What is heat recovery ventilation (MVHR)?

Heat recovery is reclaiming heat from air or water that would otherwise be lost. In homes it usually means mechanical ventilation with heat recovery (MVHR), also called HRV: warm, stale air extracted from the kitchen, bathroom and WC passes its heat in a heat exchanger to the cold fresh air supplied to living rooms and bedrooms. The two airflows do not mix. In Dutch this is called “warmteterugwinning” (WTW).

How does an MVHR unit work?

1. Two fans keep supply and extract balanced.
2. In the heat exchanger, usually a counterflow type, the airflows pass each other in thin channels.
3. Filters keep dust and pollen out of the supply air and protect the exchanger.
4. A summer bypass lets cool night air in without passing through the exchanger.

Manufacturers quote heat recovery efficiencies of around 90 percent or more for modern units; in practice results depend on commissioning, ductwork and airtightness.

Requirements

• An airtight home: with many leaks, air bypasses the unit.
• Space for ducts to every habitable room and for the unit, ideally inside the insulated envelope.
• Transfer paths: undercut doors or transfer grilles.

Maintenance

• Change or clean filters regularly; Dutch consumer body Milieu Centraal advises replacing them twice a year.
• Have the unit and bypass serviced periodically and the system rebalanced from time to time; clean ducts at longer intervals.

Common mistakes

• Adjusting the valves: the system goes out of balance.
• Forgetting the filters: higher electricity use, more noise, poorer air.
• Connecting a cooker hood to the MVHR: grease fouls the exchanger.
• Uninsulated ducts in a cold loft: heat loss and condensation.

Other forms include shower drain heat recovery and sewage heat recovery.

Related terms

• Heat exchanger
• Sewage heat recovery
• Heat pump

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