Making pump overrun a thing of the past

Making pump overrun a thing of the past

Most modern heat only boilers need to use pump overrun to dissipate residual heat and reduce the risk of water boiling in the heat exchanger when it is switched off. Andy Speake, Product Manager at Baxi, explains more about pump overrun and how Baxi heat only boilers have been designed without the need for pump overrun or permanent live wiring.

Older boilers were made of very different materials to the high efficiency models we manufacture today. When they turned off, any residual heat left in the heat exchanger could safely disperse through the casing and flue.

Today’s boilers are much more energy efficient, but because they are lighter, they are less able to disperse any residual heat. This means that when they turn off there is a danger that residual heat will cause the water in the heat exchanger to boil, causing extensive and costly damage to the heat exchanger.

Most manufacturers overcome this risk by using a pump overrun. The boiler is designed so that the pump continues to run for a short period after the boiler has switched off, circulating the water so the heat is dissipated through the heating system and won’t boil in the heat exchanger.

As the pump is usually installed in the airing cupboard next to the hot water storage cylinder, the installer needs to run a wire from the pump to the boiler, which is often situated in another part of the house. Wireless options are available, but they are costly and may not always be suitable.

Aluminium heat exchanger
Baxi uses aluminium heat exchangers for its heat only boilers. Aluminium is a great conductor of heat, and holds less residual heat. The heat exchangers for Baxi heat only boilers have been carefully designed to be thinner and slimmer, so they react more rapidly when the boiler is turned off, meaning the risk of boiling water is eliminated, as is the need for a pump overrun.

That means the installer does not need to run wires from the boiler to the pump, which often involves lifting carpet and floorboards, and then making good. This is good news for everyone as it is time consuming for the installer and inconvenient for the homeowner.

Less disruption
The Baxi 800 Heat is the newest heat only boiler to be added to the company’s range. Like all its heat only models, there is no requirement for a pump overrun that needs power when the boiler switches off or a screen or power light that needs to be on when the external controls have switched the boiler off. This saves the installer the problem of getting a permanent power supply to the boiler and means less disruption overall when completing the job.

With outputs of 16, 25 and 30kW, and weighing 17.5 kg, the 800 Heat completes Baxi’s line-up of easy-to-install and efficient heat only, system and combi boilers. Fully compatible with market-leading Megaflo cylinders, the Baxi 800 Heat is suited for homes with several bathrooms or a high requirement for hot water.

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