Quick answer
An HRV freezes up when its defrost cycle fails or cannot keep pace. Below about -5 °C, moisture from the outgoing house air frosts inside the core, and the unit is meant to clear it automatically. A stuck defrost damper, a bad temperature sensor, unbalanced airflows, a blocked drain or very humid indoor air let the frost build into solid ice. Switch it off and thaw it at room temperature.
If your HRV is freezing up, it helps to know that some frost is part of normal operation. A heat recovery ventilator passes warm, moist exhaust air from the house beside cold incoming air inside a core. The exhaust air gives up its heat, cools below its dew point and leaves water behind, and once the incoming air is cold enough that water freezes on the core. A working HRV melts it off every so often; a block of ice means that routine has broken down.
The defrost cycle: what should happen below about -5 °C
Most HRVs start defrosting when the outdoor air drops below roughly -5 °C, measured by a temperature sensor in the fresh-air stream. There are two common methods. In recirculation defrost, a motorized damper closes the outdoor air port and the unit circulates warm house air through the core for a few minutes. In exhaust-only defrost, the supply fan stops while the exhaust fan keeps pushing warm air through.
Either way the meltwater runs to the drain pan and the unit goes back to ventilating. Three parts fail and stop that from happening:
- Defrost damper motor. The small actuator sticks or strips its gears, and the damper stays open to the cold air. You stop hearing the click and change in sound that marks a defrost cycle.
- Temperature sensor (thermistor). A failed or dislodged sensor reads warm, so the control never calls for defrost.
- Control board. A board that no longer switches the damper or fan relays leaves the unit ventilating straight through a cold snap.
These are model-specific parts, and HRV and ERV repair starts with reading the control's fault indicators and watching one full defrost cycle.
Airflow, drainage and dirt: the causes you can check
- Unbalanced airflows. An HRV is set up so the supply and exhaust streams move the same amount of air. If more cold air comes in than warm air goes out, the core runs colder and frosts faster than defrost can clear. Balance drifts when filters clog, a hood gets blocked or a duct comes apart.
- Blocked condensate drain. The pan under the core drains through a tube with a trap loop. A kinked, plugged or frozen tube leaves meltwater in the cabinet, where it refreezes on the next cycle.
- Dirty core and filters. Lint on the two filters and in the core passages cuts airflow, mostly on the exhaust side that carries household dust, and tips the balance toward cold.
- Blocked outdoor hoods. Snow drifts, a frosted intake screen or fluff matted on the screen starve one side. Both hoods are on an outside wall, set apart from each other.
High indoor humidity loads the core
The more moisture in the house air, the more frost the core has to deal with. A house held at high winter humidity, with a humidifier set too high, a lot of cooking and showers, or laundry drying indoors, can out-frost a healthy defrost cycle on a -15 °C night. Windows are the gauge: steady condensation on the glass in cold weather means the indoor humidity is too high for the temperature outside, and condensation on windows in winter explains the targets.
Running a furnace humidifier hard while the HRV runs continuously is a common way to make ice: one machine adds water and the other tries to remove it. Lower the humidistat as the outdoor temperature falls.
HRV versus ERV: which one frosts first
An ERV frosts later than an HRV. Its core passes water vapour as well as heat, so part of the moisture in the exhaust air moves across to the dry incoming air instead of condensing in the core. With less liquid water in the core, an ERV tolerates colder outdoor air before frost forms, and many have no drain at all.
An ERV still has a defrost routine, and it can still ice up when that routine fails. The trade-offs between the two are laid out in HRV vs ERV.
How to thaw a frozen HRV safely, and when to call
- Switch the unit off at its wall control and unplug it.
- Open the door, put towels under it and let the ice melt at room temperature. Expect a few hours.
- If the core slides out freely, take it out and let it thaw in a laundry tub. Do not force a core that is frozen in place.
- No heat gun, hot water or chipping. Plastic cores warp and aluminium fins bend, and many ERV cores must not be washed at all, so check the label.
- While it thaws, clean both filters, clear the drain tube and check the outdoor hoods.
- Restart on low and listen for a defrost cycle during the next cold night.
Book a service call if:
- Ice returns a day or two after a full thaw with clean filters.
- The defrost damper never moves, or the control shows a fault light.
- Water is leaking from the cabinet onto the floor or into the furnace return.
- The unit has been off for weeks and the windows are running with condensation.
The diagnostic visit is $90–$150 and goes toward the repair. Damper motors, sensors and boards are priced per model, and rebalancing needs airflow measurements at both ducts, so you get a written quote once the fault is confirmed. If the unit is old enough that parts are no longer supported, we quote HRV or ERV installation alongside the repair so you can compare.
Do not leave a frozen HRV switched off for the rest of the winter: a tight house needs the ventilation, and humidity climbs quickly without it. ZK Mechanical services HRVs and ERVs across Burlington, Oakville, Milton and Hamilton. Call (647) 801-1252 or send a service request, and find related questions under air quality and ducts answers.