‘Why Do I Have Hot Water But No Heating?’ Causes, Checks, And Next Steps

Having hot water but no heating is frustrating because, in a sense, the boiler is half working. Taps and showers are fine, yet the radiators stay cold and the house never warms up. Even worse, it tends to show up at the worst possible time.

The good news is that this fault is usually diagnosable quickly. Some fixes are simple settings. Others involve internal components that need a Gas Safe registered engineer. This guide explains what’s most likely, what you can safely check, and when it’s time to stop guessing and get someone in.

What This Symptom Usually Means

In many homes (especially with combi boilers), hot water and heating are controlled in slightly different ways. Hot water is often triggered directly by a tap opening; heating relies on demand from controls like a thermostat and timer, plus the boiler’s ability to send heat into the central heating system.

That’s why having hot water but no heating often points to one of three areas:

• A control or incorrect setting issue
• A circulation or pressure problem
• A component that manages the switch between water and heating

Quick Checks That Are Safe

A few checks can be done without taking covers off or touching anything gas-related.

Check The Thermostat And Programmer

Start with the obvious. Make sure the thermostat is switched on, set above room temperature, and the programmer is set correctly for heating and hot water. Many call-outs end up being a schedule that got changed, a battery that died, or heating accidentally set to off.

Look At The Pressure Gauge

Low boiler pressure can stop heating even when the hot water still works. Most boilers like to sit around 1 to 1.5 bar when cold. If the needle is below 1 bar, that can cause problems. Some boilers show pressure digitally, but the principle is the same.

Understand How To Top Up Safely

If pressure is low, the filling loop may be used to top up, but only if you’re confident and your manufacturer's instructions are clear. Overfilling creates its own issues, so go slowly and stop at the recommended range. Pressure that keeps dropping suggests a leak, which needs professional attention.

Check Radiator Valves

Sometimes the boiler is trying to heat, but the radiators cannot accept heat. Make sure the thermostatic radiator valves are open and not stuck. A single stuck valve can cause odd symptoms, especially in smaller systems.

Consider Air In The Radiators

If radiators are cold at the top, bleeding your radiators can help. Radiators that stay cold all over, or cold at the bottom, may point to circulation issues rather than air alone.

The Most Common Boiler Causes

When basic settings and pressure look fine, “hot water but no central heating” often comes down to a part inside the boiler that prioritises one function over the other.

Diverter Valve Problems

A failing diverter valve is one of the most common reasons a combi gives hot water but no heating. The diverter valve’s job is to direct heat to either your hot water or your radiators. If it sticks or wears, it may send heat to hot water only. This can look like everything “works fine” until you realise the house is freezing.

Pump Or Flow Issues

Heating relies on water moving through the system. A fault with circulation can stop radiators heating up, even if hot water still comes through at the taps. Sludge, blockages, or a struggling pump can all contribute.

Faults In Controls Or Sensors

Boilers rely on sensors and control boards to interpret demand. If the boiler does not detect a heating demand, it may never fire for heating, even though hot water demand still triggers it. A heating engineer can test these signals safely.

When It’s Time To Call A Gas Safe Registered Engineer

If you’ve checked the pressure gauge, ensured heating is set correctly, and the radiators still do nothing, it’s time to call. Repeated resets, random temperature swings, or pressure that will not hold are all signs the fault is beyond a quick tweak.

Gas appliances should only be opened and tested by a Gas Safe registered engineer. That matters for safety and for protecting your boiler long term.

When Replacement Makes More Sense

Some faults are worth repairing, particularly on newer boilers. Older units can become a cycle of part replacements, disruption, and rising bills. If your boiler is ageing, inefficient, or out of warranty, a replacement can be the more cost-effective move.

KD Jones installs new boilers from £1900 including VAT, and 0% boiler finance is available to spread the cost. Modern boilers are more reliable, more efficient, and better matched to modern controls and heating demands.

Speak To KD Jones Before You Lose Another Day Of Heating

If you’ve got hot water on but no heating, it’s worth getting the system checked before the fault escalates into bigger damage. If you do need a new boiler, installation starts from £1900 inc VAT, with 0% boiler finance available. To book a visit or discuss your options, get in touch via the details below.

📞 Call us on 01737 211376

🌐 Visit our contact page to request a quote online

📍 Based in Redhill, we cover homes across Surrey and the surrounding areas

FAQs

Why do I have hot water but no heating?

It’s often a control issue, low boiler pressure, or a combi boiler fault like a diverter valve sticking. A quick check of settings and the pressure gauge is a good starting point.

My central heating is not working but the hot water is, should I reset the boiler?

A reset can clear a temporary fault, but repeated resets usually mean an underlying issue. If it keeps happening, a Gas Safe registered engineer should investigate.

What boiler pressure should I look for?

Many systems sit around 1 to 1.5 bar when cold. Pressure that’s too low can stop heating from running properly, even if hot water still works.

Can I top up pressure using the filling loop?

Yes in some cases, if you’re confident and follow manufacturer guidance. Pressure that drops again points to a leak or fault that needs a professional.

Could the diverter valve be the problem?

Yes. A diverter valve fault is a common cause on combi boilers, because it controls whether heat goes to radiators or hot water.

Do thermostatic radiator valves stop heating working?

TRVs can stop individual radiators heating up, especially if they’re stuck closed. They would not usually stop all heating, but they can confuse the symptoms.

Should I try bleeding my radiators?

Bleeding helps if radiators are cold at the top due to trapped air. Radiators that are cold all over often point to circulation issues instead.

Is this problem more common with combi boilers?

It is. Combi boilers switch between hot water and heating, so faults with diverter valves, sensors, or controls show up in this exact way.

Can sludge be a cause of hot water with no central heating?

Sludge and debris can restrict flow in the heating circuit, which affects radiators first. A heating engineer can advise on cleaning or a power flush if needed.

When should I replace the boiler instead of repair it?

Frequent breakdowns, poor efficiency, and repeated part failures are signs replacement may be smarter. KD Jones installs new boilers from £1900 inc VAT, with 0% boiler finance available.