Table of Contents
- The outdoor-air ratio (OAR) is the share of supply air that is genuinely fresh outdoor air: OAR = outdoor_air_flow / total_supply_air_flow A space with no recirculation runs at OAR = 1 (100% outside air).
- Some spaces cannot recirculate for safety or code reasons: Cleanrooms and graded pharmaceutical suites Pharmaceutical exhaust Laboratories with hazardous vapours Kitchens and toilet exhaust With OAR = 1 there is no return air to blend with, so every joule of condition is spent on outdoor air - and every joule of exhaust is pure recoverable energy.
- Because all supply must be conditioned from outdoor state, an AHU recovery core in a 100% OA system delivers the largest percentage saving of any configuration. This is why recovery is effectively mandatory in high-OAR designs under energy codes.
- Does recirculation remove the need for recovery? No, but it reduces the outdoor fraction, so savings are smaller; recovery still helps the OA part. Can OAR exceed 1? Only transiently (economiser boost); the steady design value is between 0 and 1. Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team - WhatsApp +86 157 5335 5505, or email kuns913@gmail.com. Frequently Asked Questions What is outdoor air ratio? It is the fraction of supply air that is fresh outdoor air versus recirculated; 100 percent outdoor-air systems bring in no return air to mix. Why do 100 percent outdoor-air systems need recovery most? With no recirculation there is nothing to temper the incoming air internally, so an energy recovery wheel or plate exchanger is the main way to cut the heating and cooling load.
The outdoor-air ratio (OAR) is the share of supply air that is genuinely fresh outdoor air:
OAR = outdoor_air_flow / total_supply_air_flow
A space with no recirculation runs at OAR = 1 (100% outside air).
Some spaces cannot recirculate for safety or code reasons:
- Cleanrooms and graded pharmaceutical suites
- Pharmaceutical exhaust
- Laboratories with hazardous vapours
- Kitchens and toilet exhaust
With OAR = 1 there is no return air to blend with, so every joule of condition is spent on outdoor air - and every joule of exhaust is pure recoverable energy.
Because all supply must be conditioned from outdoor state, an AHU recovery core in a 100% OA system delivers the largest percentage saving of any configuration. This is why recovery is effectively mandatory in high-OAR designs under energy codes.
Does recirculation remove the need for recovery? No, but it reduces the outdoor fraction, so savings are smaller; recovery still helps the OA part.
Can OAR exceed 1? Only transiently (economiser boost); the steady design value is between 0 and 1.
Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team - WhatsApp +86 157 5335 5505, or email kuns913@gmail.com.
Frequently Asked Questions
What is outdoor air ratio?
It is the fraction of supply air that is fresh outdoor air versus recirculated; 100 percent outdoor-air systems bring in no return air to mix.
Why do 100 percent outdoor-air systems need recovery most?
With no recirculation there is nothing to temper the incoming air internally, so an energy recovery wheel or plate exchanger is the main way to cut the heating and cooling load.