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Fix no hot water.
Combi or system, intermittent or constant — diverter, sensor or DHW heat exchanger fault diagnosed.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Lukewarm not hot
— diverter partially stuck.
Hot but flow weak
— restricted plate heat exchanger.
No hot water at all
— diverter or flow sensor.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Diverter valve replacement
£200–£500
Flow sensor
£150–£350
DHW thermistor
£120–£280
Heat exchanger descale
£400–£800
Plate heat exchanger replacement
£300–£600
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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When the central heating works but no hot water comes out the taps, the problem is almost always inside the boiler — specifically in the hot water path. This guide covers the common causes and what UK boiler repairs cost in 2026.
Common Causes
- Diverter valve stuck — boiler can't switch to hot water mode.
- Flow sensor faulty — boiler doesn't detect tap demand.
- DHW thermistor — sensor failed.
- Heat exchanger scale — restricting flow through DHW circuit.
- Low system pressure — boiler will run but not produce hot water.
What to Check First
- Boiler pressure (1.0-1.5 bar cold).
- Fault codes on display.
- Cold water supply to boiler (not blocked).
- Boiler set to "winter" mode (CH + DHW), not summer (CH only) or off.
Typical Repair Costs
| Issue | Low £ | High £ |
|---|---|---|
| Diverter valve replacement | 200 | 500 |
| Flow sensor | 150 | 350 |
| DHW thermistor | 120 | 280 |
| Heat exchanger descale | 400 | 800 |
| Plate heat exchanger replacement | 300 | 600 |
UK Regulations
- Gas Safe required for repair.
Common Problems
- Lukewarm not hot — diverter partially stuck.
- Hot but flow weak — restricted plate heat exchanger.
- No hot water at all — diverter or flow sensor.
Frequently Asked Questions
Heating works but no hot water?
Diverter valve almost always. Boiler can't switch modes.
How long does the repair take?
Hour for sensor; 2-3 hours for diverter or heat exchanger.
Is it worth repairing?
Yes if under £300 typically. Compare against new boiler cost.
Can it fix itself?
Sometimes — diverter valve sticks free. Worth running CH for a few minutes before calling.
Hot water lukewarm only?
Diverter partial stuck or DHW thermistor reading wrong. Engineer needed.
Combi vs system difference?
Combi has on-demand DHW; system reheats cylinder. Symptoms similar but different diagnoses.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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