No Hot Water: What to Check Before You Call an Engineer

First work out which system you actually have

Everything after this depends on the answer, and plenty of people get it wrong because the airing cupboard was boarded over years ago. Go and look. There are only four common arrangements in a London home and you can tell them apart in under a minute.

A combi boiler is a single wall hung unit with no cylinder anywhere in the property and no tank in the loft. It makes hot water on demand as you open the tap. A vented system has a cylinder in a cupboard, usually copper, fed by a cold water tank in the loft, and the hot pressure at a mixer feels weaker than the cold. An unvented system has a cylinder too, but it is a white or metal jacketed pressure vessel with a cluster of valves on the cold feed and a small open funnel called a tundish in the pipework above it, and hot and cold come out at similar pressure. An electric water heater or a heat interface unit in a flat is the fourth: no gas appliance involved, just a box on the wall fed from the building supply.

One more arrangement turns up constantly in purpose built blocks and is worth naming separately, because it looks like a cylinder from the outside and behaves like nothing else inside. A thermal store such as a Pulsacoil holds heated water in a vessel and passes your mains water through a coil inside it, so the controls, the sequence of operation and the failures are all its own. That is why Pulsacoil repair in London is treated as a job of its own rather than a variation on a cylinder fault, and why the checks further down this page apply to it only loosely.

If you find a cylinder, check whether the boiler heats it through a coil or whether the heat comes from an immersion heater screwed into the side. Most cylinders have both, and the reason that matters is simple: if the boiler side has failed you may still be able to get hot water from the immersion while you wait, and the other way round.

• No cylinder, no loft tank, one wall unit: combi.
• Cylinder plus a tank in the loft: vented.
• Cylinder with a valve cluster and a small funnel in the pipe above it: unvented.
• No gas appliance at all, just a box on the wall: electric heater or heat interface unit.

Five checks worth doing before you book anyone

These apply whatever is heating the water, and they are the ones most likely to save you a call out. Do them in this order because each one rules out a whole branch of the diagnosis.

Start with other outlets. Run the hot tap in the kitchen, then a bathroom basin, then the bath. If one outlet is cold and the rest are fine, nothing is wrong with your heating at all and you are looking at that tap or that shower. A blocked aerator, a failed thermostatic cartridge in a mixer, or a partly closed isolation valve under the basin will all produce cold water at a single point while everything else behaves. A shower in particular runs cold for reasons of its own, most of them inside the shower rather than anywhere near the boiler, and no hot water in the shower is worth reading on its own before you blame the appliance.

Then check whether the system knows you want hot water. Programmers and smart controls get nudged, batteries in wireless thermostats die quietly, and holiday modes get left on after someone goes away. Put the hot water on to constant rather than timed and give it a few minutes. If there is a cylinder thermostat strapped to the tank, check it has not been wound down to the bottom of its range by somebody chasing energy savings.

Then check the power and the fuel. The boiler or heater has a fused spur, usually nearby and often with a neon light, and that spur has a fuse in it. Look at the consumer unit for a tripped breaker or RCD. If you have other gas appliances, light a hob ring: if the hob will not light either, you have a gas supply problem rather than a boiler problem, and if you are on a prepayment meter, check the credit.

Finally, look at the appliance itself. A fault code or a red light on a boiler is a starting point worth writing down before you touch anything, because a reset wipes it. If the boiler display is blank altogether, that is electrical supply, not a boiler fault as such.

• Try more than one hot tap before assuming the system has failed.
• Set the hot water to constant and check the programmer is not in holiday mode.
• Check the cylinder thermostat has not been turned down.
• Check the fused spur, the consumer unit and, if applicable, the meter credit.
• Write down any fault code before you reset anything.

In cold weather, a boiler that has stopped with the condensate pipe frozen is common and easy to spot: the pipe is white plastic, runs outside, and the boiler often makes a gurgling noise before locking out. Thawing it with warm, not boiling, water and resetting the boiler is a legitimate thing to do yourself.

If you have a combi boiler, check the heating first

The question to answer here is whether the heating still works. Turn the heating on and put a hand on a radiator. If the radiators heat and the hot water does not, the boiler is running, the gas is fine and the fault is on the hot water side of it: usually the diverter valve that is supposed to send heat to the hot water plate instead of the radiators, the flow sensor that tells the boiler you have opened a tap, or a scaled up plate heat exchanger. None of those three are user serviceable and there is no point cycling the boiler at them. That one symptom, heating working while the hot water stays cold, narrows the fault down further than anything else you can observe from the tap, and it is worth following through properly for a combi. Our page on no hot water from a combi boiler takes that symptom apart valve by valve, including the checks that separate a diverter fault from a scaled plate exchanger.

If neither heating nor hot water works, the boiler is not firing. Look at the display. Low pressure on the gauge will lock many boilers out, and that has its own remedy which is worth reading up on separately. A fault code is more useful still, because the common ones point straight at a cause: ignition lockouts, flame loss and fan or air pressure faults all have distinct codes by manufacturer. Two resets are reasonable. Beyond that you are just relighting something that has already told you it cannot run safely.

One more combi check that costs nothing: run the hot tap fully open rather than at a trickle. Combis need a minimum flow, usually a few litres a minute, before the flow sensor triggers the burner. A partly closed service valve, a scaled aerator or a failing thermostatic shower cartridge can drop the flow below that threshold, and the boiler quite correctly does nothing. If the water goes hot when you open the tap wide, you have found it.

If you have a cylinder, check whether heat is getting in

With stored hot water there are two separate questions: is heat getting into the tank, and is the tank holding it. Feel the cylinder with the back of your hand after the system has had an hour to heat. Warm at the top and cold at the bottom is normal during a reheat. Cold throughout means nothing has gone in, and a hot water tank not heating at all has a short list of causes worth working through in order. Hot at the top when you have already used all the hot water means the tank is heating but not recovering fast enough for the household, which is a different problem entirely.

If nothing is going in and the boiler is running, the usual suspect is the motorised valve that opens to send hot water through the cylinder coil, or the cylinder thermostat that tells it to. Most motorised valves have a small manual lever on the side, and if the valve head has failed the lever will feel loose or will not hold. That is a useful observation to pass on, but changing the head is work for an engineer because it means draining down or isolating properly.

On an unvented cylinder, check the tundish, the small open funnel in the pipe above the valves. It should be dry. Water running through it means a valve is discharging, which is a fault in its own right, separate from the missing hot water, and worth chasing down quickly. On a vented cylinder, check the loft tank is filling: if the ball valve has stuck or the tank has frozen, the cylinder empties and the hot taps splutter rather than simply running cold.

Airlocks are worth ruling out on vented systems too, particularly after any work on the pipework or after the loft tank has been allowed to run dry. The sign is hot water that spits and stops rather than flowing cold and steady.

• Cold all the way down: no heat going in.
• Hot at the top only, an hour after heating: partial reheat in progress.
• Spluttering hot taps on a vented system: air, or the loft tank is not filling.
• Water in the tundish on an unvented cylinder: a valve is discharging.

If an immersion heater does the heating, check the switch first

Immersion heaters are one of the few parts of a hot water system where the honest answer is that you can do most of the checking yourself before anyone comes out. An immersion heater that has stopped working fails in a small number of predictable ways, and the checks below settle most of them without anyone attending.

Check the switch first. Immersions are wired to their own switched spur, usually a large white switch with a neon, and that switch has a fuse. Many cylinders have two elements, a short one at the top for a quick boost and a long one lower down for a full tank, each with its own switch. People switch the wrong one, or leave the boost on a timer that ran out. If the neon lights but no water heats, the element or its thermostat has gone. If the neon does not light, you have a supply problem: the spur fuse, the breaker, or the switch itself.

The other classic is the element tripping the breaker the moment it is switched on. That is an element that has failed to earth, almost always because scale has cracked off it and it has overheated. In hard water parts of London this is the normal end of an immersion heater rather than an unusual event. Leave the circuit off and get it replaced rather than resetting the breaker repeatedly. An immersion heater repair is usually a single visit where the element is a standard length and there is headroom above the cylinder to withdraw the old one, and a longer one where a scaled element has to be cut out of a tank that has never been drained.

Immersion thermostats also have a manual reset button, a small stud under the cover, which trips on overheat. Resetting it once is reasonable. If it trips again, something is making the water overheat and that needs finding, not resetting.

Do not take the cover off an immersion heater unless the circuit is switched off at the consumer unit and you have checked it is dead. The terminals are live at mains voltage and sit directly against a tank full of water.

Why there is usually no hot water, in order of likelihood

If you want the short version before working through any of the above, the causes fall into three groups, and the order below is roughly how often each one turns up on a call.

The first group is controls and power, and it is by far the largest. A programmer knocked off constant, a holiday mode left running after a trip, a dead battery in a wireless thermostat, a cylinder thermostat wound down by somebody chasing a saving, a tripped breaker, a blown fuse in a spur, a prepayment meter out of credit. None of these is a fault in the heating system. All of them feel exactly like a failed system when you stand at the tap.

The second group is a single failed part in the middle of an otherwise healthy system. A diverter valve on a combi that no longer moves across, a motorised valve on the cylinder circuit that has stopped opening, a flow sensor that does not register a tap being opened, an immersion element that has failed to earth. The common giveaway across all of them is that the heating still works while the hot water does not. That pattern is common enough to have a page of its own, no hot water but the heating works, because a single observation at the radiator rules out gas, ignition and most of the expensive possibilities in one go.

The third group is the heat source itself, and it is the smallest. A boiler locked out on a fault code, a boiler that will not fire at all, a cylinder that no longer holds the heat put into it. This is the group most people assume they are in when they pick up the phone, and it is the group they are actually in least often.

• Controls and power: the largest group, and free to rule out yourself.
• One failed part mid system: suspect it when the heating still works.
• The heat source itself: the smallest group and the most often assumed.

Where the checking stops and an engineer starts

The line is not about difficulty. It is about which faults hurt someone if they are got wrong. Anything involving gas, anything involving a sealed pressure vessel, and anything involving opening up electrics inside an appliance belongs to someone qualified for that work, and on an unvented cylinder that means an engineer holding the unvented qualification, because the safety devices on those systems are what stand between stored water above boiling point and a vessel failure.

Equally, stop if the symptom is getting worse rather than staying still. Water discharging from an outside pipe, a cylinder making a rumbling or kettling noise, scalding water at a tap, or a burning smell from the boiler or a spur are not checklist items. Isolate what you safely can and get someone out.

And stop if you have run the checks and everything passes. If the programmer calls, the boiler fires, the thermostat is up and there is still no hot water, you have narrowed it down for whoever attends, which makes the visit shorter. That is the point of doing this first.

• Any smell of gas: leave the property and call the national gas emergency line before anything else.
• Any water discharging from the tundish or the outside discharge pipe of an unvented cylinder.
• Rumbling, banging or kettling from a cylinder or boiler.
• Scalding water, or water much hotter than the thermostat setting.
• A breaker that trips again after you reset it once.
• Any fault code that returns after two resets.

We do not price a job from a description over the phone. The rate is fixed before we attend and the total depends on how long the work takes and what is found, so the better your description of the symptom, the closer the estimate you get.

What to have ready when you do call an engineer

The difference between a quick visit and a long one is usually information. An engineer who knows the make of the appliance can arrive with the likely parts on the van instead of returning the next day. Photographs sent ahead of the boiler display, the cylinder data plate and the valves around it are worth more than any description.

Say what changed as well as what is wrong. Faults that start after a power cut, after a cold night, after a bathroom was refitted or after the water was off in the street all point in different directions, and that context often names the cause before anyone opens a panel. Where the appliance itself is the remaining suspect and every check above has passed, boiler diagnosis is the visit to book, because the engineer then arrives expecting to test rather than to guess.

• The make and model from the boiler front or the cylinder data plate.
• Any fault code on the display, written down before resetting.
• Whether the heating still works.
• Whether it is every tap or one tap.
• What changed immediately before it stopped.
• Where the stopcock and the consumer unit are.