because the loft is the place nobody looks. A weeping tank connector drains into insulation, which holds it until it reaches the ceiling below. By then the hot water has been poor for weeks and been blamed on the boiler. Go up and check the level: hot water that runs out far faster than it used to, with weak cold flow at the bathroom taps, points at the tank rather than the cylinder.
Case four: a leaking or short-circuiting secondary return
Bigger houses, converted flats and most commercial buildings run a secondary return: a small pump keeps hot water circulating in a loop so it arrives at distant taps without a wait. It has two failure modes that both read as "no hot water".
- A leak on the return leg. The loop is hot pipework, often buried or boxed in, and it runs constantly. A pinhole drains hot water out of the cylinder all day. The cylinder never stops reheating, the tap runs lukewarm, and the bill rises on both fuel and water. This one is our job, and it gets misdiagnosed as an undersized cylinder more than anything else on this page.
- A passing valve letting cold across. A failed non-return valve or a worn thermostatic mixer cartridge lets mains cold push into the hot side. You get cold out of the hot tap at one outlet, or a cylinder that never reaches temperature. Nothing leaves the building — it leaks sideways, between circuits — so a water meter test shows nothing.
Those need different people. The first is a trace and a repair. The second is a valve replacement.
Water that is being lost but is not a leak
Two things on a hot water system throw water away by design when something else has failed, and both get reported as leaks.
The first is the tundish under an unvented cylinder — the small open funnel that lets you see a discharge. If it drips, either the expansion vessel has lost its charge or the temperature and pressure relief valve is passing. Building Regulations Approved Document G sets out that discharge arrangement precisely, so that a cylinder fault announces itself visibly rather than quietly. It is not a leak in your pipework, and it needs a G3-qualified engineer, not a detection visit.
The second is condensate. Every condensing boiler makes acidic water that runs to a drain. A blocked or frozen condensate pipe backs up, trips the boiler out and puts water on the floor under it. Usually a five-minute fix, and free to check first.
What we do, and what we will tell you not to pay for
We trace and repair leaks, and we put the floor, ceiling or wall back afterwards. We do not fit diverter valves, immersion elements, programmers or plate heat exchangers, and we will not pretend a control fault is a leak to win a job.
If the check above landed on row one — heating fine, hot water not — say so on the phone and we will point you at a heating engineer. If it landed on a pressure loss, a coil, a loft tank or a secondary return, that is a detection visit: the circuit is isolated and pressure-tested to prove the loss and size it, then traced with thermal imaging and, on a drained circuit, tracer gas, so the floor is opened once at a marked point rather than searched blind.
The rate is £150 per hour, agreed before we attend, with no call-out fee. If we do not find the leak, you do not pay for the time we spent looking. The visit includes a written report prepared to support a trace and access claim — your insurer decides what the policy covers, but the report gives the adjuster what they ask for. Detail on pricing, and the repair side on boiler and heating leak repair.
If every tap runs hot and only the shower does not, the fault is in the shower valve rather than the system: see no hot water in the shower.
