Heat recovery ventilation summer bypass: what can it do, and what can it not do?

A summer bypass helps regulate heat transfer in a ventilation unit. Under suitable conditions, for example when it is cooler outside than indoors in the evening, supply air can be routed so that extracted air does not warm it unnecessarily. ^S20

This is not a separate refrigeration unit. If it is hot outside, opening the bypass alone will not produce colder air. The airflow of a typical domestic ventilation system also does not by itself guarantee removal of all heat from the sun and other sources.

Different heat exchangers and controllers implement summer mode differently. It is therefore incorrect to give every heat recovery unit the same damper or rotor control instructions. Follow the operating manual for the specific unit. ^S20

Customers should coordinate ventilation with other summer comfort measures: window shading, solar control and separate cooling where necessary. Passive ground source cooling and a heat recovery unit's summer bypass are different things.

Before ordering: ask how the selected unit will work in summer. Assess actual outdoor conditions, controls and expected room heat load, not just the feature name.