Hot Water On But No Heating? Get Expert Repairs Now

If you’re dealing with a situation where the hot water is working perfectly but your heating isn’t, you’re not alone. This is one of the most common boiler-related problems homeowners face especially during colder months when you need heating the most.

I’ve personally seen many cases where people assume their boiler is completely broken, only to find out later that the issue is something small and fixable. The good news? If your hot water is still running, your boiler is at least partially working, which makes diagnosing the issue much easier.

Understanding the Problem: Hot Water But No Heating

Modern boilers, especially combi boilers, are designed to provide both:

• Hot water for taps and showers
• Central heating for radiators

Even though they come from the same unit, these two functions often rely on different internal components. So when one works and the other doesn’t, it usually points to a specific fault rather than a full system failure.

1. Faulty Diverter Valve (The Most Common Cause)

If you search for boiler on but no heating, this is the issue that comes up again and again—and for good reason.

What is a diverter valve?

It’s a component inside your boiler that controls where the hot water goes:

• To your taps → hot water
• To your radiators → heating

What happens when it fails?

Sometimes, the valve gets stuck sending water only to taps. That’s when you end up with hot water but no heating.

Real-world insight:

In one job I handled, the homeowner had been dealing with freezing rooms for days but still had hot showers. The diverter valve had partially seized, meaning it couldn’t switch to heating mode.

Signs of a faulty diverter valve:

• Hot water works normally
• Radiators stay completely cold
• Boiler fires up when you use taps, but not for heating

Solution:
This isn’t a DIY fix. A professional engineer will either:

• Repair the valve
• Or replace it entirely

2. Thermostat Problems

Sometimes, the issue isn’t the boiler, it’s the thermostat not telling the boiler to turn on the heating.

Common thermostat issues:

• Incorrect temperature setting
• Dead batteries
• Faulty wiring
• Calibration issues

What to check:

• Turn your thermostat up to maximum
• Listen for a clicking sound
• Check if the boiler responds

If nothing happens, your thermostat may not be sending the signal.

Solution:

• Replace batteries
• Reset the device
• Upgrade to a smart thermostat if needed

3. Timer or Programmer Faults

Your heating system relies on a timer or programmer to control when heating turns on and off.

The problem:

Sometimes the timer stops working properly, even though hot water continues to function.

Signs:

• Heating doesn’t follow schedule
• Works in manual mode but not automatic
• Boiler ignores heating commands

Solution:

• Reset the programmer
• Switch to manual mode temporarily
• Replace the unit if faulty

Low Boiler Pressure

Low pressure is a simple but very common reason for boiler working but no heating.

Why pressure matters:

Your heating system needs pressure to circulate hot water through radiators.

Ideal pressure:

• Between 1.0 and 1.5 bar (when cold)

Signs of low pressure:

• Heating not working
• Pressure gauge below 1
• Boiler may show warning

Solution:
Repressurize the system using the filling loop. Most homeowners can do this themselves in a few minutes.

5. Airlocks and Circulation Problems

If air gets trapped inside your system, it can block the flow of hot water.

Symptoms:

• Cold radiators (especially at the top)
• Gurgling or bubbling noises
• Uneven heating

Real-life example:

I’ve seen cases where one radiator heats perfectly while another stays ice cold, classic airlock issue.

Solution:

• Bleed your radiators
• Check for trapped air
• Ensure proper water flow

6. Motorised Valve Failure (System Boilers)

If you have a system or conventional boiler, motorised valves control water flow between heating and hot water.

Problem:

If the valve fails, your heating won’t activate, even though hot water still works.

Signs:

• Heating doesn’t turn on
• No response from heating controls
• Boiler seems fine otherwise

Solution:
This requires a qualified engineer to repair or replace the valve.

7. Sludge or Blockages in Radiators

Over time, central heating systems can build up sludge—especially in older homes.

What this does:

• Blocks water flow
• Prevents radiators from heating
• Reduces system efficiency

Signs:

• Radiators cold at the bottom
• Some radiators work, others don’t
• Dirty water when bleeding

Solution:

• Power flush the system
• Clean or replace affected radiators

8. Heating But No Hot Water (The Reverse Issue)

Sometimes, the problem flips: heating works, but hot water doesn’t.

This can be caused by:

• Diverter valve stuck the other way
• Faulty hot water sensor
• Cylinder thermostat issues

The diagnosis process is similar but in reverse.

9. Boiler Working But No Hot Water or Heating

If neither heating nor hot water works, the issue is more serious.

Possible causes:

• Ignition failure
• Gas supply issue
• Faulty PCB (circuit board)
• Pump failure

In this case, don’t try to fix it yourself. Call a professional immediately.

Step-by-Step Troubleshooting Checklist

Before calling an engineer, go through this:

• Check thermostat settings
• Replace thermostat batteries
• Ensure programmer is working
• Check boiler pressure
• Bleed radiators
• Make sure radiator valves are open

If everything checks out and the problem persists, it’s likely a mechanical fault.

When Should You Call a Professional?

From experience, you should call an expert if:

• You suspect a diverter valve issue
• There are strange noises from the boiler
• Error codes appear
• You’ve tried all basic fixes

Trying to fix internal boiler components without proper knowledge can lead to bigger (and more expensive) problems.

Conclusion

Dealing with hot water on but no heating can feel stressful, especially in winter—but in most cases, the issue is fixable without replacing the entire boiler.

From my experience, the most common causes are:

• Diverter valve problems
• Thermostat faults
• Low pressure

Start with the simple checks first. If the issue still doesn’t resolve, don’t wait too long—getting professional help early can save you time, money, and a lot of discomfort.

Frequently Asked Questions