Hot Water But No Heating A Homeowner's Troubleshooting Guide
It’s one of the most common and frustrating boiler problems you can face, especially in a cold snap: you’ve got plenty of steaming hot water for a shower, but the radiators are stone-cold. Before worrying about a complete boiler breakdown, take a breath. This is a frequent issue, and the fix is often more straightforward than you might think.
When your taps are hot but the heating is off, the most likely culprit is a small but vital component in your combi boiler called the diverter valve. Its job is to direct hot water to either your taps or your central heating. If it gets stuck, it’ll usually default to providing hot water, leaving your radiators out in the cold.
Hot Water, Cold Radiators: What’s Going On?
This is a classic UK heating headache. A jammed diverter valve is a top cause, often made worse by sludge build-up from a system that hasn't had a recent service. You can discover more insights about this common boiler issue from our heating experts.
But it’s not always the diverter valve. This guide will walk you through the most common reasons you have hot water but no central heating. We’ll cover everything from a confused thermostat and low boiler pressure to airlocks and dodgy radiator valves, helping you diagnose the problem.
To get started, here's a quick reference table to help you identify the potential cause behind your chilly radiators.
Quick Diagnosis: Common Causes of No Heating
| Potential Cause | Common Symptoms | DIY Fix or Professional? |
|---|---|---|
| Diverter Valve Fault | Lukewarm radiators, heating only works when hot water is on | Professional |
| Timer/Controls | Heating won't come on at scheduled times, display is blank | DIY Check First |
| Thermostat Issue | Radiators don't heat up even when thermostat is turned up | DIY Check First |
| Low Boiler Pressure | Pressure gauge is below 1 bar, boiler shows an error code | DIY (if comfortable) |
| Airlock in System | Gurgling noises from pipes, some radiators are cold at the top | DIY |
| Stuck TRVs | One or more specific radiators are cold while others are hot | DIY |
While some of these issues can be tackled yourself, anything involving opening up the boiler casing, like a diverter valve replacement, is strictly a job for a Gas Safe registered engineer.
Investigating the Common Causes of Heating Failure
Steaming hot water at the taps but stone-cold radiators? The culprit is usually one of a few usual suspects.
More often than not, the problem traces back to a faulty diverter valve. This component acts like a set of points on a railway, directing hot water either to your taps or to your central heating. If it gets stuck in the ‘hot water’ position, your radiators will never warm up. We have a whole guide explaining the boiler diverter valve if you want to dive deeper.
Beyond the diverter valve, here are other common causes:
Low Boiler Pressure: Take a quick look at the pressure gauge on your boiler. If it’s dropped below 1 bar, there might not be enough pressure to push hot water around your heating circuit.
Faulty Thermostat: Your thermostat is the brain of the operation. If it's not working, it might not be sending the "turn on" signal to your boiler, no matter how high you set the dial.
Airlocks in the System: Trapped air can form a stubborn blockage in your pipework, stopping hot water before it can reach your radiators.
These faults are incredibly common and account for a huge number of call-outs every winter.
Your Safe DIY Troubleshooting Checklist
Before calling an engineer, it’s worth running through a few quick checks. Often, when you have hot water but no central heating, the fix is surprisingly easy. This checklist will guide you through the basics without ever needing to open your boiler’s casing.
First, check the thermostat and timer. It sounds simple, but you’d be surprised how often a dead battery or an accidental setting change is the cause. Make sure they’re both switched on and calling for heat.
Next, look at your boiler’s pressure gauge. You’re looking for a reading between 1 and 1.5 bar. If the needle has dropped into the red zone, the pressure is too low for the system to push hot water to your radiators. Topping up the pressure is usually straightforward; our guide on using a boiler filling loop walks you through how to do this safely.
Finally, check your thermostatic radiator valves (TRVs). Someone might have turned them down or off. A quick twist to open them back up could be all it takes.
When to Call a Go Assist Engineer
Knowing when to call a pro is vital for your safety and your boiler's health. If your DIY checks haven't worked or you feel out of your depth, it’s time to call us.
Certain signs are a giveaway that you need an expert. If the boiler’s display is flashing an error code you don’t recognise, or if you suspect an internal part like the diverter valve has failed, you’ve reached the limit of what you can safely do.
Strange noises like loud gurgling, kettling (a boiling sound), or banging are another red flag. These often point to a serious issue like a sludge build-up, which requires specialist equipment. You can learn more in our guide to power flushing a central heating system.
Never, ever try to open up your boiler or fix internal components on your own. It's not just extremely dangerous—working on a gas appliance is illegal unless you are on the Gas Safe Register. If you’re in any doubt, don't risk it.
Don’t let a heating problem leave you in the cold. Book a boiler repair online and one of our certified engineers can be with you for a same-day or next-day appointment.
Simple Habits for a Healthy Heating System
The best way to deal with a heating failure is to prevent it. A few simple habits can save you the stress and expense of an emergency call-out.
The single most important step is an annual boiler service. This is a professional health check where an engineer can spot developing problems—like a struggling diverter valve or a failing pump—long before they leave you in the cold. It’s like an MOT for your boiler, ensuring everything runs safely and efficiently.
Beyond the yearly service, these quick checks make a huge difference:
- Keep an eye on the pressure. Glance at your boiler's pressure gauge occasionally. You want to see it sitting steadily between 1 and 1.5 bar. If it drops, it’s an early warning sign.
- Bleed your radiators. Trapped air is a common culprit behind cold spots. Bleeding your radiators once or twice a year is a quick job that releases this air and prevents blockages.
Putting these habits into practice is key to a reliable system. For those with newer setups, this also includes ensuring proper heat pump maintenance to prevent costly breakdowns.
Frequently Asked Questions About Heating Problems
When you've got hot water but cold radiators, it's natural to have questions. Here are quick answers to the most common queries.
Why do combi boilers prioritise hot water?
A combi boiler is built to give you hot water instantly. The moment you turn on a hot tap, the diverter valve redirects all of the boiler's heating power to your taps, which is why the central heating takes a temporary backseat. It can't do both jobs at once, so it always prioritises your hot water supply.
Will resetting my boiler fix the problem?
It's always worth a shot. A simple reset can sometimes clear a minor electronic hiccup. However, if the issue is mechanical, like a seized diverter valve or low boiler pressure, a reset won’t solve the root cause. The problem will likely return until the underlying fault is fixed.
How often should I get my boiler serviced?
An annual service is absolutely vital. Most manufacturers require it to keep your warranty valid. More importantly, it’s the best way for a qualified engineer to spot and sort out small issues before they become a complete breakdown, leaving you with no heating or hot water when you need it most.
