Pros and Cons of Heat Recovery Ventilators

HEAT RECOVERY

Energy Recovery Systems

Energy Efficiency and Energy Recovery

Fresh air is essential to healthy people – and healthy buildings. That’s why commercial buildings are required to bring in fresh air – typically 15-20 cubic feet per minute (cfm) for every occupant. This unconditioned air greatly increases your building’s air-conditioning load – and since an equal amount of air must be vented outdoors, you’re basically “throwing away” air you’ve paid to cool.

Energy recovery ventilation systems – or “ERVs” – help reduce this waste and lower your energy costs. What’s more, an FPL incentive program now helps businesses pay for this technology – so you can save even more when you install a qualifying ERV unit on a new or existing HVAC system.

Energy recovery systems typically incorporate heat exchange equipment to reduce energy costs by extracting heat from the facility’s exhaust air stream before it is vented outside. Energy recovery from the laboratory’s exhaust should be considered when significant portions of operating hours are at ambient temperature of 50°F (10°C) and below. Another recoverable energy source is provided by chiller/DX condensers. Water cooled condensers can be piped to reject waste back into the labs HVAC system to provide reheat capacity, to augment run-around coil systems, and to dry regenerative heat wheels. When properly designed, these energy recovery systems can reduce installed HVAC system capacity by one-half; reduce operating energy from one-third to two-thirds, depending upon mode of operation; and have life-cycle cost paybacks from immediate to three years. The four major energy recovery systems include run-around coil systems, regenerative heat wheels, heat pipes, and fixed-plate exchangers

Plate heat exchangers (recuperators)

A minimum effectiveness of 50% (based on sensible energy transfer under balanced flow conditions).
Internal leakage < 1% (for units >0.2m3/second rating)
Pressure drop <250 Pascal (Pa) NOTE: where pressure drop data is not documented, the fan power consumption must be consistent with a pressure drop within this limit.

Thermal wheels

A minimum effectiveness of 70% (based on sensible energy transfer under balanced flow conditions).
Internal leakage < 5%
Pressure drop <200 Pa

Run around coils

A minimum effectiveness of 45% (based on sensible energy transfer under balanced flow conditions).
Pressure drop <100 Pa across each coil
Water side pressure drop < 25 kPa per coil

Heat pipes

Disadvantages of Energy Recovery Ventilation.

Unlike air handling units on unit ventilators, energy recovery systems do not have the capability to provide sufficient outside air for cooling overheated rooms. This can lead to overheating, especially in the late spring and early fall. Any building employing energy recovery ventilation should have a by-pass sytem for ERV ‘s or operable windows (or air conditioning) to provide cooling during warm weather.

Energy recovery ventilation is not a good choice for interior spaces unless those spaces are air-conditioned.

A second area of concern involves the fact that there is little standardization in the heat recovery industry. Manufactures and products come and go, and there is some concern that products specified today may not have manufactures’ support a few years from now.

Heat exchangers to recover heat increase the pressure drop of the air handling system, and increase the power demand of the fans.
This is however typically only 5-10 % of the recovered energy.

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