Three appliances, one mechanical room: how we made a central heat pump, an HRV, and a humidifier coexist
At Noyan, in the Haut-Richelieu RCM, a few kilometers from Missisquoi Bay and the U.S. border, we replaced the central system of an entirely electric home with a 4-ton GREE FLEXX R32 central heat pump. The appliance was not alone in the mechanical room: a NU-AIR HRV already occupied the upper corner of the room, and a Honeywell humidifier had to be connected to the ductwork. Three air-moving appliances in a finished basement closet measuring approximately six by seven feet.
This is the kind of project that separates a proper installation from one that actually works. The appliance is selected based on a technical data sheet; the integration cannot be improvised. Here is how we approached it, and what a homeowner in Montérégie should require from their contractor before signing.
Noyan: no natural gas, lake winds, and a long service trip
Noyan is a small agricultural municipality in the Richelieu Valley, neighboring Clarenceville, Lacolle, and Saint-Georges-de-Clarenceville, backed by Lake Champlain. Three local realities shape every central-system design in this area:
- No natural gas network. Supplemental heating will be electric or oil-fired. In an all-electric home, the question of fuel does not even arise: it is an integrated electric heating coil in the air-handler cabinet, which the customer instinctively calls “the electric furnace.”
- Exposure to wind from the open waters of the lake and the farmland, which increases air infiltration and therefore the actual heating load compared with an identical home in a sheltered urban setting.
- The distance. We are an hour's drive from Montreal and Longueuil. A service call for an adjustment that should have been made during commissioning is costly for everyone. On this project, we therefore devoted a disproportionate amount of time to fine-tuning rather than basic installation.
The appliance: GREE FLEXX R32, 4-ton configuration
- Air handler cabinet: GUD48AH2/G-D(U)
- Outdoor unit: GUD60W2/NhE-D(U)
- Certified AHRI matched-system number : 217120761
- Cooling: 48,000 BTU/h nominal, modulation from 34,000 to 50,000 BTU/h
- Heating: 48,000 BTU/h nominal, modulation from 34,000 to 52,000 BTU/h
- Efficiency: 19 TRÉS2 (SEER2)—the best rating in the FLEXX lineup—, 10 CPSC2 (HSPF2), 12 TRÉ2 (EER2), COP 3.4
- ENERGY STAR certified
- Heating range: down to -30 °C; cooling range: -15 °C to 54 °C
- R32 charge: 4.6 kg factory charge
- Airflow: approximately 1,200 CFM at a nominal external static pressure of 125 Pa (0.5 in. w.c.)
- Electrical: cabinet MCA 7.1 A / MOCP 15 A; outdoor unit MCA 39.9 A / MOCP 45 A
- Cabinet dimensions: 24 4/5 x 52 1/16 x 21 1/4 in. (630 x 1320 x 540 mm), 90.5 kg
- Control: conventional 24 VAC thermostat, factory-installed Wi-Fi, included A2L leak-detection sensor
- Warranty: 10 years standard, plus 2 additional years upon registration with proof of installation
Why 4 tons rather than 5
In a rural home exposed to wind, the temptation is to move up to the larger configuration “to avoid running short of heating.” The load calculation did not justify it, and two technical arguments settled the decision.
The first: the 4-ton configuration has a rating of 19 TRÉS2, compared with 18.5 for the 5-ton unit. This is the best cooling efficiency in the entire lineup.
The second is more subtle. An oversized modulating unit does not modulate better: it operates more often near its minimum capacity, where actual efficiency collapses and dehumidification becomes mediocre in summer. The modulation range of the 4-ton unit drops to 34,000 BTU/h; that of the 5-ton unit reaches only 35,000 BTU/h. The difference at the low end of the range is negligible, but the difference at the high end completely changes the unit's behavior during the shoulder seasons. The real comfort improvement came from integrating the HRV and the humidifier, not from adding another ton.
Vertical (upflow) installation in a multiposition cabinet
The cabinet is installed here vertically, in upflow configuration: return air enters from below through a custom-built base plenum—which our technician also annotated by hand with an “Air Flow” arrow—and discharge exits at the top toward the supply plenum.
Two points deserve the reader's attention.
The first is a documentation warning: the FLEXX technical data sheet describes a “horizontal (side) airflow design,” whereas the cabinet's dimensions themselves (52 in. high by 24 4/5 in. wide) and the actual installation are vertical. We treat this appliance as a multiposition cabinet, and we systematically validate the selected position and drain-pan configuration in the installation manual rather than in the commercial data sheet. A commercial data sheet is read; an installation manual is followed.
The second is practical: the selected position determines how the primary and secondary drains from the pan are connected. A vertically mounted cabinet does not use the same openings as a horizontal cabinet, and a secondary drain left blocked by a factory plug is useless when the primary drain becomes clogged.
The mechanical room: what changes when everything fits in a finished closet
A mechanical closet in a finished basement presents three constraints that an unfinished basement does not.
Service clearance. The control panel, fan compartment, and coil must remain accessible. On this project, the cabinet was positioned so that its front was clear over its entire height, even at the cost of sacrificing storage space. An appliance that cannot be opened becomes an appliance that is not maintained.
Noise. A 1,200 CFM fan in a closet adjoining an occupied room can be heard. We carefully designed the duct transitions, avoided abrupt changes in cross-section at the plenum outlet, and verified that the cabinet was not resting rigidly against the partition. Noise from a central system rarely comes from the appliance itself; it comes from excessive air velocities in an undersized duct.
Filter access. In an upflow configuration, the filter is located in the return, beneath the appliance. If the filter drawer cannot be accessed without moving a shelf, the customer will stop replacing it before the second year.
The A2L calculation for an appliance connected to a duct network
The R32 is a refrigerant A2L, mildly flammable, and a closed mechanical closet is the smallest volume one could imagine around an appliance charged with 4.6 kg.
The nuance that few articles explain correctly: for a device fully connected to a duct network, it is not the volume of the closet that governs the analysis, but the volume of the space served by the network and the system's ability to disperse a leak. A cabinet that discharges freely into a mechanical room and a cabinet connected to a plenum that distributes air throughout the house are not treated the same way. That is exactly why the factory-supplied A2L sensor controls the indoor fan start-up: detect, then circulate the air.
We checked the sensor's location, its power supply, and its response during commissioning, and explained to the client what it means when a fan starts without a thermostat call. On a jobsite an hour from our offices, a client who understands how their equipment behaves can avoid an unnecessary service call.
The open ceiling that must be properly closed
Running the new ductwork required opening the drywall ceiling in the mechanical room. Closing it properly is not an aesthetic finishing detail: in a finished basement, the mechanical-room partition and ceiling contribute to fire separation and noise control. We systematically document the openings left for the client and what must be closed, by whom, and with which material.
The NU-AIR HRV: the point almost everyone misses
The house was equipped with a NU-AIR HRV (heat recovery ventilator), with its four identified ducts: fresh air from outdoors, stale air to outdoors, fresh air to the building, and stale air from the building. Many installers replace the central unit without ever touching the HRV or checking how the two systems communicate. This is a costly mistake.
Interlocking with the cabinet fan
When an HRV is connected to the ductwork — the most common configuration in Quebec — fresh air is injected into the central system's return. If the cabinet fan is not running while the HRV is operating, this fresh air, at -20 °C minus the recovered heat, stagnates in the return plenum and rises toward the coil without being distributed.
The HRV must therefore control the indoor fan startup. On a modulating system such as the FLEXX, this requires correctly wiring the interlock to the 24 VAC control and selecting an appropriate continuous circulation speed. Without this, you end up with a house that has an HRV on the wall but is not ventilated.
Where to draw stale air from and where to deliver fresh air
The connection must maintain sufficient distance between the fresh-air injection point and the stale-air extraction point in the ductwork. If they are too close, they short-circuit: the HRV draws in the air it has just injected, runs, consumes energy, and renews nothing. We measure this separation at the plenum and prioritize a stale-air intake that actually captures air from the house.
Balancing and defrosting, specifically in Noyan
An HRV must be balanced: the supply airflow and exhaust airflow must be adjusted relative to each other at the balancing ports. An unbalanced HRV puts the house under negative or positive pressure, which moves moisture into the walls and can disrupt the draft of other appliances.
In Noyan, where periods below -20 °C are not uncommon, the core defrost cycle of the HRV is especially important: during this cycle, the unit stops introducing fresh air. The nominal airflow must therefore be selected with the understanding that it will not be maintained 100% of the time in January.
The Honeywell humidifier and a heat pump's supply-air temperature
This was the part of the project that required the most explanation for the customer, and it is probably the most useful information in this article.
A bypass humidifier works through evaporation: warm air from the supply plenum passes through a wet pad and becomes charged with vapor. Its performance depends directly on the supply-air temperature.
A furnace typically delivers air at 50 °C or higher. A central heat pump delivers much cooler air, often between 32 °C and 43 °C depending on the outdoor temperature and modulation rate. The consequence is mechanical: the same humidifier produces significantly less vapor downstream of a heat pump than downstream of a furnace.
What we do in response to this reality:
- Warn the customer before installation, rather than receiving a call in January. A house that showed 40% relative humidity with the old system may stabilize at a lower level with the new one, at the same setting.
- Size the humidifier according to the actual supply-air temperature, not the default value from a chart designed for furnaces.
- Make sure the bypass is open when it should be and closed during cooling. A bypass damper left open in summer causes air to circulate in a short circuit around the coil and reduces dehumidification.
Adjusting the humidistat according to the cold
The classic mistake is choosing a percentage and never adjusting it. The adjustment chart attached to the unit exists for a reason: the colder it is outside, the lower the indoor humidity level should be; otherwise, vapor condenses on the windows and migrates into wall assemblies. In a well-sealed country house in Haut-Richelieu, leaving a humidistat set to 45% in January causes water to run down the glazing and, over time, leads to damage.
We set the humidistat during commissioning and show the customer how to adjust it according to the season, with one simple rule: persistent condensation on the windows means it must be lowered immediately.
The humidifier drain
An evaporative humidifier continuously discharges water during the heating season. This drain must be connected, not simply directed onto the slab. On this project, the slab had precisely an old runoff stain, which we discuss below.
Electrical: 45 A outside, plus auxiliary heat
The GUD60W2/NhE-D(U) outdoor unit requires an MCA of 39.9 A and an MOCP of 45 A. This is a significant increase compared with the 2- and 3-ton configurations in the same range, which require only 30 A.
This is in addition to the electric auxiliary heat coil integrated into the cabinet, on its own circuit. The values on the specifications sheet for the indoor unit alone (7.1 A MCA, 15 A MOCP) no longer apply once a coil is installed: the calculation must be redone.
In a fully electric rural home — water heater, range, dryer, and possibly an EV charger —, adding these two circuits requires a panel load calculation before the contract is signed. We have seen too many projects where the question of the electrical service arises on the morning of installation. It is never a good day, for the customer or for us.
Finally, the auxiliary heat must be locked out so that it engages only below the home's balance point and during defrost. Auxiliary heat that responds to every call for heat defeats the purpose of a unit with a COP of 3.4 and turns the winter energy bill into an argument against the heat pump.
The stain on the slab: what we inspect before installing, not afterward
The concrete slab in the utility room had a large rust-colored runoff stain. We never install a new unit over a mark like this without investigating its source: an old water heater, a former humidifier drain, improperly connected condensate, or water infiltration through the slab. Identifying the source before installation prevents two things — the customer attributing a pre-existing problem to the new system, and the new system reproducing the same problem.
The FLEXX condensate was connected with a trap sized for a coil under negative pressure, a continuous slope, and a cleaning access point. A 4-ton cooling coil produces a significant amount of actual water for several months each year.
What this Noyan project changes for your project
The Mistakes We Correct Most Often on This Type of Installation
- Replacing the central unit without reviewing the HRV or its interlock with the blower.
- Leaving an unbalanced HRV, causing the house to be pressurized or depressurized year-round.
- Injecting fresh air too close to the stale-air intake on the plenum, causing the HRV to operate ineffectively.
- Keeping a humidifier without accounting for the lower supply-air temperature of a heat pump, then blaming the heat pump for dry air.
- Leaving the humidifier bypass damper open in summer.
- Choosing the higher-capacity configuration “for safety”, which degrades modulation, dehumidification, and efficiency.
- Discovering that the electrical panel is inadequate on the day of installation.
- Blocking access to the filter with storage in a finished mechanical room.
Certification, Rebates, and Warranty
The installed combination carries AHRI number 217120761 and ENERGY STAR certification. These are the two references required by financial assistance programs, along with the detailed invoice and proof of installation by a licensed contractor. Programs, amounts, and criteria change from year to year: we verify the eligibility of the exact model and exact configuration before signing, rather than announcing a figure that may no longer apply when the application is submitted.
The FLEXX range comes with a standard 10-year warranty, with 2 additional years when the unit is registered and proof of installation is provided. We complete this registration for our customers.
Where We Serve
At AirGreen, we provide HVAC installation and HVAC maintenance for central systems in Noyan, Clarenceville, Lacolle, Saint-Paul-de-l'Île-aux-Noix, Henryville, Saint-Jean-sur-Richelieu, and throughout Haut-Richelieu, as well as in Montreal, Laval, Longueuil, on the South Shore, and on the North Shore. All-electric rural homes with HRVs and humidifiers integrated into the system make up a significant portion of our clientele in Montérégie—and these are precisely the installations where commissioning matters more than the brand printed on the cabinet.
If your mechanical room already contains an HRV, humidifier, or both, ask your contractor how they plan to interconnect them with the new unit. The quality of their answer will tell you more than any quote.