Each update to ASHRAE/IES Standard 90.1 has lowered the threshold at which exhaust air energy recovery (EAER) is required in connection with outdoor air systems. In some instances, conventional EAER may not be practical, either because the exhaust air is dirty, or there isn’t enough exhaust air, or the exhaust airstream is too far away from the outdoor airstream. This article reviews Standard 90.1 requirements for hydronic series energy recovery, provides practical guidance on equipment selection and reports on measured performance.