What we'll cover...
Having central heating but no hot water can be one of the biggest household frustrations that can occur. Very few annoyances compare to switching on the tap for a shower to be met with icy disappointment, especially when your radiators are happily warming the house at the same time.
This guide explains what you need to know if your hot water isn’t working but your heating is, including the most common causes, which checks you can complete safely by yourself or when a Gas Safe engineer is required, and whether the issue signifies a bigger upgrade decision.
Why is my heating working but hot water isn't?
If your radiators are heating up as they should but your taps and shower remain cold, the issue is usually isolated to the domestic hot water side of the system, as opposed to the central heating loop.
In most UK homes, heating and hot water share the same boiler supply but operate on separate circuits. That means one can work perfectly while the other develops a fault.
The cause often falls into one of three categories:
- Heating control or thermostat issue
- Water flow or pressure problem
- Internal boiler component failure (most commonly the diverter valve)
To begin with, we’ll explore the checks you can safely do yourself.
What to check first if hot water isn't working
Before calling an engineer, run through the following checklist.
1. Check the thermostat and hot water settings
Although it may sound obvious, one of the most common causes of hot water issues is a problem with the thermostat or hot water settings.
If you’ve adjusted your heating settings recently (particularly during seasonal changes), the hot water thermostat might have been unintentionally turned down or switched off.
What to do:
- Check the hot water temperature control on the boiler
- Check any separate hot water programmer or timer
- Ensure the hot water schedule hasn’t been disabled
- Confirm that Eco mode hasn’t restricted hot water output
- Compare thermostat settings with the boiler control panel
If you use smart heating controls, double-check the app – zone settings can sometimes override manual controls.
Consult our guide on how to set up a smart heating system if you’re not entirely sure how your heating controls are set up to operate properly.
2. Check the boiler power supply
Any recent power outage or interruptions to voltage could reset boiler settings.
Ask yourself:
- Has there been a recent power cut?
- Is the boiler display showing any error codes?
- Has the hot water temperature been reset to minimum?
Even if the heating is working, a reset might have altered hot water settings.
3. Try the classic fix: Switch it on and off again
Yes, really. Modern boilers contain electronic control boards that can be susceptible to an occasional glitch, so the simplest of fixes is always worth a try.
The correct way to reset:
- Switch the boiler off at the main power switch
- Leave it off for at least 30 minutes
- Switch it back on
- If your diverter valve is on a separate circuit, reset the valve as well
Allow time for the system to restart properly. It’s surprising how often such a simple method can resolve temporary communication faults between components.
4. Check for frozen or leaking pipes
Especially during the colder winter months of the year, frozen pipes are a frequent culprit of hot water failure. Even if your heating works, a frozen hot water pipe can prevent water from reaching taps.
Signs of frozen pipes:
- Gurgling noises
- Frost on exposed pipework
- Reduced water flow before complete loss
Also check visible pipework for leaks. Even a small drip can cause a reduction in system pressure.
If you’re unsure about pipe layouts, our ultimate guide to heating pipework offers deeper insights into different pipe system configurations.
5. Check for trapped air or valve problems
Airlocks or incorrectly positioned radiator valves can reduce pressure and interrupt hot water flow.
Look for:
- Isolation valves that have been recently adjusted
- Partially closed stop valves
- Signs of trapped air
6. Check if there's an issue with your local water supply
It’s worth ruling out the simplest explanation before you begin commencing with any fixes. Ask a neighbor if their hot water or taps have been affected.
A local water supply interruption can sometimes appear like a heating issue and could be influencing the performance of hot water and heating of several homes in the same area.
The most common cause: A faulty diverter valve
If you’ve ruled out the causes outlined above, the most likely culprit is a broken diverter valve. This is particularly common in combi boiler systems.
What does a diverter valve do?
The diverter valve essentially acts as a traffic controller for your boiler system.
It switches the boiler’s heat output between:
- The central heating circuit (radiators)
- The domestic hot water supply (taps and showers)
When you switch a hot tap on, the valve will redirect heat from the radiators to your hot water.
If the valve becomes stuck, worn, or fails:
- Heating continues working
- Hot water is not prioritised
- Taps run cold
Essentially, the system is unable to cross from one function to the other, or ‘switch lanes’ to return to the traffic analogy.
Signs of a faulty diverter valve
A combination of the following indicators suggests a faulty diverter valve could be causing an issue with your hot water supply.
- Heating works fine
- No hot water emerges from taps
- Lukewarm water at best, if any does emerge
- Boiler fires up, but there is no temperature increase at the tap
- Clicking noises emanating from inside the boiler when taps are turned on
It is strongly recommended to enlist the services of a Gas Safe registered engineer to deal with a faulty diverter valve; improper DIY repair work can cause potential boiler damage and/or gas leakages, and could also pose a risk to your boiler warranty.
Hiring a professional to fix your hot water issue
If the above checks don’t identify or resolve the issue, it’s time to contact a Gas Safe registered engineer to fix your hot water issue.
A professional will:
- Perform a diverter valve check
- Check the internal components on your boiler
- Inspect the heat exchanger
- Assess pump performance
- Confirm the integrity of your system pressure
While a diverter valve replacement is often a straightforward process for a qualified heating engineer, locating and accessing it does require experience and expertise.
You should never attempt to remove boiler casing or repair internal components yourself.
Could a lack of hot water indicate a bigger problem?
In some cases, a lack of hot water could relate to a:
- Heat exchanger blockage
- Pump failure
- Build-up of sludge
- Faulty motorised valve (in system boilers)
- Internal sensor malfunction
If your boiler is:
- More than 15 to 20 years old
- In need of regular repairs
- Making unusual noises
- Struggling with pressure levels
…it may be worth considering whether repair or a boiler replacement makes more financial sense in the long run.
Our guide to buying a new boiler can help you weigh up upgrade options and estimated costs for different types of systems.
Is it worth replacing an old boiler?
Many homeowners will as if it is worth replacing an old boiler when their hot water supply fails but heating still works fine.
If your boiler is approaching 20 years old, you may find:
- Parts are harder to source
- Significantly lower efficiency levels than modern models
- Repair costs can spiral if multiple components require attention
Modern systems offer:
- Enhanced energy efficiency
- Better compatibility with smart heating controls
- Faster hot water response
- Less maintenance is needed
Plus, if you are taking the plunge with a boiler upgrade, it can make sense to update your whole heating system with radiators or heated towel rails in turn too.
Browse our guide on how to improve radiator efficiency & performance for practical advice on how to optimise your system.
No hot water DIY troubleshooting checklist
Before calling an engineer, run through these steps to potentially save you time and money:
- Ensure your hot water thermostat is correctly set
- Check the programmer/timer is active
- Recall there have been no recent power reset issues
- Attempt a boiler reset (allow 30 minutes switched off)
- Make sure you have no frozen or leaking pipes
- Confirm neighbours’ water supply is unaffected
- Ensure there are no visible valve issues
If you’ve proceeded with all these steps and still have no hot water, a diverter valve or internal component issue are likeliest.
Solve your home heating issues with BestHeating help
If your heating is working properly but the hot water supply isn’t, a total system failure is rarely the diagnosis.
In many instances, the solution can be:
- A settings adjustment
- A simple reset
- A diverter valve repair
The key is to work logically through safe checks before escalating to seeking out professional help.
There are all sorts of resources in the BestHeating Advice Centre that will help you work through your heating troubles, including what to do should the opposite issue arise, and you have hot water without heating.
And if you do need to upgrade your heating components, BestHeating offers a wide range of modern, efficient radiators designed to pair perfectly with contemporary boiler systems, helping you get the most out of your heating system on the whole.
Let us know all about your modern heating upgrades via Instagram, Facebook or X.
