E25: Sensible Heat Recovery from Exhaust Air – Above Green

Requirement Summary

This measure can be claimed if a Sensible Heat Recovery device with at least 60% efficiency is installed in the ventilation system to reuse the heat from the exhaust air.

Intention

Recovering sensible heat from the exhaust air helps buildings to reduce fossil fuel consumption, and lower operating costs by providing useful heat for space heating and in some cases for space cooling. Buildings that use energy for heating or cooling with fresh air supply have the potential to benefit from the application of heat recovery systems for ventilation.

Approach/Methodologies

When buildings include an HVAC system and the main load of the building is due to space heating, installing sensible heat recovery on the ventilation system reduces energy consumption by preheating the incoming fresh air with the outgoing exhaust air. Alternatively, in cooling mode, the incoming fresh air is cooled with the exhaust air from the air-conditioned space.

To qualify, the design team must demonstrate that the HVAC system has a sensible heat recovery device on the fresh air supply system. No heat recovery system is included in the base case. EDGE uses Temperature Transfer Efficiency (TTE) as the measure of efficiency, which is either quoted by the manufacturer or can be calculated with the following formula:

Temperature Transfer Efficiency (TTE):

Where:

μt = Temperature Transfer Efficiency (%)

T1 = Outside air temperature before heat exchanger (ºC)

T2 = Air temperature after heat exchanger (ºC)

T3 = Exhaust air temperature before heat exchanger (ºC)

Potential Technologies/Strategies

Heat recovery aims to collect and reuse the heat arising from a process that would otherwise be lost. In the case of sensible heat recovery in buildings, it involves transfer of energy between an exhaust airstream that preheats (winter mode) or precools (summer mode) the supply airstream. As air contains moisture, the heat contained within the air can be sensible heat (affects the temperature) or latent heat (includes water vapor). Some energy recovery devices transfer both sensible and latent heat (also called “total heat recovery”), and some only transfer sensible heat, which is the technology covered by this measure.

Sensible Heat Recovery occurs when the temperature of the cooler air stream exchanges heat with the temperature of the warmer air stream. Moisture level is not impacted unless condensation occurs.

In some areas of the building where condensation is expected, such as restaurants, spas and pools, this technology is ideal as the materials are anti-corrosive. It is also convenient for light ventilation systems as it offers low pressure drops.

Relationship to Other Measures

Sensible heat is recovered from the exhaust air, reducing the heating load in heating mode and therefore decreasing consumption in “Heating Energy.” The same principle applies to the cooling load if the building is predominantly in cooling mode; then the reduction is in “Cooling Energy.” The energy due to “Fans” also decreases slightly as less air is moved. However, in climates where both heating and cooling are used, savings appear in the “heating energy,” but the “cooling energy” increases due to some heat trapped during midseason.

Assumptions

The HVAC systems included in the base case do not include heat recovery systems. The improved case is assumed to have a sensible heat recovery device with a Temperature Transfer Efficiency (TTE) of at least 60%. If the actual TTE value is higher or lower than 60%, the value must be entered in EDGE. It is assumed that at least 75% of all exhaust air in the building is being passed through the heat recovery system.

Compliance Guidance

To demonstrate compliance, the design team must provide documentation to support the claims.

Design Stage

Post-Construction Stage

At the design stage, the following must be used to demonstrate compliance:

Manufacturer’s data sheets for the sensible heat recovery device specifying Temperature Transfer Efficiency; or
A calculation to demonstrate efficiency in case the manufacturer’s data does not specify the TTE; and
Mechanical and electrical layout drawings showing the location of the heat recovery technology and indicating % of total air passing through the heat recovery system.

At the post-construction stage, the following must be used to demonstrate compliance:

As-built mechanical and electrical drawings with the location of recovery technology, if changed from design; and
Delivery notes showing that the specified recovery technology has been delivered to the site; and
Manufacturer’s data sheets for the sensible heat recovery technology using the specified efficiency (TTE) if changed from design; or
Photographs of the installed heat recovery system.