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The Use of Frost Protection in AHU Design

If you have not considered an adequate frost protection strategy, then you could be putting your ventilation equipment at risk unnecessarily

By Alun Thomas, Engineer Director, October 2016

Units subjected to high fresh air volumes and sub zero conditions are likely to experience one or more of the following: ruptured or burst LPHW coils, and associated water damage to the building structure. This can occur when there is a time-lag between the room sensor and damper sequence in rooms with a high gain in winter or for systems utilising hydronic coils with small water volumes or low flow rates. The results are frozen, blocked or collapsed filters as the condensed water freezes on the filter media and the associated fouling of internal components. This in turn effects unit performance, heating efficiency and maintenance frequency (Filters and their frames which have collapsed when exposed to excessive pressure differential).

CIBSE guidelines state that “frost protection is required upstream of the filters in both full fresh air and recirculated systems”, ideally monitored from a downstream thermostat to control the air to above 4⁰C. This ensures that the air into the system does not damage any of the components as it passes through the various sections, while still providing the maximum temperature differential through heat recovery sections.

It is most important to maintain the required ventilation rates irrespective of the outdoor temperature and in most cases, shutting off the ventilation equipment when subjected to freezing conditions is not a viable control strategy – especially in an active building such as an office, theatre, or leisure centre. Nuaire’s Ecosmart control system has been designed with frost protection in mind to ensure that the air is kept above the recommended 4⁰C. These LPHW or Electric Heater Batteries are placed upstream of the filter banks as per CIBSE guidance and ensure that the air into the system has been tempered for proper unit operation. Should this system fail for any reason, Ecosmart has a secondary failsafe system, which constantly monitors the air inside the unit. If this was to fall below 4⁰C, the system safely shuts down and energises the LPHW coils to warm the casing back up, while emitting a frost alarm.

At Nuaire, we use only the highest quality components in our ventilation equipment. Take our Eurovent Certified G4 panel filters for instance. These are manufactured from an Aquakote® card frame which has a higher resistance to collapse or disintegrate when exposed to high moisture levels. The filter media is a polypropylene based product which is hydrophobic and will not load with moisture. The adhesive used has a high acrylic base which, unlike other adhesives, retains adhesion even when wet. In the interest of Safety and Good Practice, Nuaire would recommend fitting frost coils in all instances where units might be subjected to freezing conditions and also suggests maintaining units with equivalent high quality filters to prolong the life of the unit.

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