Glossary · Heating, cooling & ventilation
Defrost cycle (heat pumps and HRVs)
Also called: defrost · defrost mode · defrosting
A short, automatic cycle that melts frost off a heat pump's outdoor coil or an HRV's core in freezing weather; it's a normal part of winter operation.
What it means
When it's near or below freezing, moisture in the air freezes on a heat pump's outdoor coil, so the unit briefly switches to cooling mode, with the outdoor fan off, to send hot refrigerant through the coil and melt the frost. Supplementary heat may run meanwhile so the house doesn't get cool air. Most HRVs have an automatic defrost mode too, which starts when incoming air is colder than about −5°C (23°F) and clears frost from the core.
Why it matters in an audit
Defrost uses energy and lowers winter efficiency, which is one reason seasonal and cold-weather ratings matter more than mild-weather numbers, and repeated freezing can signal a fault.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| Heat pumps (NRCan) | Frost forms when it's near or below freezing; the reversing valve switches to cooling to melt it, and supplementary heat may be needed | natural-resources.canada.ca ↗ |
| HRVs (NRCan) | Most have an automatic defrost mode that starts when incoming outdoor air is below 23°F (−5°C) | publications.gc.ca ↗ |