Water not getting hot

Lukewarm, cooling, or stone cold tells you the fault

The single most useful thing you can do before anyone attends is describe the pattern properly, because the pattern is the diagnosis. Engineers ask this first for a reason.

Hot at first and then cooling off after a minute or two means heat is being stored or made, and running out. On a cylinder that is a heating coil that is no longer transferring enough heat, a boiler that has stopped satisfying the hot water demand, or simply a cylinder that has been drained by an earlier bath. On a combi it usually means the appliance is modulating down or cutting out under a demand it cannot sustain.

Lukewarm from the very start, and staying lukewarm no matter how long you run it, is a different animal. Nothing is running out, because nothing got properly hot in the first place. That points at a temperature control set too low, a scaled or partly blocked heat exchanger, a diverter valve that is only half committing, or hot and cold getting mixed together somewhere in the pipework.

Stone cold, with the heating perfectly fine, means the hot water side alone has failed. On a combi that is the diverter valve or the plate heat exchanger. On a cylinder it is the coil, the motorised valve feeding it, the cylinder stat or the wiring that ties them together. If the heating is also off, you are not looking at a hot water fault at all, you are looking at a boiler that has stopped. The stone cold case with working radiators has a page of its own, no hot water but the heating works, which sorts a combi from a cylinder system before anything gets dismantled.

What to check yourself before you call anyone

A fair share of the calls we take are things that cost nothing to rule out and take two minutes. There is no point paying an engineer to turn a dial you could have turned, and any decent engineer will say so on the phone rather than let you book.

Do these in order, and run the hot tap closest to the heat source between each one, because the tap furthest away can be cold for reasons that have nothing to do with the fault.

If all of that is in order and the water still will not get hot, the fault is in the appliance, the cylinder or the pipework, and it is worth booking someone.

• The hot water temperature control. On a combi it is a dial or a digital setting separate from the heating temperature, and it gets nudged down by accident. On a cylinder the stat is a dial strapped to the side of the tank, usually a third of the way up, under the insulation.
• The programmer or timer. Hot water and heating are separate channels on most controls, and the hot water channel can be off or on a timed period that ended hours ago. Put it to constant and see what happens.
• Whether the heating works. This is the fastest split there is. Heating fine and hot water cold narrows the fault to one branch immediately.
• Your other hot taps. If the kitchen is hot and the bath is not, the appliance is fine and the problem is downstream of it.
• The immersion heater switch, if you have a cylinder. Some have a dedicated switch on the wall with a neon that tells you whether it is calling, and it may have been left off after someone reset a tripped breaker.

Do not open a cylinder, disturb the pressure relief valve or the tundish, or take the case off a boiler. Those are sealed for a reason and an unvented cylinder holds stored water well above scalding temperature under mains pressure.

Water not getting hot enough from a combi boiler

A combi makes hot water on demand, and there is no store to draw on, so every litre you run has to be heated as it passes through the appliance. That puts a hard ceiling on what it can do. Temperature and flow trade against each other: the more water you pull through per minute, the less the boiler can lift its temperature. A combi rated for a given output at a given flow will produce lukewarm water the moment you ask it for more flow than that, and running two outlets at once is exactly how people discover it.

So the first question on a combi that will not get properly hot is whether it is failing at all flow rates or only at high ones. Half open the hot tap and see. If a gentle flow comes out genuinely hot and a full flow comes out tepid, the appliance is working and you are simply at its limit, which in winter is entirely normal. Incoming mains water in London sits several degrees colder between December and March than it does in August, and the same boiler that was fine in summer is being asked to do noticeably more work for the same tap temperature.

If it is lukewarm at every flow rate, the usual culprit is the plate heat exchanger. This is a compact stack of stainless plates inside the boiler where heat crosses from the central heating water to the domestic hot water without the two ever meeting. The domestic side is fed by raw mains water, the channels through it are narrow, and in a hard water area they scale up. As the scale thickens, heat transfer drops and the passages restrict, so you lose both temperature and flow together. The classic pattern is a slow decline over a year or two, then a sharp complaint once it crosses the line from acceptable to unusable.

The other common cause is the diverter valve. When you open a hot tap, a combi has to stop sending hot water round the radiators and send it through the plate heat exchanger instead. That switch is made by a valve with a moving element, and the elements stick. A valve that has half stuck gives you hot water that is weaker than it should be, or hot water that only appears once the heating has been off for a while, or heating radiators that stay warm while you are running a bath. Fully stuck on heating and you get no hot water at all, which is worked through appliance by appliance on our page for no hot water from a combi boiler.

• Lukewarm only at high flow, hot at low flow: the boiler is at its output limit, and colder winter mains make it worse.
• Lukewarm at every flow rate: scaled or blocked plate heat exchanger, most likely.
• Weak hot water while radiators stay warm: diverter valve sticking part way.
• Boiler does not fire at all when a hot tap opens: flow turbine, sensor or the diverter not signalling demand.

Hot water not hot enough from a cylinder

A cylinder is a store, so the question changes from how fast heat can be made to how well it is getting into the tank and how long it stays there. Most cylinders are heated by a coil, a length of pipe coiled inside the tank carrying hot water from the boiler, with an immersion heater as a backup or a secondary method.

When a coil-heated cylinder produces water that is warm rather than hot, work back along the chain. Is the boiler actually firing for hot water, or only for heating? Is the motorised valve on the hot water circuit driving open, and can you hear the boiler and pump respond when the cylinder calls? Is the cylinder stat set where it should be and actually switching? The whole path from control to valve to boiler to coil has to work, and a failure anywhere in it gives you the same lukewarm result at the tap. The hot water tank not heating page walks that path in order, from the programmer to the motorised valve to the coil.

Stored water should be brought to at least 60 degrees. That is not an arbitrary comfort figure, it is the temperature at which Legionella bacteria are controlled in stored water, and it is why a cylinder that is being run at 45 degrees to save money is a bad idea rather than a clever one. What comes out of the tap can be, and often should be, lower than that. Many installations have a blending valve after the cylinder specifically to drop the delivered temperature to a safe level while the store itself stays hot.

If the tank is heating but the hot runs out very quickly, suspect the coil rather than the controls. A coil that has scaled on the outside, or a cylinder that has silted up around it, transfers heat slowly. You will get a top layer of usable hot water and then a rapid fall away, and recovery after a bath takes far longer than it used to. The same symptom appears when the boiler flow temperature has been turned down for efficiency on the heating side without anyone noticing that the cylinder depends on that same flow temperature to charge properly.

On an immersion heater the pattern is different again. The element either goes open circuit and gives you nothing, or the thermostat built into it fails and cuts out early. An immersion that heats only the top third of the tank is normal if it is a short upper element, and a fault if it is a full length one. How an element and its thermostat are actually tested is on the immersion heater not working page.

Stored hot water at 60 degrees will scald in seconds. If there are small children or elderly people in the property and no blending valve is fitted after the cylinder, have one fitted rather than turning the store temperature down.

Why the water goes lukewarm rather than properly cold

Lukewarm is the most misread symptom of the lot, because people assume it is a weak version of no hot water when it is very often a mixing problem. Cold water is getting into the hot pipework somewhere, and the hot you are making is being diluted before it reaches you.

The commonest route is a failed thermostatic mixer. A shower valve, a bath filler or a blending valve after a cylinder all contain a cartridge that mixes hot and cold to a set temperature. When the cartridge fails, or when scale jams it, it can sit permanently part open on cold. Worse, on many installations a failed mixer lets cold push back into the hot pipe under pressure, so every hot tap in the property goes lukewarm and the fault appears to be the boiler. If the hot water is fine until someone uses a particular shower or tap, or if disconnecting one mixer restores everything, that is what you have found.

The second route is a crossed connection, usually after someone has worked on the plumbing. A single tap plumbed the wrong way round or a flexible tail crossed under a basin will feed the hot system with mains cold whenever the cold side is at higher pressure. It is not rare in converted flats where the pipework has been altered more than once.

The third is simply an appliance running short of heat, which is where you end up once mixing has been ruled out. Lukewarm at every outlet, no mixer involved, and nothing in the pipework crossed means the heat input is genuinely too low, and you are back to the plate heat exchanger, the coil or the controls.

• One outlet lukewarm, the rest hot: that outlet or its mixer cartridge.
• Everything lukewarm, and it started after a shower or tap was fitted: crossed connection or a mixer passing cold back into the hot.
• Everything lukewarm, nothing changed recently, gradual onset: scale in the heat exchanger or coil.
• Everything lukewarm since the controls were adjusted: the temperature setting or the boiler flow temperature.

Limescale is the answer more often than not

Most of London sits on chalk, and the water is properly hard. Every time that water is heated above roughly 60 degrees, dissolved calcium comes out of solution and deposits itself on whatever surface is hottest. That is why scale forms in kettles, on immersion elements and inside heat exchangers rather than in cold pipes.

What makes it a diagnosis rather than a nuisance is where it lands. Scale inside a combi plate heat exchanger costs you temperature and flow at the same time. Scale on an immersion element insulates it, so the element runs hotter than it should, heats the water more slowly, and eventually fails early. Scale around a cylinder coil slows the charge rate so the tank never quite catches up with demand. In all three cases the complaint you arrive to is the same: the water is not as hot as it used to be, and it has been getting worse.

The tell is history. A fault that came on overnight is a component or a control. A fault that has crept up over eighteen months in a hard water area, with the shower noticeably weaker as well as cooler, is scale until proven otherwise. Some scaled plate heat exchangers can be chemically flushed, many are past that and are replaced, and an immersion element that has scaled badly is replaced rather than cleaned. Where a property is on a hard supply and the same problem has already happened once, fitting scale protection on the incoming main is the thing that stops it happening a third time. What scale does inside a cylinder, and what genuinely helps, is set out in our piece on hard water and hot water cylinders in London.

Hot at one tap and not at another: what that means

When the hot water is fine somewhere and not somewhere else, stop suspecting the boiler. Heat that reaches the kitchen sink is heat that was made successfully, and the fault is between there and the outlet that is failing you.

Aerators and flow restrictors in tap spouts silt up with scale and debris, particularly on basin taps, and a tap that trickles will also feel cooler because the water has more time to lose heat on the way. Unscrew the aerator, rinse it and put it back, and it is a genuinely common fix.

Shower valves deserve their own suspicion. A thermostatic shower has a cartridge with a wax or bimetallic element that fails gradually, and once it does, the shower runs cooler than the setting says and the control at the top of its travel does nothing. On an electric shower the fault is different again, because it heats its own water from cold and has nothing to do with your hot water system. Both cases, and the check valves that let cold cross into the hot side, are covered on our page for no hot water in the shower.

Water takes too long to get hot

Slow rather than cold is its own complaint. On a combi the delay is the volume of water sitting in the pipe between the boiler and the tap, plus the second or two the appliance takes to respond. A bathroom at the far end of a flat delivers a slug of cold before the hot arrives, and that is not a fault. It becomes one when the run has a dead leg left over from an old alteration, at which point you can run the tap for a minute and still get nothing better than warm.

On a cylinder, slow generally means slow recovery rather than slow delivery. If the tank empties into a bath and then takes hours to be usable again, the heat is going in too slowly: coil scaling, a boiler flow temperature set too low, a motorised valve that is only partly opening, or a cylinder that is simply undersized for how the household actually uses water. A family of four with one bathroom and a small tank will run out no matter how healthy every component is, and the honest answer there is capacity rather than repair. What counts as a normal reheat is set out on our page for how long hot water takes to heat up, which is worth reading before you assume anything has failed.

What the engineer checks, and in what order

The order exists so that each step clears a group of causes. Jumping to the interesting part is how a plate heat exchanger gets replaced on a system whose real fault was a blending valve under a bath.

It starts with the same evidence you would gather yourself, gathered properly. Temperature is measured rather than felt, at the nearest outlet and at the furthest, with the flow rate measured at the same time so the two numbers can be read together. A combi that lifts water by a good margin at four litres a minute and barely at all at twelve is telling you something quite specific.

Then the heat source itself. On a combi, whether it fires on hot water demand, whether it modulates up to full rate, what the flow and return temperatures do under a running tap, and whether the diverter valve is actually moving. On a boiler and cylinder setup, whether hot water demand from the cylinder stat reaches the boiler, whether the motorised valve drives fully open and makes its end switch, and what the primary flow and return temperatures across the coil are doing. A coil that is passing heat properly shows a meaningful drop between flow and return; one that has scaled shows almost none.

Last, the distribution: blending valves, mixer cartridges, any point where hot and cold meet, and the actual pipe routes. This is where the mixing faults live, and it is also where the fault turns out to be when everything upstream measures healthy.

• Delivered temperature and flow rate measured together at more than one outlet.
• Does the heat source respond to hot water demand, and does it reach full rate.
• Flow and return temperatures under load, at the boiler and across a cylinder coil.
• Diverter valve or motorised valve proved to be moving, not assumed.
• Cylinder heat distribution top to bottom, stat setting and switching, immersion tested at the element.
• Blending valves, shower and bath mixers, and any crossed connection in the pipework.

What is worth repairing and what is not

Most of what causes lukewarm water is a repair. Mixer cartridges, immersion elements, diverter valves, motorised valves, cylinder stats and blocked aerators are all parts, and fitting the right one cures the fault outright. A plate heat exchanger is a part too, and on a boiler that is otherwise sound it is well worth changing.

The picture changes when the same appliance has already had two or three significant parts, when the boiler is old enough that parts support is ending, or when a cylinder is leaking as well as underperforming. A cylinder that has corroded through is not repairable and no amount of work on the controls will change that. Where that is the situation you will be told plainly, with what has actually failed and why patching it does not stack up, and the decision stays yours.

There is one case with no choice attached. Any appliance or cylinder found to be immediately dangerous gets turned off and labelled, and you will be told exactly what was found. Unvented cylinders in particular are pressurised vessels with safety devices that are not optional, and work on them is carried out by an engineer qualified to do it.