17% humidity displayed on the wall: what happens when an air exchanger works too well
The screen of the wall control photographed at this Westmount job displays a single numerical reading: RH: 17%.
For a homeowner, it is a number without context. For us, it is a complete diagnosis. A dwelling at 17% relative humidity is significantly drier than the air in a temperate desert, and the cause is almost never a defect in the unit. It is a unit doing exactly what it was asked to do, for too long and too intensely.
Westmount homes—stone and brick, two or three stories, original woodwork, lath-and-plaster, and wide hardwood floors—are precisely the ones where this dryness costs the most.
Why ventilation inevitably dries the air
The physics is simple and rarely explained to customers.
Outdoor air at -15 °C, even saturated at 80% relative humidity, contains very little water vapor in absolute terms: cold air does not have the capacity to carry much of it. When this air enters the house and warms to 21 °C, its capacity increases dramatically, but the amount of water it carries does not change. Its relative humidity plummets—often below 10%.
Every cubic foot of fresh air introduced in January is therefore a cubic foot of drying air. An air exchanger running at high speed all winter does more than improve air quality: it methodically dries out the house, room by room.
The target comfort level is between 30 and 40% in winter, and closer to 30% in an older home whose windows develop condensation beyond that level. At 17%, it is well below this range.
What dryness damages in an older home
- Hardwood floors: the boards shrink, the joints open, and the movement repeats every seasonal cycle until it becomes permanent.
- Original woodwork and doors: cracks at miter joints, splitting panels, and moldings coming loose.
- Lath-and-plaster: hairline cracks at the corners and around openings.
- Musical instruments and antique furniture: solid wood does not forgive extremes.
- Everyday comfort: constant static electricity, air perceived as cold at the same temperature, irritated mucous membranes.
The usual reflex is to add portable humidifiers in every room. This treats the symptom while paying twice: once to remove moisture through ventilation, and once to put it back.
The real lever: modulate, don't stop
Turning off the air exchanger solves the dryness and recreates all the problems it was meant to correct—stagnant humidity in bathrooms, odors, and CO₂ buildup in closed bedrooms. The right approach is to adjust the airflow to the season, which is precisely what the control installed here allows.
The EHC 2.5 control: three modes
Ventilation mode — Normal operation. Stale air is exhausted, fresh air enters, and the core recovers the heat.
Recirculation mode — The unit circulates and filters indoor air without introducing outdoor air. Useful during an episode of poor outdoor air quality, and useful during extreme cold to maintain air movement without worsening dryness.
Standby mode — The unit remains inactive unless indoor relative humidity exceeds the setpoint configured on the control. This is the mode that addresses the situation at hand: during a cold snap, the unit ventilates only when humidity actually rises—after a shower, while cooking, when the house is full—rather than continuously.
Four speeds
Reduced, medium, normal, or 20 minutes per hour. The last setting is most useful in January in a lightly occupied home: one-third of the operating time, sufficient air exchange, and drying reduced by the same proportion.
Our reference seasonal settings
- Extreme cold (-15 °C and below): reduced speed or 20 min/h, standby mode with a humidity setpoint.
- Normal winter (-15 to 0 °C): continuous reduced speed, with boosts to high speed using the bathroom timer.
- Shoulder season: continuous medium speed; this is when the unit works best.
- Humid summer: normal speed, keeping in mind that an HRV introduces outdoor humidity as is.
We leave these settings in writing for the client. A control whose modes no one understands always ends up being left on just one.
Choosing the Greentek SOLACE 2.5H-EC
The decisive criterion for this home was not maximum airflow, but the modulation range. A large Westmount home needs a lot of air during the shoulder seasons and very little in January. A unit whose minimum airflow is already high forces you to choose between under-ventilating and over-drying.
- Type: HRV, counterflow core
- Airflow range: 31 to 252 ft³/min at 0.4 in. water column
- Connections: 6 in., round, on top
- Dimensions: 24 7/8 in. (H) x 27 7/8 in. (W) x 15 3/8 in. (D)
- Weight: 57 lb
- Power supply: 120 V, 6.4 A max
- Power consumption: 34 W at low speed, 260 W at high speed
- Fan efficiency: 2.5 ft³/min per watt
- Sensible recovery: 82% at 0 °C
- High-efficiency EC motors
- Defrosting: by internal recirculation
- Filtration: MERV 8 or optional MERV 13
- Injected-foam cabinet, insulated and soundproofed
- Suspension chains and wall bracket included
The ratio between 31 and 252 ft³/min is what makes the seasonal strategy possible. A conventional unit whose minimum is around 75 ft³/min simply cannot go low enough in January, regardless of the control settings.
EC motors add the economic argument: 34 W in continuous operation, meaning negligible annual consumption for a unit that never stops. The counterflow core recovers more than a cross-flow core—82% versus 74 to 77% for cross-flow series—which matters in a large-volume home.
Westmount-specific constraints
Exterior penetrations. Westmount strictly regulates modifications visible from public roads. We systematically place intake and exhaust hoods on rear or side elevations that are not exposed, and we confirm the applicable requirements before work begins rather than afterward. On a stone facade, a poorly placed penetration is not a mistake you can simply erase.
Hot-water heating. A large portion of the area's older homes are heated by radiators, with no existing duct network. Ventilation therefore requires a fully dedicated system: stale-air extraction in the bathrooms and kitchen, and fresh-air supply in the bedrooms and living areas.
The three floors. Vertical routes run through stacked closets, existing plumbing chases, or a discreet new bulkhead. Each additional foot of ductwork adds pressure loss, bringing us back to the unit's modulation range: a unit with reserve capacity is preferable.
Mistakes to avoid
- Running the unit at normal speed all year. This is the direct cause of the 17% shown on the display.
- Compensating for dryness with humidifiers without adjusting the airflow. You pay to remove the humidity, then to put it back, and excess humidification eventually condenses on older windows.
- Blocking supply grilles in bedrooms because the air feels cold. The system becomes unbalanced and the room is no longer ventilated.
- Setting the humidity setpoint just once. It must be readjusted at least twice a year.
- Ignoring balancing readings. Supply and exhaust must be within 10% of each other, with the readings recorded and provided to the client.
- Putting off maintenance. Filters checked every three months, the core washed annually, and exterior hoods cleared of snow.
Our approach to ventilation in older homes in Greater Montreal
At AirGreen, we install and adjust ventilation systems in Westmount, Hampstead, Outremont, Mont-Royal, and Côte-des-Neiges, as well as throughout Montreal, Laval, Longueuil, the North Shore, and the South Shore. Heritage homes require a different approach from newer construction: a fully dedicated system, vertical routing that must be devised, untouchable façades, and an envelope that reacts strongly to changes in humidity.
Our method does not end with commissioning. We balance the system, record the readings, then adjust the controls for the current season and leave the client written instructions for the others. A properly installed but poorly adjusted air exchanger produces exactly the figure shown in this photo.
For an assessment of an HVAC installation, the addition of an air exchanger, or the adjustment of an existing system in Westmount or elsewhere in Greater Montreal, our team comes to you, takes measurements, and makes the necessary adjustments.