It is a confusing fault to be faced with. The radiators are warming up exactly as they should, so the boiler is clearly working, and yet the hot tap runs cold.
The reason it feels contradictory is the reason it is usually straightforward to diagnose. Heating and hot water are two different jobs, and most boilers do them through separate paths. When one works and the other does not, the fault is nearly always in the path rather than in the boiler.
Key Takeaways
- A working heating system tells you the boiler itself is very likely fine
- On a combi boiler, a stuck diverter valve is the most common cause by some distance
- On a system boiler with a cylinder, suspect the cylinder thermostat, the motorised valve or the programmer
- A handful of checks are safe to do yourself, and one of them fixes it surprisingly often
- Anything involving gas, or opening the boiler, is a job for a Gas Safe registered engineer
First, Work Out Which System You Have
Everything else follows from this, and it takes a few seconds.
If you have a hot water cylinder, usually a tall insulated tank in an airing cupboard or loft, you have a system or conventional boiler. Hot water is heated in advance and stored.
If you have no cylinder anywhere, and hot water arrives the moment you open a tap, you have a combi. It heats water on demand as it passes through.
The two fail in completely different ways, so the checks below are split accordingly.
If You Have a Combi Boiler
The diverter valve
Inside a combi there is a valve whose only job is to decide where the heat goes: to the radiators, or to the hot water. When you open a hot tap it is supposed to switch over.
When that valve sticks in the heating position, you get exactly the symptom you are seeing. The boiler fires, the radiators heat, and the hot water never gets a look in. It is the single most common cause of this fault, and it is a repair rather than a replacement in most cases.
You can often confirm it by ear. Turn every radiator off, run a hot tap, and listen. If the boiler fires up but the water stays cold, the heat is going somewhere it should not.
The plate heat exchanger
The other frequent culprit is the heat exchanger that transfers heat into the water. In hard water areas it gradually scales up, and as it does the hot water gets weaker and slower before it disappears altogether.
Much of Berkshire and the surrounding counties sits on chalk, so the water is hard, and scaling is a normal part of a boiler’s life here rather than a sign anything was done wrong.
The tell is usually gradual decline rather than sudden failure. If the hot water has been getting less impressive for months, this is the likelier of the two.
If You Have a Hot Water Cylinder
Check the programmer first
This sounds too simple to be worth doing. It resolves the problem often enough to be worth doing first anyway.
Heating and hot water are usually on separate schedules, and it is easy for the hot water schedule to be switched off, set to a time that no longer suits, or reset entirely after a power cut. A brief power interruption you barely noticed can put a programmer back to its factory settings, which frequently means heating on and hot water off.
Set it to constant for an hour and see whether anything changes before going further.
The cylinder thermostat
Strapped to the side of the cylinder, usually under the insulation, is a small dial that tells the system when the water is hot enough. If it has been knocked down, or has failed, the system is told the water is already hot and never bothers heating it.
It should be set to around 60 degrees. That is not an arbitrary number: it is hot enough to control legionella bacteria in stored water, which is why HSE guidance settles on it. Lower is not safer, and higher brings a scalding risk at the tap.
The motorised valve
On most cylinder systems a motorised valve directs hot water from the boiler either to the radiators, to the cylinder, or to both. It does the same job as a combi’s diverter valve, just as a separate component sitting near the cylinder or airing cupboard.
These fail in two ways. The motor itself can go, or the internal switch that tells the boiler to fire can. Either way the result is the same: the radiators get everything and the cylinder gets nothing.
A scaled or airlocked coil
Inside the cylinder, a coil of pipe carries hot water from the boiler through the stored water. If that coil scales up over years, it transfers heat far less effectively, and the water never reaches temperature no matter how long the system runs.
This one tends to arrive slowly, and on older cylinders it can be the point at which replacing the cylinder makes more sense than nursing it. If yours is reaching that stage, an unvented cylinder is usually what replaces it.
What You Can Safely Check Yourself
None of these involve tools, gas or removing any covers.
- Check the programmer. Is hot water actually scheduled on, and is it set to a sensible time? Try constant for an hour.
- Check the cylinder thermostat, if you have a cylinder. It should read around 60 degrees.
- Look at the boiler display. A fault code changes the picture entirely, so note it down before doing anything else.
- Check the boiler pressure. It should usually sit between 1 and 1.5 bar when cold. Very low pressure can stop the boiler working properly.
- Run a hot tap and listen. Does the boiler fire up in response? That one answer narrows the diagnosis considerably.
If none of that changes anything, the fault is mechanical and needs an engineer.
What Not to Do
Do not open the boiler casing. Beyond being dangerous, it is illegal for anyone who is not Gas Safe registered to work on gas appliances, and it will invalidate your warranty.
Be wary of advice to simply turn the cylinder thermostat up well past 60 degrees to compensate. It does not address the fault, it wastes energy, and it introduces a genuine scalding risk at the taps.
When to Call an Engineer
Call once the safe checks are done and nothing has changed, or immediately if the boiler is showing a fault code, if you can smell gas, or if there is water where there should not be.
If you smell gas, leave the property and ring the National Gas Emergency Service on 0800 111 999 before you call anyone else.
Most of these faults are repairs rather than replacements. A stuck diverter valve, a failed cylinder thermostat or a spent motorised valve are all common parts that get changed regularly, and the job is usually done in a single visit.
Getting It Sorted
The good news in this fault is what it tells you. If your heating is working, the expensive part of your system is almost certainly fine, and what has failed is a component rather than a boiler.
If the checks above have not brought your hot water back, get in touch and we will diagnose it properly rather than guessing at parts. We cover Reading and the wider area including Wokingham and Crowthorne, and most boiler repairs of this kind are resolved on the first visit.
Once it is running again, an annual boiler service is what catches a tired diverter valve or a scaling heat exchanger while it is still a small job rather than a cold shower.