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How an ERV gives a tight house fresh air.

Energy recovery ventilators, explained: what they do, when a house needs one, and how they compare with opening a window.

Two streams that never mix.

Stale air leaves from kitchens and bathrooms; fresh air comes in and is filtered. Inside the core they pass each other through thin walls, so the outgoing air warms (or cools) the incoming air.

How an energy recovery ventilator works Outside air enters on the left, passes through the core in the ventilator and is delivered to the rooms on the right. Stale air from the kitchen and bathrooms enters on the right, crosses the core in the other direction and is exhausted outside on the left. The two streams cross in the core without mixing. OUTSIDE INSIDE Exterior wall Fresh air in Stale air out Stale air from kitchen & baths Fresh, filtered air to rooms The core heat (and some moisture) passes across; air does not
  • Fresh air
  • Stale air
Simplified drawing. Real units have fans and filters on both streams and connect to ducts; many also have a bypass for mild weather.

Three ways to get fresh air.

Open a window

Free and immediate. But it lets in noise, pollen and smoke, and in winter the heat goes out with the stale air.

Fine on mild days

Bath & kitchen fans

Remove moisture and smells where they start. In a tight house, the replacement air leaks in wherever it can, unfiltered.

Always keep these

An ERV

Balanced, continuous fresh air through a filter, with most of the heat kept in. Quiet, and runs in the background.

Best for tight, remodelled homes
Homes

When a house needs one.

  • After new windows, insulation or air sealing
  • Condensation on windows, or stuffy bedrooms in the morning
  • Allergies, or wildfire-smoke days you'd rather keep outside
  • New homes: California's energy code (Title 24) already requires whole-house mechanical ventilation
A compact energy recovery ventilator mounted in a clean attic with insulated ducts attached
Businesses

When an office needs more air.

  • Meeting rooms that get stuffy in the afternoon
  • Studios and small offices in older buildings with sealed windows
  • Rooftop units whose outdoor-air dampers were closed years ago
A small design studio with desks and a ceiling supply grille, daylight through a large window

Smoke days.

When wildfire smoke drifts in, an ERV can be set to pause or reduce outdoor air, and the main system can recirculate through its best filter. We set both up so you know which switch to use.

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A simple wall control for a ventilation system beside a bright kitchen doorway