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Quick answer
Every condensing boiler produces condensate, mildly acidic water from the flue gases, which must drain away. The preferred method is a gravity drain to an internal waste pipe or external drain. Where there is no suitable fall, such as a boiler in a basement, a cellar or a room far from any drain, a condensate pump collects the water and pumps it to a drain. Condensate pumps need power, can fail, and should be checked during the annual service. A failed pump can make the boiler lock out or cause water to spill.
Key takeaways
- Condensing boilers produce a steady flow of acidic condensate that must be drained.
- A gravity drain is simplest and most reliable where possible.
- A condensate pump is used where there is no gravity route.
- Pumps can fail or block, causing lockouts or leaks.
- Some pumps have a safety switch that stops the boiler if the reservoir overfills.
Why condensate needs draining
A condensing boiler cools flue gases enough that water vapour condenses, recovering extra heat. That water collects in the boiler’s condensate trap and must be removed continuously. On a cold day with high demand, a boiler can produce several litres a day.
Gravity drains first
Installers prefer to run condensate by gravity to an internal soil or waste pipe, which avoids freezing problems. External runs need to be kept short, insulated and of a larger diameter to reduce freezing risk. See condensate pipe checks during a service and frozen condensate pipe.
When a pump is needed
- Boiler in a basement or cellar below drain level.
- Boiler in a location where pipework would have to rise to reach a drain.
- Loft installations or flats without a suitable drain nearby.
- Where external routes would be long and exposed to frost.
How a condensate pump works
Condensate drains into a small reservoir with a float switch. When the float rises, the pump runs and pushes the water through a small pipe to a drain. Many pumps include a safety switch wired to the boiler so that if the pump fails and the reservoir fills, the boiler shuts down before water overflows.
Common faults
- Pump not running because of a power or float fault.
- Reservoir or pipe blocked by sludge from the condensate.
- Noisy operation from worn parts.
- Water leaking from the reservoir.
If your boiler locks out and there is a condensate pump, check whether it has power and whether the reservoir is full. Then call an engineer.
Maintenance
The condensate trap and pump reservoir should be inspected and cleaned during the annual service. Book an annual boiler service.
Book with Gasmaster
Condensate pump giving trouble? Gasmaster Plumbing & Heating, Gas Safe registered (220932), services and repairs boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service.
Frequently asked questions
Is a condensate pump noisy?
They make a short humming sound when they run. Loud or constant noise suggests a fault.
Can a condensate pump freeze?
The pump is usually indoors, but external discharge pipes can freeze.
Does a condensate pump need its own power supply?
It needs power, often from a fused spur or the boiler supply, as specified by the manufacturer.
Sources
- Manufacturer installation manuals for condensing boilers
- Building Regulations Approved Document H, drainage (gov.uk)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
