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Dunham-Bush - Low Surface Temperature (LST) Radiators & Convectors

Low Surfact Temperature - LST - Radiators - Natural Convectors - Heating Systems

Safe Heating for Buildings

Safety is paramount in all buildings; designers have a duty of care to eliminate or minimise all risks to building occupants as is reasonably practical. Buildings such as day centres, playrooms, surgeries, nurseries and care homes are used by people of all ages who are vulnerable. Risk assessment, provided by the Management of Health and Safety at Work Regulations 1999, will enable the designer to identify hazards, assess the risk of injury and decide appropriate measures. All occupants are deemed to be at risk and some are more vulnerable than others to scalding or burning from hot surfaces or water temperatures; research has shown that partial thickness burns to skin can be inflicted by solid surfaces at temperatures of 45°C or more for a period in excess of 2 hours (a possible occurrence if the occupant is rendered immobile or unconscious).

Heating systems are typically designed using Low Pressure Hot Water (LPHW) with a flow temperature of 80°C and a return temperature of 60°C; if the surface temperature of a heating device is between these two, then the average person could receive a partial thickness burn in about one second, with a full thickness burn being inflicted after approximately ten seconds. Hence, a heating device should operate with a maximum surface temperature when running at design output, which can be achieved by installing a heating device designed to have a low surface temperature.

NHS Estates Health Guidance Note ‘Safe’ Hot Water & Surface Temperatures 1998 provides guidance for designers on safe surface temperatures of components and services. The guidance recommends that heating devices should have a maximum surface temperature of 43°C when operating under design conditions.

Heating devices should be designed so that there are no surface ‘hot spots’. All openings (e.g. grilles) should prevent small hands entering the device and touching hotter surfaces inside the device. They should be designed to be easily cleaned as a routine maintenance activity, to prevent dust and debris build-up and also to control infection. Consideration should also be given to exposed horizontal or vertical pipework within 2m of floor level, since this will present the same risk as the heating device; therefore pipework to the device should be boxed or insulated.

For most heating devices, the outlet grille has the highest surface temperature. But a risk assessment may deem the outlet grille does not present a significant hazard. Therefore, higher grille temperatures may be acceptable since casing temperatures will always be significantly lower than grille temperatures.

Standards for Testing

CE marking on heating devices is a mandatory requirement for construction products and can only be applied to products that have been successfully tested to an EU standard by a notified body, accredited under the Construction Products Regulations (305/2011/EU-CPR).

The only accredited body in the UK is BSRIA, who test in accordance with the harmonised standard:

BS EN 442 Specification for Radiators and Convectors

Part 1 1996 Technical Specifications and Requirements

Part 2 1997 Test Methods and Rating

Part 3 2003 Evaluation of Conformity

Part 1 provides technical specifications for radiators and convectors which are permanently installed in a building and fed with hot water or steam below 120°C from a remote heat source.

Part 2 specifies the laboratory apparatus, test method and criteria for selecting test samples. It also specifies the correct information that a manufacturer shall provide from the test data.

Part 3 specifies how radiators and convectors are evaluated to comply with the standard.

A radiator or convector can be tested to BS EN 442 and its thermal output can be measured at various water temperatures. This enables characteristic equations to be determined for different sizes or models of the same type.

Similarly, surface temperatures are measured at various water temperatures and characteristic equations can be derived to predict surface temperatures at different operating conditions.

Low Surface Temperature (LST)

Natural Convectors

The table below shows performance data for a typical Low Surface Temperature (LST) natural convector which is wall mounted with a casing width of 1500mm. The casing includes an outlet grille on the casing top and inlet grille on the casing bottom, with a hot water emitter connected to LPHW flow and return. Air temperature is taken at 0.75m above finished floor level.

A review of the results can yield the following conclusions:

  1. Greater output is achieved from the stack effect; an increase in the height of the casing will induce better airflow and heat transfer, leading to higher thermal output.
  2. This in turn leads to reduced maximum surface temperatures at the outlet grille on top of the casing.
  3. Reducing the return temperature will only lead to a lower mean water temperature; this reduces output, but will not reduce grille surface temperature which is determined by the water flow temperature.
  4. Reducing the water flow temperature will reduce output as well as bring the grille surface temperature down to an acceptable value when selected in conjunction with an appropriate casing height.
  5. reduced room design temperature will increase heat output by virtue of a greater mean temperature difference across the emitter, and also lead to reduced grille surface temperatures.

Click the PDF image to view the technical information in full

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