The heat exchanger is the heart of the boiler. It transfers heat from burning gas to the water circulating through your system — without the two ever mixing. Water passes through a coiled pipe surrounded by hot combustion gases, rapidly warming before being pumped around your home.

⚡ A faulty heat exchanger is one of the most common causes of boiler inefficiency. Look for leaks, corrosion, or carbon deposits.

Shop Heat Exchangers

Component 02

Burner

The burner mixes gas and air in precise proportions, then ignites the mixture to produce a controlled flame. Modern burners extract maximum heat while minimising waste gases. The PCB controls when the burner fires and at what intensity.

🔥 Blocked burner jets cause yellow or orange flames instead of blue — a sign the burner needs cleaning or replacement.

Shop Gas Valves

Component 03

PCB / Controls

The Printed Circuit Board is the boiler's brain. It monitors sensors throughout the system, controls the burner, pump and valves, and displays fault codes when something goes wrong. Always check the error code on the display before diagnosing any fault.

🧠 PCB failures can cause a wide range of symptoms — often mistaken for other faults entirely.

Shop PCBs

Component 04

Pump & Diverter Valve

The pump circulates hot water around the central heating system. The diverter valve in combi boilers directs water to either radiators or hot taps — but not both at once. A seized pump or stuck diverter valve causes loss of heating or hot water.

💧 Diverter valve faults are among the most common combi boiler repairs — often presenting as no hot water or no heating.

Shop Diverter Valves

Component 05

Flue

The flue safely expels combustion gases to the outside while drawing fresh air in for combustion. Condensing boilers extract so much heat that flue gases cool enough to condense — which is why you see steam from modern boiler flues in cold weather.

⚠️ A blocked or damaged flue is a serious safety hazard. Never obstruct your boiler flue — risk of carbon monoxide poisoning.

Shop Flue Fans

Component 06

Expansion Vessel

As water heats it expands. The expansion vessel absorbs this extra volume using a pressurised rubber diaphragm, preventing dangerous pressure spikes. A failed vessel causes the pressure relief valve to release water — leading to a gradual drop in system pressure.

📉 Constantly dropping boiler pressure is often caused by a failed expansion vessel diaphragm — not always a visible leak.

Shop Expansion Vessels

Component 07

Pressure Gauge

The pressure gauge shows the water pressure in the central heating system. Most boilers operate between 1 and 1.5 bar when cold. Too low and the boiler won't fire; too high and the pressure relief valve will discharge water. Always check this first when diagnosing faults.

✅ Ideal pressure: 1–1.5 bar cold, up to 2 bar when fully heated. Anything outside this range needs attention.

Shop Pressure Gauges

Did You Know?

We're Proud of Our Impact.

We're proud to have helped over 100,000 low-income households to access high-quality refurbished boiler parts, saving a total of £15 million and significantly reducing the industry's carbon footprint.

100,000+

Low-income households helped with quality refurbished boiler parts

£15M

Total savings for customers

3.8M kg

CO₂ emissions prevented

Our Brands

National Boiler Spares are part of Heating Trade Supplies Group, a leading boiler spare parts company who delivers quality boiler parts at competitive prices.

Our aim is to become your go-to choice for all your boiler spares. We offer a massive range of brand new, fully compatible, and remanufactured boiler parts. As the only company to hold BSI Kitemark for remanufacturing in the UK, our PCBs, diverter valves, and fans have been independently and repeatedly tested by BSI experts to ensure the quality and performance of our products meet that of a newly manufactured product. All our reconditioned products come with a 1-year warranty. We also remanufacture pumps, heat exchangers, and other parts.

Our sustainable approach to the circular economy has dramatically reduced carbon emissions. Over the last 5 years, we have reconditioned over 70,000 PCBs and over 40,000 fans, in turn preventing the emission of 3,865,000 kg of CO2 into the environment.

Get In Touch

We have over 20,000 products available now via our online store.

We offer free next day delivery on all orders over £50 (to most of the UK, on orders placed before 3pm Monday–Friday). To speak to us about an order or for any other information, please contact us at:

Phone +44 191 263 3708

Whatsapp +44 7834 398673

Email info@heatingtradesupplies.com

Address National Boiler Spares, Tyne View Terrace, Wallsend, NE28 6SG , Great Britain

National Boiler Spares — Wallsend, NE28