Noise is the main reason an air exchanger ends up unplugged—and it is almost never the unit itself that is to blame.
When we ask a homeowner why their air exchanger is turned off, the answer rarely comes in technical terms. It is more likely to sound like this: “We could hear it in the bedroom”, “It was whistling in the hallway”, “It was making the living room floor vibrate”.
Three complaints, three completely different physical causes, and none caused by a defect in the unit.
The project described here revolved entirely around this issue. A home in Terrebonne, in a residential area built in the 2000s, where the utility closet had been reduced to the bare minimum by the builder to maximize the finished basement's living space. The unit had to fit into a closed niche, against a party wall adjoining a bedroom, beneath a finished ceiling.
In other words: the worst possible location from an acoustic standpoint, and impossible to move without demolition.
We installed a LIFEBREATH RNC5-HEX-TPD there, a compact top-port HRV, and spent half the day not installing it, but making sure it could not be heard.
The four sources of noise, and how to tell which one you are hearing
A ventilation system can produce four distinct types of noise. Confusing them leads to unnecessary interventions—you replace a quiet unit because the noise is coming from a grille.
1. Aerodynamic noise at the grilles
What you hear: a constant, localized whistle or whooshing sound coming precisely from the ceiling or wall register.
Cause: air is moving too quickly through a grille that is too small for the airflow assigned to it. This is by far the most common cause, and the one least often diagnosed.
2. Fan noise transmitted through the ducts
What you hear: a low hum present in several rooms at once, which changes pitch as the unit speeds up.
Cause: motor noise travels through the air column inside the ducts and emerges through the grilles. A rigid sheet-metal duct is an excellent acoustic conductor.
3. Structural vibration
What you hear: a low-frequency vibration perceived more in the floor or wall than in the air, often louder in the room above or beside the mechanical room than in the mechanical room itself.
Cause: the unit is rigidly attached to the structure. Motor vibrations are transmitted to the joists and studs, which act like a membrane.
4. Noise regenerated by elbows and dampers
What you hear: intermittent noise, sometimes a slight clicking or rustling, that appears only at high speed.
Cause: a tight elbow immediately downstream of the unit casing, or an almost-closed balancing damper, creates turbulence. The noise is then generated in the duct itself, downstream of the unit.
On this Terrebonne project, the first three sources were all potentially at play because of the niche backing onto a bedroom. We addressed them one by one.
Air velocity at the grilles: the number that determines everything
This is the most decisive parameter for a system's acoustic comfort, and it appears on none of the quotations.
Our working thresholds
| Grille velocity | Perception |
|---|---|
| Below 300 ft/min (1.5 m/s) | Inaudible, including in a bedroom at night |
| 300 to 500 ft/min (1.5 to 2.5 m/s) | Acceptable in common areas |
| 500 to 800 ft/min (2.5 to 4 m/s) | Perceptible whistling |
| Above 800 ft/min (4 m/s) | Pronounced whistling, source of complaints |
The demonstration, with the numbers to prove it
Let's consider a grille that must deliver 30 CFM in a bedroom.
With a 6-inch round grille:
- Gross area: approximately 28.3 in²
- Actual free area (the slats take up space): approximately 75%, or 21.2 in² = 0.147 ft²
- Velocity: 30 ÷ 0.147 ≈ 204 ft/min
Result: quiet, even at night.
With a 4-inch round grille, for the same airflow:
- Gross area: approximately 12.6 in²
- Actual free area: approximately 9.4 in² = 0.066 ft²
- Velocity: 30 ÷ 0.066 ≈ 457 ft/min
Result: noticeable in a quiet bedroom.
The airflow is identical. The unit is identical. Only the grille changes—and with it, whether the occupants keep the system running.
That is why we size grilles based on the airflow assigned to each room, rather than the diameter of the duct serving it. A 6 in. grille supplied by a 4 in. duct remains quiet; the reverse does not.
The six measures applied on this project
1. An insulated flexible section at the unit outlet. The first few feet of each branch are made with insulated flexible duct rather than rigid sheet metal. The flexible duct breaks the direct acoustic transmission between the fan and the rest of the system. It is the most effective measure per dollar spent.
2. No elbow in the first few inches. We systematically leave a straight section at the collar outlet before any change in direction. An elbow attached directly to the unit creates turbulence exactly where pressure is highest.
3. Suspension decoupled from the structure. The unit is mounted on its original straps and hooks, with isolators, without rigid contact with the enclosure framing. A unit screwed directly into a wood frame turns the wall into a resonance chamber—and here, that wall faced a bedroom.
4. Grilles oversized by one size in the bedrooms. The speed calculation above determined the choice, room by room.
5. Remote balancing dampers. Rather than throttling a damper located directly behind a bedroom grille—which regenerates noise directly into the room—we place the adjustment components upstream, in the section of the system that passes through the closet or suspended ceiling.
6. The treated but unsealed enclosure. We lined the inside of the enclosure with sound-absorbing material and installed a solid door. However, we did not seal the enclosure: a confined service space must be able to dissipate heat from the motors, and a technician must be able to access it.
The mechanical closet: what to plan before closing up the wall
Maintenance clearances that are non-negotiable
A closed enclosure is an excellent acoustic and aesthetic choice. It becomes a trap if it is designed without thinking about the next ten years.
What we require before closing it up:
- Full opening of the unit door, without removing a duct. On the RNC5-HEX-TPD, the front door provides access to the filters and core; if it hits a partition halfway open, maintenance will not be possible.
- Straight-line core removal. Sufficient clearance is needed in front of the unit to extract it, not just to open the door.
- Visual access to the balancing ports on the door, allowing airflow rates to be revalidated without disassembly.
- Access to the electrical outlet. The unit is powered by a cord; the outlet must remain reachable for unplugging before any maintenance.
A niche that is too tight turns a thirty-minute maintenance task into a two-hour job. In practice, it mainly turns regular maintenance into maintenance that never gets done.
The two drains and finished-floor protection
The RNC5-HEX-TPD uses damper-based recirculation defrost. A motorized damper closes off the outdoor air intake during very cold weather and circulates indoor air through the core to defrost it. The house is not depressurized during the cycle—a real advantage over units that simply shut off the supply fan.
The trade-off is real: this unit produces condensate and requires two connected drains. In the installation photo, both drain connections are visible beneath the cabinet, with their braided hoses.
Our method in a finished space:
- Two separate traps, primed during commissioning. A dry trap lets air through and stops draining.
- A continuous slope toward the discharge point, with regular supports to prevent sags where water can collect.
- A drip pan beneath the unit. Whenever a water-producing appliance is above a finished floor or living space, a second line of defense is essential. The cost is marginal compared with replacing a floating floor.
- Visible drainage. We avoid blind drainage into a cavity; a leaking drain should be seen before it is smelled.
Features of the LIFEBREATH RNC5-HEX-TPD
| Specification | Value |
|---|---|
| Type | HRV (heat recovery ventilator) |
| Core | Patented counterflow HEX, removable for cleaning |
| Net supply airflow | 110 CFM at 0.1 in. w.c. · 104 at 0.2 · 95 at 0.3 · 89 at 0.4 · 78 at 0.5 |
| Apparent sensible effectiveness (ASE) at 0°C | 82 % |
| Sensible recovery efficiency (SRE) at 0°C | 75 % |
| Power supply | 120 V, 0.5 A |
| Power consumption | 36 W (low speed) / 55 W (high speed) |
| Defrosting | Damper-controlled recirculation — two drains required |
| Configuration | Top ports |
| Weight | 50 lb (22.7 kg) |
| Filters | Washable, on both airstreams |
| Balancing | Ports on the unit door |
| Suspension | Adjustable straps and S-hooks |
The key figure to remember here is 36 W at low speed.
An air exchanger must operate continuously to fulfill its role. At 36 W, continuous operation costs a few dollars per month—less than leaving a light bulb on continuously in an earlier generation of lighting. This is an argument we consistently use with clients who hesitate to leave the unit running: the savings from turning it off are negligible, while the cost in humidity and air quality is not.
Low energy consumption also has a direct acoustic benefit: less motor power means less energy available to generate noise. A properly sized compact unit running continuously at low speed is structurally quieter than an oversized unit forced to cycle at high speed.
Mistakes to avoid
- Choose the grille based on duct diameter rather than airflow. This is the number one source of whistling.
- Rigidly attach the unit to the framing of a wall adjoining a bedroom. Structural vibration is the most difficult complaint to correct afterward.
- Install an elbow immediately at the collar outlet. Turbulence, pressure loss, and regenerated noise, continuously.
- Throttle a damper located directly behind a bedroom grille. The adjustment then comes at the expense of acoustic comfort.
- Close up the niche without checking the core-removal clearance. Maintenance then becomes theoretical.
- Omit the drain pan above a finished floor.
- Turn off the unit to save energy. At 36 W, there is nothing meaningful to save.
Maintenance
- Every 3 months — wash the filters on both airstreams. They are washable, eliminating a recurring cost, provided it is actually done.
- Every 6 months — remove and clean the counterflow core, check both traps and their priming, inspect the drain pan, and check the defrost damper.
- Annually — clear the exterior hoods, inspect the vapor barrier on the exterior ducts, and revalidate airflow rates at the door ports.
For installations in enclosed niches, we recommend keeping the niche door open for the half-hour following maintenance, allowing the unit to return to its normal operating condition and the technician to confirm that no new noise is present. An improperly reinserted core or misaligned filter will first make itself known acoustically.
Compliance and programs
For new homes covered by the Quebec Construction Code, balanced mechanical ventilation is required and is a condition of the Novoclimat program. For an existing home, replacing or correcting an air exchanger generally does not qualify for a rebate program — these programs primarily target insulation, air sealing, and heat pumps. When a larger project is being considered, we confirm the order of the work with the client because the sequence affects eligibility for the programs in effect.
For your project in Terrebonne or elsewhere on the North Shore
At AirGreen, we treat noise as a design criterion, not as a consequence to be tolerated. This means sizing the grilles according to the actual airflow of each room, decoupling the unit from the structure, avoiding elbows at the cabinet outlet, relocating adjustment components, and checking the result by ear in every room before leaving the site.
If your air exchanger is currently turned off because it was bothering you, there is a good chance the cause can be corrected without replacing the unit. It is one of the most satisfying services we perform.
We carry out HVAC installations — air exchangers, wall-mounted and central heat pumps, air conditioners, and heating systems — in Terrebonne, Lachenaie, La Plaine, Mascouche, throughout the North Shore, as well as in Montreal, Laval, Longueuil, and the South Shore. Licence RBQ # 5645-9605-01.
To have your installation assessed or to obtain a quote for a LIFEBREATH air exchanger, call us at (514) 316-2973 or email sales@airgreen.ca.