In hot water

There are an estimated 16 million immersion heaters in operation in the UK and one million are replaced annually. Of these, some 10 000 fail with the thermostat contacts closed. A system full of boiling water is the inevitable result.

The consequences of this can be disastrous where polymer header tanks are used. They are not capable of withstanding the elevated temperatures caused by the system venting into them. Eventually the tanks collapse and this can result in damage to property and cause injuries.

The old standard applying to fixed immersion heaters has not been reviewed for nearly 30 years and the European standards authorities have been concerned for some time that the risks associated with these immersion heaters are too great.

In April 1999 the BSI published a new standard: European Standard EN 60335-2-73 replaces BS 3456: Part 2. It introduces new requirements of construction and, for the first time, stipulates a maximum water temperature for installations using a fixed immersion heater.

The most significant clauses within EN 60335-2-73 are 19 and 22 and any person or organisation that repairs, replaces or installs immersion heaters should be aware of them:

• Clause 19, Abnormal operation, requires that the immersion heater is able to withstand an input of 15% over the rated power input and during this test the water temperature in the cylinder must not exceed 988C.

• Clause 22, Construction, requires that any over-temperature protection device that is used in compliance with Clause 19 must be ‘non-self resetting’. This means that the cut-out must remain an open circuit until it is manually reset.

A further requirement of Clause 22 is that the sensing device and safety cut-out contacts should operate independently of the thermostat or temperature control contacts.

The BSI has allowed a transition period during which immersion heaters and thermostats to the old standard may be sold alongside those complying with the new. However, that does not eliminate the duty of the installer to offer the client the best possible advice.

It may be tempting to compare the price of the new and old thermostat and decide to stick with the old on the grounds of cost. However, although any damage or injury may be caused by sub-standard polymer tank installation, it is a strong possibility that the installer of the immersion element and rod thermostat will be held liable.

All organisations dealing directly or indirectly with the public are required, by law, to exercise a ‘duty of care’, ie goods supplied must be of a type that will not cause injury to the user. That liability now extends to the directors and officers of the organisation, personally.

The British public is following the trend of its American cousins and becoming more and more litigious and so an ill thought out installation or even merely poor advice can cost a company a lot more than its reputation.

A conventional defence to ‘duty of care’ claims is that the technology available at the time of sale was not sufficiently advanced to enable an alternative, safer product to be offered. This defence will no longer work in the event of an injury resulting from a failed immersion heater and thermostat, as safety immersion heater thermostats are now readily available.

Safety thermostats incorporate a temperature control plus an over-temperature device and prevent water in the system from boiling. There are no excuses for not upgrading an old installation to comply with the new Clauses 19 and 22, as most safety thermostats are suitable for direct replacement of the old rod thermostats.

There are basically two types of safety thermostat on the market today. One-shot devices are literally that – they cut out once when the water temperature exceeds the set safety limit and then have to be replaced. They are susceptible to cut out caused by external heat sources such as a central heating boiler and should therefore be avoided in all but the simplest immersion heater applications.

Suitable for most applications are the types that incorporate an independent cut-out that can be reset and re-used. Although the initial price is a bit higher, the cost in the longer term can be much less as this is generally a fit and forget device.

A little extra time taken to explain clearly to the client the advantages of fitting a safety thermostat will provide them with a safer, more secure installation and you, the installer, with peace of mind.