Chris White, research and development director of VES Andover, is surprised how many ventilation systems are underperforming - and explains how the solution may be simpler than people think
Remarkable efficiencies are being claimed for plate-heat exchangers.

Efficiencies of 80% are being highlighted in advertisements and trade literature.

But can the suppliers prove it? Under what conditions might that efficiency be achieved?

In the UK, we are fortunate in having the BSRIA test laboratories, to simulate and record environmental conditions, test product duties, and issue a certificate for approval to BS or EN
standards.

Test facilities

One or two of the large manufacturers also have certified test facilities. Similarly, AMCA testing and approval is of a high standard, as are some test facilities in France and Germany.

However, when manufacturers or distributors rely on a set of
calculations by a lesser-known third party, should they publish the data without questioning it? How does that information comply with
ISO or CE documentation?

Clearly, pressure is on suppliers of heat-recovery ventilation
equipment to provide low-energy products with maximum heat recovery.

The first challenge is the maximum specific fan power (SFP), detailed in Building Regulations 2006.

For a supply-and-extract unit with heat recovery, this set a limit of 2.5W/l/s for the two fans combined in new buildings, and 3.0W/l/s in older buildings. A typical double-inlet forward- curved direct-driven fan, as used by the majority of the industry in the UK, will not meet this criteria.

What VES Andover has decided to do is to replace this type of fan with ones that have a combined SFP within the limits and that save energy. Then the updated range of Ecovent Energy heat-recovery units will have the lowest energy components.

A project like this is a major research and development exercise.
The designer may be tempted to succumb to the attractions of the thermal wheel. These have high efficiency due to latent recovery.

They also contain a dessicant-coated wheel that absorbs
moisture on extract and releases it on the supply side.

Heat recovery

However, the standard type of thermal wheel normally installed has coiled aluminium-strip media, which has comparable heat-
transfer properties to the plate-heat exchanger. The higher levels of heat recovery only occur in latent-transfer applications.

Now, look at the construction of the thermal wheel - it is a heavy wheel, mounted on a small shaft, driven by a tiny motor and a rubber band. What will be the life expectancy of such a combination?

With heat wheels, there is a certain amount of cross-contamination between supply and extract airflows. The amount depends on the difference in air volume and pressure. How much will this increase several years down the line, when the rubber seals or brushes start to perish?

As an organisation that refurbishes ventilating and air-handling equipment, VES Andover has been asked to replace failed heat wheels in air-handling units. Wareham Sports Centre in Dorset was a typical example. In one unit the shaft had corroded and broken away from the wheel, in the other unit the wheel was clogged, and the bearings had failed. This was after ten years of life.

The wheels were large and difficult to remove from the units, and the price from the manufacturer was £3,200, each, for replacements. The decision was to cut the wheels up and modify the air-handling unit cases to accept plate-heat exchangers. This was a considerably cheaper option, and one that required little
maintenance.

Controls system

Finally, a good controls system is essential to get the best out of a heat-recovery air-handling unit. It needs three air sensors - one to monitor the fresh air intake, one for the supply air, and one for the extract air or room temperature. From this the face and bypass damper on the plate-heat exchanger will automatically be adjusted for heat recovery, free cooling or cooling recovery.

Another feature which VES provides is intelligent start-up. The controls monitor the time it takes for the space to reach the required temperature over a number of days, and automatically adjusts the start time each day, minimising energy use.

Going one step further, incorporating an air-quality sensor into the system reduces air volume to the minimum necessary, providing further energy savings.There is a fine line with controls for heat recovery, between an easy-to-use system with the essential features, and one that is too complex, and not set up properly because it was not fully understood.

VES Site Services undertakes ventilation systems controls
surveys, checking the operation, settings, looking for faults and energy-saving possibilities. It is often surprising how poorly a
system is functioning, and how easy it is to put it right.

www.ves.co.uk