No hot water but the heating works

Why working heating tells you where the fault is

Every wet central heating system splits into two jobs at one point. Water is heated once, in the boiler, and then it either goes round the radiators or it goes to heat your hot water. Everything before that split is common to both. So when the radiators are getting hot you have already proved, without touching a tool, that the boiler is lighting, the fan and the flue are clear enough for it to run, the pump is moving water, the system pressure is high enough for the boiler to allow itself to fire, and the electrical supply and the earth are sound.

That is most of a boiler ruled out in one observation. The reason this fault feels so alarming is that nothing looks broken. No leak, no noise, often no code on the display, and the house is warm. The boiler is behaving exactly as designed while one component refuses to send the heat where you want it, which also means the repair is usually a single part rather than an appliance.

Combi or cylinder? Work out which one you have first

The two mechanisms behind this symptom have almost nothing in common, so the first thing to establish is whether your hot water is made on demand or stored. Go and look in the airing cupboard, the loft and the boiler cupboard.

If there is no cylinder anywhere and no tanks in the loft, and the hot tap normally runs hot indefinitely without a stored supply to run out, you have a combi. Everything on the hot water side lives inside the boiler casing, and the repair is a boiler repair.

If there is a tall insulated cylinder, usually in an airing cupboard or a hall cupboard, you have a system or a heat only boiler. Look at the pipework above the cylinder for one or two small boxes clamped to a valve body, each with a manual lever on the side. Those are the motorised valves, and they are the first thing an engineer will look at with this exact symptom. A cold water storage tank and a smaller feed and expansion tank in the loft means it is a vented cylinder. No tanks, a copper or stainless cylinder, and a small white plastic funnel with a pipe running outside means it is an unvented cylinder, which is pressurised mains water and is the type that needs a qualified engineer for anything beyond the controls.

In a purpose built flat, particularly a newer block, you may have neither. A heat interface unit or a thermal store such as a Pulsacoil takes heat or hot water from a communal supply and behaves differently again, with its own pumps and controls inside a cupboard rather than a boiler on the wall.

• No cylinder, no loft tanks, instant unlimited hot water: combi boiler.
• Cylinder plus two loft tanks: vented cylinder on a system or heat only boiler.
• Cylinder, no loft tanks, plastic tundish and a discharge pipe to outside: unvented cylinder.
• A cupboard unit with pipes in and out and no gas appliance: heat interface unit or thermal store.

Heating on but no hot water on a combi: the diverter valve

A combi has one heat exchanger and has to share it. When you open a hot tap, a flow sensor in the cold inlet notices water moving and tells the control board, which fires the burner and operates the diverter valve. The valve swings the heated primary water away from the radiator circuit and sends it through the plate heat exchanger instead, where it gives its heat to the mains water passing the other way. Close the tap and the valve springs back to the heating position.

The valve is a brass body with a spool or a cartridge inside it, moved either by a small electric actuator on top or by water pressure acting against a spring, depending on the model. It is a moving part sitting in hot water in a hard water city, and it fails in a predictable way. Scale and debris build around the spool and the O rings harden, the spool stops travelling its full stroke, and eventually it sticks. If it sticks in the heating position, your radiators carry on perfectly and your hot tap gives you cold water forever.

There are tells worth noticing before anyone arrives. If the boiler fires when you open a hot tap, and the radiators start warming up even though the heating is switched off, the valve is either stuck mid travel or stuck towards heating. If the boiler fires on hot demand and the water never gets beyond tepid no matter how long you run it, that points at the valve or at the plate heat exchanger. If the boiler does not react at all when you open the hot tap, no fan, no ignition, nothing, then the fault is more likely the flow sensor that is supposed to tell the board you asked for water, or the strainer in the cold inlet being blocked.

If there is a code on the display, write it down before you press reset. A repeated code and a boiler that has been reset thirty times tell an engineer two different stories. There is a longer walk through the same appliance, flow sensor and plate heat exchanger included, on our page for no hot water from a combi boiler.

When a combi fires but the water is only lukewarm

This variant is worth separating out, because water that is warm rather than cold usually means the heat is getting to the right place and simply not getting through fast enough. The plate heat exchanger is a stack of thin steel plates with mains water running through one set of channels and hot primary water through the other. The channels are narrow by design, which is what makes the transfer so efficient. It is also what makes them vulnerable in London, where the water is hard and every litre that passes leaves a little calcium behind.

A scaled plate heat exchanger produces a very specific pattern. The hot water is usable at a trickle and disappears when you open the tap fully, because at higher flow rates the water is through the exchanger too quickly to pick up much heat. Temperature that changes with how far you open the tap is the single most useful clue you can give an engineer on the phone, and it is something you can test in ten seconds. A stuck diverter valve, by contrast, tends to give the same disappointing result at any flow rate.

A few other things produce lukewarm rather than cold. A thermostatic mixing valve, fitted on some cylinders and on a lot of showers and basins, can fail and blend in too much cold. A scaled shower cartridge does the same thing in one location only. And if the boiler is cycling on and off rapidly while you run the tap, the problem may be circulation or a sensor rather than the hot water side at all, which is a different fault and a different fix. Water that is lukewarm at every tap rather than cold at all of them is its own diagnosis, and the page on water not getting hot separates the mixing causes from the heat input ones.

Do not try to cure a scaled plate heat exchanger by turning the boiler flow temperature to maximum and leaving it there. It masks the symptom for a while, accelerates the scaling, and raises the risk of scalding at the tap.

Cylinder systems: the motorised valve, the coil and the stat

With a cylinder, the boiler does not heat your bath water directly. It heats water that runs through a coil of pipe inside the cylinder, and that coil gives its heat to the stored water around it. For that to happen, something has to divert the boiler water into the cylinder branch when the programmer asks for hot water. That something is a motorised valve, and with this symptom it is where an engineer looks first.

Two arrangements are common. In an S plan there are two separate two port valves, one for heating and one for hot water, each with its own motor. In a Y plan there is a single three port mid position valve that can send water to the heating, to the cylinder, or to both. Inside the plastic head is a small synchronous motor that drives the valve open against a return spring, plus a microswitch that only closes once the valve has actually travelled, which is what tells the boiler to fire.

The failure modes here match the symptom exactly. On a Y plan, the valve rests in the heating position when it is not being powered, so a dead motor or a seized spindle leaves you with heating that works flawlessly and hot water that never arrives. On an S plan, the hot water valve can fail while the heating valve carries on. In both cases the boiler may not even fire on a hot water demand, because the microswitch never closes and the call never reaches it. A valve head that hums without moving, or a valve body so stiff that the manual lever will not slide across, is a valve that has failed.

If the valve is proving good, the next candidates are the cylinder thermostat strapped to the cylinder body under the insulation, which stops calling if it fails or if someone has turned it below the temperature of the water already in there, and the coil itself. A coil furred up with scale, or one with a sludge deposit around it, transfers heat slowly, so the cylinder takes far longer than it should and often only warms at the top. A coil that has genuinely blocked or split is a cylinder replacement rather than a repair. Where the cylinder is stone cold from top to bottom rather than simply slow, the page on a hot water tank not heating follows the same chain in more detail.

• Motorised valve motor failed, or the brass spindle seized in the heating position.
• Microswitch inside the valve head not closing, so the boiler never gets the hot water demand.
• Cylinder thermostat failed or set below the current water temperature.
• Coil scaled or sludged, so the cylinder heats slowly and only at the top.
• Airlock or a partial blockage in the cylinder primary flow and return.

Is the hot water channel on the programmer switched off?

This is the one that embarrasses people, and it should not, because it is genuinely common. A system with a cylinder has two independent channels on the programmer, one for heating and one for hot water. They can be set to completely different times, and one can be off while the other is on. That produces this exact symptom and costs nothing to rule out.

Power cuts are the usual cause. A lot of programmers, particularly older mechanical ones and cheap electronic ones without a backup battery, lose the time and revert to a default schedule when the supply is interrupted. The heating pattern happens to catch your evening and the hot water pattern does not, so you get warm radiators and cold taps and no explanation. Set the hot water to constant, wait, and see whether anything changes. If the cylinder warms up, your fault was a schedule rather than a component.

Wireless controls add their own version. A room thermostat or a programmer that has lost pairing with its receiver next to the boiler will keep controlling whichever channel is still talking. Flat batteries in the wall unit do the same. If the receiver has indicator lights, watching them while you force a hot water demand tells you immediately whether the call is even leaving the controls.

On a combi none of this applies to hot water. A combi makes hot water whenever a tap is opened, regardless of the programmer, and there is no hot water schedule to get wrong. If someone has told you to check the timer on a combi for a hot water fault, they have misunderstood the appliance.

If only one tap or only the shower has gone cold

Before blaming the boiler, establish whether it is the whole house or one outlet. Run the kitchen tap, then a basin upstairs. If one of them is hot, your hot water is being made correctly and the fault is local to the outlet that is not. Where that outlet is the shower, the causes are different enough that we have written them up separately under no hot water in the shower.

Showers are the most common single point failure, and they are their own subject. A thermostatic shower has a cartridge with a wax element inside that senses temperature and blends hot and cold. In hard water it scales, sticks, and fails safe towards cold. An electric shower has nothing to do with the boiler at all and runs off the cold mains with its own heating element, so an electric shower going cold while the taps are fine is an electrical fault in that unit.

A single cold tap can also be a failed flexible tail or a partly closed service valve underneath it. A tap that gives nothing but a dribble on the hot side is a supply problem at that tap, not a heating fault.

Use the immersion heater to prove where the fault is

Most cylinders have at least one immersion heater fitted, a straight electric element screwed into the side or the top, with its own switch nearby and usually its own fused spur. It exists so you have a way to make hot water when the boiler side is unavailable, and it is worth knowing where the switch is before you need it.

It is also the quickest test there is for splitting the fault in two. Switch the immersion on, leave it, and come back. If you get properly hot water from the immersion, the cylinder itself is sound, it holds water, and the problem is on the boiler side: the motorised valve, the controls, the pump or the coil. If the immersion produces nothing either, you are looking at the cylinder or its own electrical supply, and the immersion heater not working page covers how an element and its thermostat are tested.

Treat it as a short term measure. An immersion heats slowly and is expensive to run compared with the boiler, and an element in London water scales up until it fails, which is why so many houses discover theirs died long ago at the moment they need it.

If the immersion trips the breaker as soon as you switch it on, leave it off and do not keep resetting it. That usually means the element has failed to earth and it needs replacing before it is used again.

Flats with a heat interface unit or a thermal store

If you live in a block with communal heating, there is no boiler of your own to blame. A heat interface unit takes hot water from the building circuit and passes it through plate heat exchangers inside your cupboard, one for your radiators and one for your taps. The same split exists and the same logic applies: heating working proves the building supply is arriving, so the fault is on the domestic hot water side of the unit, typically a scaled plate, a failed actuator on the internal valve, or a stuck flow sensor.

A thermal store such as a Pulsacoil is a different arrangement again. It stores heated water and heats your mains water on the way to the tap, with its own pump and control board. On these, hot water gone while the heating is unaffected usually points at the pump or the board, and hard water shortens the life of the heat exchange parts noticeably.

One practical point before you call anyone in a flat: check with a neighbour or the building manager. If the communal supply is off or running cool, the whole block is affected and no work inside your flat will fix it.

What you can check yourself before calling anyone

Some of this genuinely is worth doing first, because it either fixes the problem or it makes the visit shorter and better informed. None of it involves opening an appliance. The same sequence written out as a straight list is in our no hot water checklist before you call.

If you have worked through all of that and the taps are still cold, the fault is a component, and it needs someone with test equipment rather than more guessing.

• Try more than one hot tap, so you know whether it is the whole house or one outlet.
• On a cylinder system, set the hot water channel on the programmer to constant and wait.
• Check the cylinder thermostat dial has not been knocked down, and that the room thermostat and programmer batteries are not flat.
• Find the manual lever on each motorised valve above the cylinder and see whether it slides freely, then put it back where it was.
• Switch the immersion heater on for an hour to see whether the cylinder will heat at all.
• On a combi, note whether the boiler reacts when you open a hot tap, and whether the water temperature changes when you open the tap further.
• Write down any code on the boiler display before you reset it, and note whether the same code keeps coming back.

Leave the casing on the boiler and leave the unvented cylinder controls alone. An unvented cylinder holds mains pressure hot water and the safety devices on it are not user serviceable.

Which branch of this fault you have, and where each one is written up

Everything above this point is common ground. Past it the fault splits into branches that have almost nothing to do with each other, and each branch has a page that goes further than there is room for here. It is worth knowing which one is yours before you spend an evening reading about a mechanism you do not have.

With no cylinder in the property, the whole hot water side lives inside the boiler casing, and the page on no hot water from a combi boiler takes the flow sensor, the diverter valve and the plate heat exchanger in the order they are actually tested rather than in the order they are usually guessed at. With a cylinder that is stone cold from top to bottom, the page on a hot water tank not heating follows the coil, the motorised valve and the cylinder thermostat as one chain, because on a cylinder system the electrical path and the water path have to be proved together.

Where hot water does arrive but arrives wrong, the branch changes again. The page on water not getting hot separates the faults that cut the heat going in from the ones that blend cold back in at the outlet, which is the distinction that decides whether the boiler is worth looking at at all. The page on no hot water in the shower is for the narrower case where one outlet has gone cold while the rest of the house is fine, and that is far more often a scaled cartridge or an electric shower with its own element than anything upstream of it.

Two more are worth having open while you work. The page on an immersion heater not working covers testing the one heat source that bypasses the boiler completely, which remains the fastest way to prove whether the cylinder itself is sound before anyone is paid to open anything. And the no hot water checklist before you call is this same sequence written out as a plain list, so that nothing gets skipped standing in a cupboard with a torch.

What an engineer does on the visit

The order is the same one you have just been reading, because it rules out the most for the least effort: establish which system it is, confirm the heating really does work, then force a hot water demand and watch what responds.

On a cylinder system that means following the demand electrically: is the programmer sending the call, is it arriving at the wiring centre, is the valve being powered, is the valve actually travelling, and does the microswitch close and tell the boiler to fire. A multimeter at the wiring centre answers most of that in a few minutes. Then the water side: flow and return temperatures across the cylinder primaries while the system runs, which shows whether heat is going into the coil and being absorbed or simply going round and coming back at almost the same temperature.

On a combi it means proving the flow sensor sees the demand, watching whether the diverter actuates, then measuring temperature in and out of the plate heat exchanger on both the primary and the mains side at a known flow rate. Those readings separate a stuck valve from a scaled exchanger without anyone having to buy both parts to find out.

Most of these faults come down to a single component, and the parts involved are stocked. The rate is fixed before we attend and the total depends on how long the job takes, so a fault that is proved quickly is a short visit. Where a repair is not sensible, for example a cylinder with a blocked coil or a boiler that is out of parts support, we will say so rather than fit something that will not last.