Exhaust Air Energy Recovery

Exhaust air energy recovery provides one of the most economical ways to meet Indoor Air Quality (IAQ) requirements by allowing for the introduction of outdoor air for the purposes of dilution, while recycling waste energy to reduce the heating, cooling and humidification loads required by the space. This is the relationship between energy recovery and ventilation.

Ventilation Air GraphVentilation Air

ASHRAE Standard 62.1 prescribes necessary outdoor air ventilation rates in order to maintain acceptable levels of IAQ based on a variety of factors. Introducing outside air into occupied spaces for the purposes of dilution reduces occupant discomfort and complaints, and a well designed and ventilated area will result in high levels of productivity, health and satisfaction. For this reason, an increasing number of high performance building standards and guidelines allocate additional points when ventilation rates are increased by 30% above the minimum requirements to increase the number of air changes, and provide more fresh air.

Minimum Energy Recovery Requirements

Naturally, this additional outdoor air comes at a cost, which is why the latest ASHRAE Standard 90.1 recommends the use of exhaust air energy recovery in every climate zone down to as little as 30% outdoor air. In addition, the enthalpy recovery effectiveness needs to be a minimum of 50% (which is defined as a change of enthalpy between the outdoor air and return, equal to greater than 50%). These recent changes to the latest version of the standard will require air-to-air energy recovery in many systems and applications, where exhaust air energy recovery was never considered before.

DOE Climate Zone
% Outdoor Air at Full Design Airflow Rate
>30% and <40% >40% and <50% >50% and <60%
>60% and <70% >70% and <80% >80%
Design Supply Fan Airflow Rate (CFM)
3B, 3C, 4B, 4C, 5B NR NR NR NR >5,000 >5,000
4B, 2B, 5C NR NR >26,000 >12,000 >5,000 >4,000
6B >11,000 >5,500 >4,500 >3,500 >2,500 >1,500
1A, 2A, 3A, 4A, 5A, 6A >5,500 >4,500 >3,500 >2,000 >1,000 >0
7, 8 >2,500 >1,000 >0 >0 >0 >0
NR: Not required

Minimum High Performance Building Energy Recovery Requirements

In addition to these changes to the minimum energy standard, ASHRAE has also developed Standard 189.1, which prescribes the minimum exhaust air energy requirements for a high performance building. As a result, Standard 189.1 goes beyond the basic recommendation and expands the below table down to as little as 10% and increases the enthalpy effectiveness up to 60%.

Energy Recovery Requirement (I-P)
 Climate Zone
% Outside Air at Full Design Flow
>10% and <20%
>20% and <30%
>30% and <40%
>40% and <50%
>50% and <60%
>60% and <70%
>70% and <80%
>80%
Design Supply Fan Flow (CFM)
3B, 3C, 4B, 4C, 5B
NR
NR
NR
NR
NR
NR
>5,000
>5,000
1B, 2B, 5C
NR
NR
NR
NR
NR
>26,000
>5,000
>4,000
6B
NR
>22,500
>11,000
>5,500
>4,500
>3,500
>2,500
>1,500
1A, 2A, 3A, 4A, 5A, 6A
>30,000
>13,000
>5,500
>4,500
>3,500
>2,000
>1,000
>0
7, 8
>4,000
>3,000
>2,500 >1,000
>0 >0
>0
>0
NR: Not required 

All Venmar CES products have been designed with these considerations in mind to be the most efficient system capable of delivering a wide array of outdoor air ventilation rates in a variety of different configurations. They can be used as 100% OA systems and many can also be effectively used with minimum outside air under mixed air configurations.

Venmar CES offers the most complete array of AHRI Certified™ air-to-air energy recovery devices of any manufacturer. The complete commercial offering of technologies includes certified sensible energy recovery systems employing heat pipes, aluminum or polypropylene plates and sensible rotary wheels, and latent devices such as desiccant wheels and static membrane plate, heat and moisture cores.

External Links

AEE
ASHRAE
DOE

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USGBC