Installation of a Broan-NuTone HRV90 air exchanger (Compact Flex series) in an attic space in Anjou, Montreal

The attic is the most demanding place to install an air exchanger, and most of the rules that apply there are written nowhere on the quotation

An air exchanger suspended from the roof trusses, above the insulation, in a space where the temperature follows the outdoor temperature: this is a configuration we regularly see in Montreal, and it is the one that generates the most service calls three winters later.

The project completed in Anjou allows us to explain why this installation is sometimes the only possible option, what conditions we require before accepting it, and what happens when those conditions are not met. The selected unit is a Broan-NuTone HRV90 from the Compact Flex series, the lightest and most compact model in the range, suspended by chains from the roof trusses.

Why an attic, in an Anjou bungalow

The residential housing stock in Anjou includes a high proportion of bungalows and split-level homes built between 1955 and 1975. Many share a feature that makes mechanical ventilation difficult: there is nowhere to put the unit.

• The basements have long been finished, often without a proper mechanical room, and the little space that remains is already occupied by the hot-water tank, furnace, and central vacuum.
• Some homes are built on slabs, with no basement at all.
• The ceilings are low, and there is no usable joist cavity through which to run ducts without demolition.

In these cases, the attic becomes the only free space in the house. It offers room, direct access above all the rooms, and the possibility of connecting ceiling vents without opening a single wall.

It also provides the building’s worst thermal environment.

The temperature rule, and why it disqualifies most attics

The manufacturer is explicit on this point: an attic installation is only feasible if the temperature is maintained above 10 °C at all times.

A standard Quebec attic, insulated at the floor and ventilated through soffit and ridge vents, does not meet this condition. It follows the outdoor temperature. In January, in Anjou, it readily drops below -20°C.

An HRV exposed to these temperatures suffers three types of damage:

1. The condensate freezes. The appliance produces water in winter. A pan and drain at -20°C no longer drain: they clog, and then water overflows into the ceiling insulation.
2. The core is constantly freezing. The defrost cycle starts repeatedly, reducing the actual airflow and increasing energy consumption.
3. The recovered heat is lost before it arrives. An appliance that warms the fresh air to 14°C is useless if the ducts then cross fifteen metres of space at -20°C.

The five conditions we require before accepting an attic installation

We do not refuse this type of project. We make it conditional.

1. The appliance must be inside the thermal envelope, or in an insulated, temperature-controlled enclosure built around it. An appliance installed uncovered above the floor insulation is outside the envelope and therefore non-compliant.
2. All ducts must be insulated along their entire length, with a continuous vapor barrier sealed at every connection. No exceptions, no exposed sections.
3. The condensate drain must be protected from freezing along its entire length, with a continuous slope and no low points.
4. There must be practical access: a sufficiently large hatch, a walkway, and complete clearance in front of the service door. An appliance reached by crawling across roof trusses will never be maintained.
5. The suspension must preserve the integrity of the trusses. This point deserves its own section.

When these five conditions cannot be met, we recommend a different installation, even if the project ends up costing more in finishing work. That is less pleasant to hear in a quote than to discover after water damage.

The Broan-NuTone HRV90 at a glance

The model was selected based on two criteria specific to the attic: the weight to be suspended and the space required for the insulated enclosure to be built around it.

• Type: heat recovery ventilator (HRV), Compact Flex series
• Continuous airflow: 47 to 95 CFM at 0.2 in. w.g. — 47 to 90 CFM at 0.4 in. w.g.
• Apparent sensible efficiency: 75% at 0°C
• Sensible recovery efficiency (SRE): 66% at 0 °C and 60% at -25 °C
• Fan efficiency: 1.2 CFM/watt
• Connections: 4 inches, side or top configuration, as desired
• Dimensions (side-connection version): 16 5/16 × 19 13/16 × 15 5/8 in.
• Weight: 30 lb (approximately 13.6 kg)
• Certification : ENERGY STAR
• Warranty: 5 years on parts, limited lifetime warranty on the core

Thirty pounds is not much. On a roof truss, it is decisive: the additional load remains in the realm of an accessory rather than a structural modification.

What happens to the recovered heat between the unit and the vent

Here is the calculation we show clients considering an attic installation.

On a day at 0 °C, with the house at 21 °C, the HRV90 introduces fresh air at approximately 14 °C. On a night at -25 °C, it introduces it at approximately 3 °C.

These values are measured at the unit outlet. Between this outlet and the living-room supply vent, the air travels through a duct installed in a cold space. With a properly insulated and vapor-sealed duct, the loss amounts to fractions of a degree. With an uninsulated duct or one with an open vapor barrier, it can cancel out half the gain, and the duct will begin to condense on its outer surface—that is, it will wet the insulation in the bedroom ceiling below.

Insulating the ducts in the attic is therefore not an optional improvement. It is what determines whether the installation is viable.

Suspending from roof trusses

The unit is suspended by chains, which decouple vibrations and prevent them from being transmitted to the gypsum ceiling below. The mounting was done according to three rules.

• Attach it to the bottom chord, at the connection points with the diagonals, where the truss is designed to receive a load.
• Distribute the load across several trusses rather than suspending everything from a single one.
• Keep the unit level in both axes; otherwise, the condensate pan will not drain properly.

What you should never do to a roof truss

A roof truss is a calculated structural component. Unlike a floor joist, it cannot tolerate any modifications.

• Never cut a truss member or diagonal, even partially, even “just to run a duct”.
• Never drill through the truss members.
• Never rely on the metal connector plates, which are not anchoring points.
• Never force a duct through the plane of a truss: go around it or redesign the route.

A contractor who cuts a truss diagonal to run a six-inch duct leaves you with a structural problem that no one will see until the next major snowfall.

Maintenance access, planned before installation

An air exchanger requires filter maintenance every three months and annual maintenance for the core. In an attic, this maintenance will not be performed if an access route was not planned.

So, even before proposing the layout, we check the hatch size, whether there is a walkway or planks laid across the truss members, the lighting, and the clearance in front of the unit's access door. This is the difference between a system serviced twelve times in twelve years and one opened only once, on the day it breaks down.

Air quality in Anjou: what enters through the fresh-air intake

Anjou has a particular profile on the island of Montréal: a residential area bordered by Highway 25, Highway 40, and one of Québec's largest industrial parks.

This does not undermine the value of an air exchanger—quite the opposite, a home that is not ventilated accumulates its own indoor pollutants, which are by far the dominant ones. But it does influence two decisions:

• The placement of the fresh-air intake, which we move as far as possible from the façade facing the busiest road, and never place on the side of a parking area where an engine is idling.
• The level of filtration, which is worth increasing when occupants are sensitive to fine particles. We discuss this openly rather than reflexively selling the most expensive filter.

The mistakes we correct most often in attics

• Place the bare unit above the floor insulation, without a conditioned enclosure. This is the foundational mistake from which all the others follow.
• Leave one metre of uninsulated ductwork near the unit, where it is most difficult to access.
• Raising the condensate drain to reach a higher drain. In an attic, every low point becomes an ice plug.
• Attaching the chains to the roof decking rather than to the framing members.
• Forgetting the access hatch when calculating the price, then deciding not to enlarge it.
• Treat the attic like a basement. They are two different thermal environments.

HRV90 Maintenance

• Every 3 months: remove and vacuum-clean the filters, then wash them with warm, soapy water. Let them dry completely.
• Once a year: remove the core, soak it in a solution of lukewarm water and mild soap, rinse it, dry it completely, and reinstall it in the correct direction.
• Once a year: wipe the condensation pan and check that the drain flows completely, before the first freezes.
• Once a year: inspect both exterior hoods, remove debris and lint, and check for any accumulation of snow or ice on the grille.
• At every visit: check the condition of the ducts' vapor barrier and reseal any open connection.

HVAC maintenance for a unit in an attic is best entrusted to an annual contract rather than improvised by the homeowner. Access is uncomfortable, which is precisely why it is so often neglected.

Have your Broan-NuTone air exchanger installed by AirGreen

We design, install, and maintain residential ventilation systems in Anjou, Saint-Léonard, Montréal-Nord, Rivière-des-Prairies, Mercier, in eastern Montréal, as well as in Laval, Longueuil, on the North Shore, and on the South Shore.

Our position on difficult installations is consistent: we would rather explain why a configuration requires additional work than deliver an installation that will work for two winters. An air exchanger installed in an attic can age very well—as long as the unit is kept within a suitable temperature range, the ducts are insulated, the drain is protected, and access is practical.

For an assessment of your ventilation needs, a quote for HVAC installation, or an HVAC maintenance contract, contact our team. We come to you, climb into the attic, and tell you what is feasible before selling you anything.