How an HRV keeps a tight house fresh without big heat loss

How does a heat recovery ventilator work?

Two fans move two air streams through a heat exchange core. Stale, warm air from bathrooms, kitchens and living areas passes on one side of thin walls, while cold outdoor air passes on the other. Heat moves across the walls but the air does not mix, so fresh air arrives pre-warmed and odours leave.

A simple example shows the effect. With indoor air at 21 degrees C, outdoor air at minus 20 and a heat recovery efficiency of 75 percent, the supply air leaves the core at roughly 11 degrees C: minus 20 plus 75 percent of the 41-degree gap. A real unit's efficiency varies with temperature and airflow, so use the tested ratings for any specific model.

HRV or ERV?

An energy recovery ventilator, ERV, transfers some moisture as well as heat. An HRV transfers heat only. In dry, cold prairie winters an ERV can keep more of the indoor humidity from leaving, while in humid-summer regions it can limit moisture coming in. Many Canadian houses with a normal moisture level use HRVs, particularly where winter indoor humidity runs high and exhaust moisture control is desirable. The choice depends on climate, house tightness and occupants, and the designer or installer should justify the pick.

Why do efficient houses need one?

The better the shell, the less accidental air exchange there is, and moisture and indoor pollutants build up unless they are removed on purpose. Building rules for new houses require mechanical ventilation, and efficiency standards such as R-2000 built the idea in from the start. A blower door test tells you how much natural leakage you are replacing with a controlled system.

Opening a window instead is fine for a short time but does not give steady, quiet, filtered ventilation in January.

What affects real-world performance?

Ducting design matters as much as the box. Long runs, tight bends and undersized ducts lower airflow and raise fan power. Poorly placed supply registers can leave the bedrooms stale, and exhaust points far from the bathrooms can leave moisture behind. The unit also needs an even flow on both sides, because unbalanced flows pull in unconditioned air through cracks. Commissioning with measured airflow, not just turning it on, is the reliable fix. Ask the installer to leave you the measured flows and the filter schedule in writing.

An energy advisor doing a home energy audit can often measure the ventilation flows in the same visit, which makes it a handy check on an installer's work.

Maintenance: what to do and how often

• Filters. Check and clean or replace them on the schedule in the manual. Clogged filters cut airflow and efficiency. Every few months is a common starting point.
• Core. Rinse or vacuum it as the manufacturer directs, usually once or twice a year. Never use harsh solvents.
• Outdoor hoods. Keep intake and exhaust hoods clear of snow, leaves and nests, and check the mesh.
• Condensate drain. Make sure the drain is open and the trap holds water if the unit has one.
• Balance. A technician can check that supply and exhaust flows match; do this at commissioning and after changes.

Frost on the core in very cold weather is normal; units use a defrost mode that briefly recirculates indoor air. If it does not clear, call for service.

How does ventilation tie in with heat pumps and district heating?

Ventilation load is a large part of the heat an efficient house still needs. Recovering it lowers the peak the heating system must supply, which matters for a cold-climate heat pump, and it matters for low-temperature heat sources too. At Drake Landing the 52 homes were served by a loop typically at 40 to 50 degrees C, and each house had its own air handler to move that heat into the living space. Reducing heat loss through ventilation helped such a gentle supply keep up.

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