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Got more questions about boiler Not Heating Radiators? Describe the bit you're unsure about — the AI explains options, sequencing and what to ask the boiler engineer.
Fix cold radiators.
Pump, motorised valve, sludge or air — what the heat-pattern tells you and what a fair fix costs.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Hiring a boiler engineer, without the regret.
Four moves that separate a smooth job from a nightmare.
Some rads work, some don't
— system imbalance or stuck valves.
Cold radiator with hot top
— air at top, bleed it.
Cold top, hot bottom
— sludge at bottom; power flush.
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
Pump replacement
£250–£500
Motorised valve replacement
£180–£400
Power flush
£350–£700
Air vent / bleeding
£80–£180
Pipe blockage clearance
£200–£500
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Boiler Engineer explained
When the boiler runs but radiators stay cold, the problem lies between boiler and radiators — pump, valves, or system. This guide covers what's likely wrong and what UK repair costs are.
Common Causes
- Pump failure — water can't circulate.
- Motorised valve stuck — diverter or zone valve doesn't open.
- System sludge — water can't flow through radiators.
- Wrong thermostat setting — programmer/timer off.
- Air-locked system — needs bleeding.
What to Check First
- Programmer/thermostat — heating switched on.
- Room thermostat set above current temperature.
- Bleed valves on radiators — release air.
- Boiler pressure (1.0-1.5 bar cold).
- Listen for pump (whirring sound).
Typical Costs
| Issue | Low £ | High £ |
|---|---|---|
| Pump replacement | 250 | 500 |
| Motorised valve replacement | 180 | 400 |
| Power flush | 350 | 700 |
| Air vent / bleeding | 80 | 180 |
| Pipe blockage clearance | 200 | 500 |
UK Regulations
- Gas Safe for boiler/system work.
- Bleeding radiators is DIY-friendly.
Common Problems
- Some rads work, some don't — system imbalance or stuck valves.
- Cold radiator with hot top — air at top, bleed it.
- Cold top, hot bottom — sludge at bottom; power flush.
Frequently Asked Questions
Why are radiators not warming?
Most common: pump failure or stuck valves. Less common: severe sludge.
How do I bleed radiators?
With bleed key on radiator valve. Open slowly until water runs out (not air). Close. Top up pressure.
Pump or motorised valve?
Pump if no rads heat. Motorised valve if heating selected on programmer but boiler stays off.
Is a power flush always needed?
If sludge is severe yes. Magnetic filter or basic flush may suffice for milder cases.
Can I run boiler with cold rads?
Briefly to diagnose; not long-term. Boiler will short-cycle which damages it.
Quick DIY checks?
Bleed rads, check valves are open, check thermostat. Then call engineer.
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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