No hot water from a combi boiler

Check these three things before you call anyone

A fair share of the calls we take for no hot water are not faults. They are something on the wall or under the sink that anybody can check, and a decent engineer would rather tell you that than take the booking. Give these two minutes first. The same sequence written out as a straight list is in our no hot water checklist before you call.

Look at the pressure gauge on the front of the boiler. If the needle is sitting on or below the red band at the bottom, the boiler will refuse to fire for anything, hot water included, and it will normally show a low pressure code with it. That is its own separate subject and it points at a top up or at a leak somewhere on the system, not at the hot water side.

Then look at the controls. A combi should give you hot water regardless of whether the heating is timed on, but a programmer that has been knocked into a summer or off setting can be set to disable hot water entirely on some models, and a boiler in a holiday or frost protection mode will do the same. Check the display for anything that is not the normal standby screen.

Last, prove the gas. If you have a gas hob, light a ring. If the hob lights normally, the gas supply is fine and the fault is inside the boiler or on the water side. If the hob does not light, or lights weakly and goes out, the problem is the supply or a prepayment meter that has run out of credit, and no amount of work on the boiler will fix it.

• Pressure gauge on the front of the boiler: needle in the red means the boiler will not fire at all.
• Programmer, room thermostat and any summer or holiday mode on the boiler display.
• Gas at the hob. No gas at the hob means no gas at the boiler.
• The electrical supply: the fused spur beside the boiler and the breaker in the consumer unit.
• Whether it is every tap or only one. One cold tap is a tap problem, not a boiler problem.

If you can smell gas at any point, stop. Turn the gas off at the meter, open windows, leave the electrics and the boiler alone, and call the national gas emergency line on 0800 111 999 before you call a plumber.

Combi boiler no hot water but the heating works

This is the classic one, and it has a classic cause. A combi has a single heat exchanger heating the water that goes to your radiators, and a second, smaller plate heat exchanger that takes heat out of that water and puts it into your mains cold on its way to the tap. A component called the diverter valve decides which of the two gets the flow. Heating demand, and the valve sits one way. Hot water demand, and it swings over and sends everything to the plate. Hot water always takes priority on a combi, which is why the radiators go lukewarm while somebody is in the shower.

When that valve sticks on the heating side, you get exactly what you are describing. The boiler fires. The radiators heat beautifully. The taps run cold no matter how long you leave them, because the heat that is being produced is never being sent to the plate. The valve itself is usually either a brass body with a motorised head on top, or a cartridge with a diaphragm inside, depending on the make. The failure is normally mechanical: the shaft seizes in the body because the system water is full of debris and the valve has not moved far for months, or the rubber diaphragm perishes and no longer pushes the shaft across.

There is a tell that separates a sticking valve from an electrical fault. Ask somebody to open a hot tap while you stand at the boiler. If the fan speeds up and the burner lights, the boiler has recognised the demand and is producing heat, so the demand side is working and the heat is simply going to the wrong place. If nothing happens at all when the tap opens, the boiler has not seen the demand and you are looking at the flow sensor instead. The second tell is the radiators. If they warm up every time somebody opens a hot tap, even with the heating switched off at the programmer, the valve is stuck on the heating side and the boiler is pouring hot water round the circuit it should have closed off.

Sometimes the valve is fine and the motor or the actuator that drives it has failed, or the board is not sending it the signal. That is why this is not a part to buy on a hunch. The valve, the motor and the board all present as the same symptom from the outside, and on most makes the valve body means draining down and breaking into the pipework, which is a much bigger job than swapping a head off the top. If you are not certain you have a combi at all, the page on no hot water but the heating works sorts a combi from a cylinder system before you go any further.

• Heating hot, taps cold, boiler fires when a tap opens: diverter valve stuck or its motor failed.
• Radiators warm up whenever a hot tap runs, with heating off: valve is not closing the heating side.
• Boiler completely silent when a tap opens: not the valve, look at the flow sensor.

The flow sensor, and why the boiler never knows you opened the tap

A combi has no way of knowing you want hot water except by detecting that water is moving through it. That job belongs to a flow sensor, sometimes a small turbine with a magnet in it that spins as water passes, sometimes a flow switch with a diaphragm and a microswitch on older appliances. Water moves, the sensor reports it, the board calls for a burn and drives the diverter valve over. No signal from that sensor and the boiler simply sits there, because as far as it is concerned nobody has asked for anything.

The symptom is very distinctive: absolute silence at the boiler when a hot tap is opened. No fan, no click from the valve, no ignition, nothing on the display. The heating works perfectly because heating demand comes from a completely different route, the programmer and room thermostat wiring.

Turbine sensors fail in two ways. Scale and debris build up round the impeller until it cannot spin freely, which is common on hard water, or the sensor itself loses its ability to read the magnet. Either way the part is usually accessible on the cold inlet side of the appliance and the repair is not a heavy one. The diaphragm type fails differently: the rubber goes hard, stops moving under pressure and never presses the switch.

Before condemning the sensor, an engineer will prove there is actually enough flow reaching it. Every combi has a minimum flow rate below which it will not fire, often somewhere near two or three litres a minute depending on the appliance. A partly closed isolation valve on the cold mains feed, a blocked filter in the inlet, or a stopcock that somebody half shut years ago will drop the flow under that threshold and produce identical symptoms to a dead sensor. The order matters: prove the water first, then the electrics, then condemn the part.

Hot water lukewarm and never getting properly hot

If the taps are not stone cold but never get beyond tepid, and the flow out of the hot tap has quietly dropped off over the last year or two, the plate heat exchanger is the first suspect. This is the part that does the actual work of heating your tap water, and it is a stack of thin corrugated plates with your mains water running through one set of channels and the hot primary water running through the other.

London water is hard, and hard water is the enemy of a plate heat exchanger. Every time the boiler heats the mains water it drops a little calcium out of it onto the inside of the plates. The deposit builds slowly, narrows the channels and insulates the metal, so less water gets through and less heat crosses into it. That is why this failure creeps in rather than arriving overnight. People often describe it as the shower having got worse over a couple of winters before it finally stopped being usable.

There are two related tells. The first is that the hot flow rate has dropped while the cold has stayed the same, which points at a restriction on the hot side only. The second is that the boiler cycles while the tap runs, firing and dropping out repeatedly, because the water on the primary side is getting hot faster than it can shed its heat across the scaled plates and the boiler keeps hitting its overheat limit.

A scaled plate can sometimes be chemically flushed in place, but on a badly furred unit replacement is usually the honest answer, because a flush can remove enough scale to unblock a channel without restoring the surface. The related question is what caused it. If you are in a hard water area, which most of London is, and there is no scale reducer on the mains feed, the replacement part will go the same way in time. That is worth raising before the repair rather than after it. If the lukewarm water is at every tap in the property and not only at one, the page on water not getting hot separates a heat input problem from cold crossing back into the hot pipework.

Descaling chemicals sold for kettles and washing machines are not suitable for a sealed heating appliance. Putting the wrong chemical through a boiler can attack the seals and the aluminium heat exchanger, and that turns a repair into a replacement.

Low flow at the tap, not low pressure in the boiler

Two different things get called low pressure and they are worth separating, because only one of them has anything to do with the hot water.

The gauge on the front of the boiler reads the pressure in the sealed heating circuit, the water in your radiators. When that drops far enough the boiler shuts down completely, so you lose heating and hot water together. It does not cause a hot water only fault.

The one that does is incoming mains flow. A combi heats water on demand as it passes through, so the amount of hot water you get is limited by how much cold water the street can push into the appliance and how fast the boiler can raise its temperature. Weak mains, a partly closed stopcock, an old lead or narrow supply pipe, or a building where everybody is drawing water at once will all reduce the flow reaching the boiler. Below the minimum the appliance needs, it will not fire for hot water at all. Just above it, you get a thin trickle of very hot water and no useful shower.

This is common in flats and in converted houses where the mains has been extended and reduced a couple of times on its way to the kitchen. It is worth checking the full run: the stopcock under the sink, the isolating valves on the boiler inlets, any inline filter, and whether a service valve has been left part closed after previous work. If a tap somewhere in the property runs strongly off the cold mains but the boiler is starved, the restriction is between the two.

There is a separate case worth knowing: a property on a stored cold water tank rather than direct mains. A combi normally wants a mains supply, and a tank fed system will often not provide either the pressure or the flow needed to run one properly. If a combi has been fitted in place of a tank system without the supply being upgraded, no amount of parts will make the hot water behave.

No hot water and no heating either: what that rules out

If both sides are dead, stop looking at the hot water parts. Nothing that is specific to hot water, not the diverter valve, not the flow sensor, not the plate, can take your heating away with it. A boiler that gives you neither has either lost something it needs to run at all, or has locked itself out to protect itself.

The common causes are prosaic. No gas, because of a prepayment meter or an isolation valve. No electricity, because the fused spur beside the boiler has blown or the breaker has tripped. A frozen condensate pipe in cold weather, which is why call volumes for dead boilers spike in the first hard frost of the year: the pipe carrying acidic condensate outside freezes, the condensate backs up into the boiler, the appliance detects it and shuts down. That one clears once the outside section of the pipe is thawed with warm, not boiling, water.

Beyond those, you are into genuine faults: a failed pump, a seized fan, an ignition fault where the boiler sparks but never establishes flame, a failed pressure sensor telling the board there is no water when there is, or an overheat thermostat that has tripped and latched. Any fault code on the display narrows it enormously, so write it down before you reset anything.

What the fault code on the display actually tells you

Modern combis tell you a good deal about themselves if you let them. The code on the display is the boiler reporting which of its own safety checks failed, and it is the single most useful piece of information you can give an engineer on the phone. Note the make, the model off the sticker inside the front panel, and the exact code including any letters.

Be careful how much you read into one, though. A fault code names the symptom the board detected, not the part that caused it. An overheat code can be a scaled heat exchanger, a failed pump, a stuck valve or a genuinely faulty thermistor. A no flame code can be gas supply, an electrode, a gas valve or a blocked condensate trap. That is precisely why the order of testing matters and why people who buy the part the internet associates with the code so often end up buying a second one.

Where we have written up a specific code, the page for it sets out what the boiler was actually checking at the moment it stopped. The Vaillant F28 page and the Ideal L2 page both cover ignition lockouts, which is the family of codes a boiler shows when it has tried to light and given up.

Hot water that runs hot, then goes cold

A separate symptom worth calling out, because the causes differ from a boiler that gives you nothing at all. Hot for thirty seconds or a couple of minutes, then cold, then sometimes hot again, is usually the boiler cutting out and relighting rather than the hot water being switched away.

If the boiler is overheating and dropping out, look at anything that restricts the transfer of heat or the movement of water: a scaled plate heat exchanger, a pump that is running but not moving much, air trapped in the primary circuit, or a blocked filter. The pattern is heat, then a bang or a kettling noise or simply silence, then a wait, then the burner relighting.

If the boiler stays running and the water still goes cold, suspect the diverter valve creeping back towards the heating side under pressure, which happens when the shaft is partly seized or a diaphragm is on its way out. It holds the changeover briefly and then gives up.

Then there is the case where it only happens in one room. A thermostatic mixing valve inside a shower or on a bath tap can fail and start blending in more cold as the hot side comes up to temperature. If every other tap in the property runs properly hot and only one fixture misbehaves, the boiler is not your problem and there is no sense taking the front off it. That case is worked through from the bathroom end on our page for no hot water in the shower.

What the engineer actually does on the visit

The point of the visit is to end with the fault proved rather than guessed, because on this symptom the same complaint has four or five possible causes and the parts are not interchangeable in price or in labour.

It starts at the boiler with the front off and the stored fault code and fault history read. Then the electrical side gets confirmed: supply at the terminal strip, the earth, and whether hot water demand is reaching the board when a tap opens. Somebody opens a hot tap and the engineer watches the appliance react, or fail to react, which on its own splits the fault between the demand side and the delivery side within a minute.

Then the water side. Flow rate through the appliance is measured to prove it is above the minimum the model needs. Flow and return temperatures are read while the boiler runs, because a rapid rise with almost no difference between them means water is not moving and points at the pump or at a restriction rather than at anything to do with hot water. The diverter valve is tested in operation, not just looked at: does the actuator drive, does the shaft move, does the heating side actually close.

Gas and combustion are checked where the boiler is firing poorly rather than not at all: standing and working pressure measured at the inlet under full load, the ignition sequence observed through fan, spark and flame establishment, and combustion read with a flue gas analyser. And on any appliance in a hard water area the plate heat exchanger gets specific attention, because it is the most likely single part to be at fault on a combi whose hot water has faded rather than stopped.

At the end of it you should be told which part has failed, how that was proved, what putting it right involves, and whether this particular appliance is worth spending the money on. The hourly rate is fixed and agreed before we attend, and what a job costs in the end depends on how long it takes and what parts it needs.

• Fault code and stored fault history read from the appliance.
• Electrical supply proved and hot water demand confirmed as reaching the board.
• Boiler behaviour watched live with a hot tap open.
• Incoming flow rate measured against the model minimum.
• Flow and return temperatures read under load to prove circulation.
• Diverter valve tested through an actual changeover.
• Plate heat exchanger assessed for scale where flow has dropped.
• Gas pressure and combustion tested where the burner is at fault.

When a repair stops being worth doing

Most combis with no hot water are worth repairing. A diverter valve head, a flow sensor or a plate heat exchanger are ordinary parts on an appliance that is otherwise sound, and replacing one is far cheaper than replacing the boiler.

There are three cases where we will say otherwise. The first is parts support. Manufacturers stop making components for older ranges, and once the part you need is not available anywhere the decision has been taken for you. The second is a failed primary heat exchanger, which on many combis costs enough as a part and enough in labour to make the sums point at a new appliance. The third is the third major failure inside a year, which almost always means the system is doing the damage rather than the boiler being unlucky: debris circulating from old steel radiators, no system filter, untreated hard water, or a chronic circulation problem that has been quietly cooking the appliance. Where the sums genuinely do point away from a repair, our boiler replacement page sets out what changing the appliance involves and what has to be put right around it.