Forty feet of flexible duct replaced with galvanized steel, and a sound level meter to prove it
The initial measurement, taken in the primary bedroom of a home in the Symbiocité area in La Prairie, was 41 dBA at night, with the air exchanger running at medium speed. The owner, an engineer by training, had purchased a sound level meter before calling us. He wanted a number, not an impression.
The second symptom was subtler: the back bedroom, at the end of the system, received barely perceptible airflow, while the office fifteen feet from the unit was blasted by a cold draft in January.
The same culprit explained both: the ductwork system.
What we found above the suspended ceiling
The original installation, completed during construction, included approximately forty feet of flexible duct on the supply side. Three observations, all common:
- Sagging between suspension points. The flexible duct hung in festoons, with each dip forming a pressure pocket.
- Overly tight bend radii. Two right-angle elbows were folded back on themselves, with a radius of less than one diameter.
- Compression at penetrations. The duct was crushed between a joist and a plumbing pipe, reduced to perhaps 60% of its cross-section.
A properly stretched flexible duct already creates significantly more airflow resistance than a smooth rigid duct of the same diameter. When sagging, tightly bent, and compressed, the difference becomes considerable, costing you in three ways: lost airflow, generated noise, unnecessary electricity consumption.
Why we favor rigid galvanized ductwork
The system we installed in La Prairie uses rigid galvanized steel for nearly its entire length, with short flexible sections only where they genuinely serve a purpose.
What rigid ductwork brings
- A smooth interior surface. Less turbulence, less pressure loss, less airflow noise.
- Stable geometry over time. A properly suspended rigid duct maintains its cross-section for twenty years. A flexible duct sags from the very first year.
- Cleanability. A rigid system can be brushed and vacuumed. Corrugated flexible duct traps dust in every groove and tears under the brush.
- Controlled airtightness. Screwed joints sealed with approved aluminum tape or sealant, which can be visually inspected.
Where flexible duct remains the right choice
Flexible duct is not a bad material. It is a material that is often used incorrectly.
We use it deliberately:
- As a short sleeve at the unit outlet, twelve to eighteen inches long, to isolate cabinet vibrations from the rigid ductwork. This is visible at our La Prairie job site, at the unit outlets.
- For the final connection to a grille, when the finished-ceiling geometry requires it.
- To get around a specific obstacle that a rigid elbow cannot clear cleanly.
In all three cases, we apply the same rules: the duct must be stretched tight, never coiled for spare length; supports about every four feet; a bend radius of at least one duct diameter, preferably one and a half; no compression.
Sealing: the detail that determines everything
Gray cloth tape, commonly called “duct tape,” has no place on a ventilation system. Its adhesive dries out, cracks, and comes loose within two to four years in a warm service cavity. We seal with approved aluminum tape or duct sealant, using screws at the joints.
A system with 15% leakage delivers 15% less air to the bedrooms while consuming just as much energy. The leak is invisible and permanent.
The key decision: three ways to connect an air exchanger
This is the question the owner in La Prairie asked us after the assessment, and it is the question most readers ask when comparing quotes on the South Shore. The three methods have different costs and produce different results.
Method 1 — Simplified installation
The air exchanger draws stale air from the furnace return plenum and injects fresh air back into it.
- Cost: the lowest
- Distribution: entirely dependent on the furnace fan
- Major constraint: the central fan must run continuously or be interlocked with the air exchanger. Without this interlock, fresh air stagnates in the plenum and falls back into the basement.
- Risk of short-circuiting: if the stale-air intake and fresh-air supply are too close together in the plenum, the unit recirculates air back onto itself. We space them well apart, generally at least three feet, and the supply is installed upstream of the furnace.
Best reserved for tight budgets and homes already equipped with a well-designed forced-air network.
Method 2 — Partially dedicated installation
Exhaust is handled by dedicated grilles in the bathrooms, laundry room, and kitchen. Supply air continues to be injected into the return plenum.
- Cost: intermediate
- Real advantage: humidity and odors are captured at the source, where they are produced
- Limitation: fresh-air distribution remains dependent on the central fan
This is the compromise we recommend most often for renovations, when the bedroom ceilings are finished and installing a complete supply network would involve significant drywall work.
Method 3 — Fully dedicated installation
Two separate networks: supply to the bedrooms, living room, and office; exhaust from the bathrooms, kitchen, and laundry room. The air exchanger operates independently of the heating system.
- Cost: the highest
- Performance: the best, without comparison
- Autonomy: the unit ventilates even when the furnace or heat pump is off, which matters a great deal during the shoulder seasons
This is the method we chose in La Prairie, in a high-performance home where the airtight building envelope makes mechanical ventilation non-negotiable.
The detail that makes a difference to comfort in January: the duct heater
A fully dedicated installation blows fresh air directly into the bedrooms. Even after passing through the recovery core, this air remains colder than the ambient air during severe cold snaps on the South Shore.
Venmar offers two electric duct heaters:
- 1604103 (EDH 6.1) — 6 in. diameter, 1 kW, 120 VAC, adjustable single-stage internal thermostat
- 02120 — 6 in. diameter, 2000 W, 220 V, adjustable temperature from 0 °C to 40 °C
Installed on the main supply run, downstream of the unit, it tempers incoming air and eliminates the draft sensation at bedroom grilles. It is the difference between a system occupants leave running and one they unplug on January 1.
The network components we specify
The Venmar catalog covers the entire airflow path:
- Whisper air vents — 4 in. (01269), 5 in. (02863), 6 in. (01271), and 8 in. white register (03585)
- Grilles and dampers — 4 in x 10 in white damper with flap (00913), 8 in x 12 in indoor grille (04790) and its 8 in adapter (04400)
- Transitions — 8 x 8 x 8 T (05261), 4 in 90° elbow (01662), insulated Y Tandem transition (04319)
- 24 VAC air control dampers — 6 in (1603104), 7 in (1603106), 8 in (1603109), normally open and convertible to normally closed
- Insulated flexible duct — 8 in x 30 ft (04399), 5 in x 20 ft (04316) — and uninsulated duct for indoor use (05247, 04396)
24 VAC dampers deserve special mention: they allow a portion of the system to be interlocked, for example to shut off supply to an unoccupied basement, or to properly interconnect the air exchanger and the central system.
Power and controls
In the jobsite photo, two distinct elements cross beneath the appliance:
The dedicated 120 V outlet. The appliance is plugged into a reserved receptacle within reach of the cord, without an extension cord. This is the configuration expected by the manufacturer.
The low-voltage control cable. The three-conductor wire connecting the wall control to the appliance is left with a service loop rather than pulled tight. We keep it physically separate from the 120 V wiring: running both through the same joist hole introduces electrical noise into the control line, and the resulting symptoms — a display that resets, a speed that changes on its own — are frustrating to diagnose afterward.
Identification. Each duct is permanently labeled with its function, directly on the metal, before the ceiling is closed. The next technician who works on it ten years from now will not have to guess.
The errors we correct most often in ventilation systems
- Use flexible ducting for the entire run because it installs faster.
- Stack two elbows back to back without a straight section between them. The resulting turbulence is equivalent to several additional feet of ducting.
- Reduce the diameter below that of the appliance collar. An appliance with 6-inch collars connected to 5-inch ducting loses airflow from the very first section.
- Seal with cloth tape.
- Omit the central fan interlock in a simplified or partially dedicated installation.
- Leave a duct uninsulated along a section carrying air at outdoor temperature. Condensation is guaranteed, followed by ceiling stains.
- Suspend the ducts with wire instead of wide straps, which deform the cross-section.
- Do not leave service clearance in front of the unit door.
Measured result
After replacing the supply ductwork with rigid ductwork, adding the duct heater, and completing documented balancing, the homeowner's sound meter displayed 31 dBA in the primary bedroom at the same speed, and the airflow to the back bedroom had gone from nearly zero to a value consistent with the planned distribution.
The unit had not changed. The ductwork had.
Certifications and financial assistance
Venmar AVS and vänEE units are HVI- and cUL-certified, and several carry ENERGY STAR certification, valid in Canada. Venmar has been a member of the Home Ventilating Institute since 1991 and a member of HRAI. Warranties vary by appliance family, from two to five years on parts, with a limited lifetime warranty on the heat recovery core of HRVs.
Replacing an HRV may qualify for the Rénoclimat program, provided the initial energy assessment takes place before the work.
What the La Prairie project highlights
An HVAC installation quote that does not specify the connection method or duct type cannot be compared with another. Two contractors can offer the same unit at the same price and deliver systems whose performance differs by as much as twofold.
The three questions to ask before signing:
- Simplified, partially dedicated, or fully dedicated installation?
- Rigid or flexible ductwork, and over what proportion of the route?
- Will the airflow rates be measured and provided in writing at the end of the work?
At AirGreen, we answer these three questions in the quote, before the work begins. We provide HVAC installation and HVAC maintenance in La Prairie, Candiac, Saint-Constant, Delson, Sainte-Catherine, Saint-Philippe, Brossard, and Longueuil, throughout the entire South Shore, as well as in Montreal, Laval, and on the North Shore.