No Hot Water But Heating Works? What's Wrong

When your heating works but you have no hot water, the cause depends on your boiler type. This guide helps you diagnose the problem and decide if you need a Gas Safe engineer.

First: Identify Your Boiler Type

Combi Boiler

• Heats water directly from mains
• No hot water tank
• Hot water on demand
• Most common in UK homes

System/Regular Boiler

• Heats water stored in a cylinder
• Has hot water tank (usually in airing cupboard)
• May have header tank in loft

The causes and fixes differ for each type.

Combi Boiler: No Hot Water But Heating Works

1. Diverter Valve Stuck

What It Is: The diverter valve directs heat to either radiators OR the hot water heat exchanger.

Symptoms:

• Heating works normally
• Hot water tap gives cold or lukewarm water
• May have worked fine then stopped

The Fix: Diverter valve needs freeing, cleaning, or replacing. Professional repair needed (£200-£350 typical cost).

2. Hot Water Heat Exchanger Scaled Up

What Happens: Hard water deposits block the plate heat exchanger that heats tap water.

Symptoms:

• Water gets warm but not hot
• Hot water flow reduced
• Problem gradually got worse

The Fix: Heat exchanger cleaning or replacement. May need system descale (£200-£400).

3. Hot Water Thermostat Set Too Low

Simple Check:

• Find boiler controls (on boiler or digital controller)
• Check hot water temperature setting
• Should be 50-60°C for most uses
• Increase if set lower

4. Microswitch Failure

What It Does: Tells boiler that a hot tap has been opened

Symptoms:

• Boiler doesn't "fire up" when tap opened
• No flame or ignition sound
• Heating works fine

The Fix: Microswitch replacement (£150-£250).

5. Flow Sensor Problem

What It Does: Detects water flowing when tap is opened

Symptoms:

• Similar to microswitch failure
• Boiler doesn't respond to taps

The Fix: Clean or replace flow sensor (£100-£200).

System/Regular Boiler: No Hot Water But Heating Works

1. Immersion Heater Off or Faulty

Check:

• Is immersion heater switched on? (usually a switch near cylinder or fuse board)
• Has the fuse tripped?
• Try resetting the switch

If Switch Is On: Immersion element may have failed (£150-£250 replacement).

2. Motorised Valve Stuck

What It Does: Controls flow between heating and hot water circuits

Symptoms:

• Can hear boiler running
• Cylinder doesn't get hot
• Valve may be buzzing

The Fix: Valve head or motor replacement (£150-£250).

3. Cylinder Thermostat Problem

What It Does: Tells boiler when cylinder needs heating

Symptoms:

• Cylinder cold despite boiler running
• Thermostat not calling for heat

Simple Check:

• Turn cylinder stat to maximum
• Listen for boiler to fire

The Fix: Cylinder stat replacement (£80-£150).

4. Programmer/Timer Settings

Simple Check:

• Is hot water set to "on" or "timed"?
• Are timer settings correct?
• Does manual override work?

5. Air Lock in Cylinder

Symptoms:

• Gurgling sounds
• Sporadic hot water
• Hot water comes and goes

The Fix: Professional bleeding of cylinder and pipework.

Quick Troubleshooting Checklist

Check	Combi	System/Regular
Hot water setting on controls	✓	✓
Gas supply OK	✓	✓
Boiler pressure 1-1.5 bar	✓	N/A
Immersion switch on	N/A	✓
Timer/programmer correct	✓	✓
Error codes displayed	✓	✓
Pilot light on (if visible)	✓	✓

DIY Fixes to Try

Reset Your Boiler

1. Turn boiler off at controls
2. Wait 60 seconds
3. Turn back on
4. Allow 2-3 minutes to restart

Check Error Codes

• Note any displayed codes
• Check boiler manual or manufacturer website
• Codes help engineers diagnose faster

Check Gas Supply

• Are other gas appliances working?
• Is prepayment meter topped up?
• Check emergency control valve is open

When to Call a Gas Safe Engineer

Call immediately if:

• You smell gas
• Boiler is leaking
• Error codes indicating CO or gas issues
• You're not confident with any checks

Book a repair if:

• Basic checks don't solve the problem
• Problem has been ongoing
• Water is warm but not hot enough

Repair Costs Summary

Component	Typical Repair Cost
Diverter valve	£200-£350
Heat exchanger	£300-£500
Motorised valve	£150-£250
Flow sensor	£100-£200
Thermostat	£80-£150
Diagnostic visit	£60-£85

Get Hot Water Restored

No hot water is frustrating, especially in Glasgow winters! Our Gas Safe engineers diagnose and fix hot water problems quickly.

Plumbers Glasgow Ltd:

• Call: +44 141 468 9930
• Same-day emergency appointments
• No hot water specialists
• All boiler makes and models

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Related Services:

• No Hot Water Glasgow
• Boiler Repair Glasgow
• Gas Engineer Glasgow

Related Articles:

• Boiler Making Noise?
• Boiler Losing Pressure?