John Suppes of Clarum Homes explains the Zehnder Heat Recovery Ventilator System at the Menlo Passive Project. One of the keys to a passive home’s efficiency is its air tightness. This means that it’s essential to have a system that brings in fresh air to the house. All air coming into the house is filtered through a Merv 13 air filter. This means all of the air in the home is clean – free of pollens and particulates.