Central Heating Not Working but Hot Water Is? Here’s Why and How to Fix It

Cold radiators but hot water still running from the taps? That combination indicates a certain set of causes, and good news is most of them can be easily checked and fixed.

If your central heating is not working but hot water is still running normally, it’s likely to be one of the five causes: a faulty diverter valve, an airlock in the radiators, low boiler pressure, incorrect thermostat settings or a circulation pump fault. Read the following steps below to determine the cause and turn the heat back on.

Step-by-Step Guide: What to Do When Hot Water Works but Central Heating Doesn’t

1. Check Your Radiators

Cold radiators typically indicate an airlock in the radiator, sludge buildup, or a system-wide fault in the boiler in pattern of which radiators are cold, which will indicate exactly which radiator you are having trouble with.

Check all the radiators in your home:

• Are all the radiators cold? This usually points to a system-wide issue, like the boiler not heating water for central heating.
• One or two radiators warm or slightly hot? This could be trapped air or sludge buildup, not a boiler fault.

Try This:

Bleed the Radiators:

• Turn off the heating.
• Use a radiator key to slightly loosen the bleed valve (turn anti-clockwise).
• Wait for the air to escape, once water drips, tighten it back.
• Use a towel or container underneath to avoid a mess.

Read our blog on how to bleed your radiators for a more detailed guide.

Radiators Cold at the Bottom but Warm at the Top?

It can be a sign of sludge buildup (a mix of rust and debris) that has accumulated at the bottom of the radiator and prevents hot water from flowing. It will be cleaned up by a professional power flush which will be done by a Gas Safe registered engineer.

Related Article: What is a central heating power flush?

2. Check Thermostat and Timer Settings

If your heating isn’t working, but the hot water still works, then the thermostat or timer has been reset, most likely because of a loss of power or a change in daylight savings time.

What to Check:

• Thermostat: Does it have a high enough setting to help the heating element work?
• Programmer or Timer: Is it set to “heating on”? Your settings may have been reset due to a power outage or daylight-saving time.
• Room Thermostat: Ensure the target temperature isn’t too low.

if your hot water works but your central heating doesn’t, your boiler is probably not the cause of the problem; it may need some adjustments.

Related Article: How does a thermostat work?

3. Can Low Boiler Pressure Stop Your Heating?

Low boiler pressure is one of the most common reasons central heating stops working while hot water keeps running normally. For boilers to pump hot water through the radiators they require the correct pressure. If the pressure drops, the heating circuit is unable to function correctly.

How to Check:

• Check the pressure gauge on the boiler; it should be between 1 and 1.5 bars.
• If it is below 1 bar, it will require repressurising the boiler; this can be done via the filling loop (refer to your user manual).

Warning: Do not open any casing. Only use controls that are accessible and follow the instructions in the boiler’s manual.

4. Reset the Boiler

A reset can ensure the boiler system is clear of minor faults and is always a good thing to try before you think it is something more serious.

To Reset:

• Use the reset button on your boiler control panel.
• Alternatively, turn the boiler off at the mains, wait 30 seconds, and turn it back on.

Note: If the problem reoccurs after reset, it is probably more serious; typically, it is a component failure. If a fault code is displayed on the boiler, note it before contacting an engineer, so they know exactly what is wrong with the boiler.

5. Could It Be the Diverter Valve?

If your hot water works fine but the radiators won’t heat up no matter what you try, the cause is most likely a faulty diverter valve, the problem is likely to be a leaking or broken diverter valve.

A diverter valve enables the hot water in the boiler to flow into either the hot water taps or the radiators. However, if it is clogged or damaged, it seals off the flow to the taps, and the radiators cool off entirely.

Signs it’s the diverter valve:

• Hot water works perfectly, but heating won’t operate at all
• You’ve had a check on the pressure, thermostat and bled the radiators, and nothing has changed.

A diverter valve repair or replacement is not a DIY job and should be performed by a Gas Safe registered engineer who will be able to identify the problem and provide a proper quote.

How much does it cost to repair a combi boiler diverter valve?

On a combi boiler, hot water but no heating is almost always the diverter valve a seized circulation pump is the other possibility. Both need a Gas Safe engineer, with repair or replacement typically costing £250–£350 including parts and labour.

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Read our guide on what is a boiler diverter valve for a full cost breakdown.

Quick Troubleshooting Summary

Use this table to quickly match your symptom to the most likely cause and the right fix:

In summary:

• If the water is still running in the radiator despite cold radiators, then the radiator may have an airlock or sludge, bleed the radiator or think about a power flush.
• If the central heating fails to switch on at all, it is likely that this is due to an incorrect thermostat or a timer setting.
• Low boiler pressure is a common cause of hot water working but no heating around the home.
• Radiators not working but hot water fine on a combi boiler almost points to a faulty diverter valve

Safety First

Only a Gas Safe registered engineer can inspect or repair Gas appliances. In the UK, it is not legal and is dangerous to carry out self-repair on gas systems, particularly the diverter valve or pump.

If you’re unsure what’s causing the fault, call 24|7 Home Rescue on 0345 3192 247 and we’ll book your repair with a Gas Safe registered engineer.

Getting Your Heating Back On

Central heating not working but hot water is? It’s usually one of five things all of which can be easily checked and fixed. A radiator airlock, wrong thermostat settings, low boiler pressure, or a faulty diverter valve.

For home emergency assistance, contact 24|7 Home Rescue on 0345 0774 177, or explore our home emergency cover and boiler cover policy options to help protect against unexpected repair costs in future.

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