The heart of the system is in the basement, and that is where everything happens
In most installation reports, attention focuses on the outdoor unit: defrosting, snow, the pad, and noise. At Saint-Césaire, the interesting part of the project was elsewhere. All the complexity was in a small basement mechanical room, around a 74 kg gray cabinet connected to an existing ductwork network, an electric heating kit, a bypass humidifier, and a 24 VAC wall thermostat.
At AirGreen, we install central systems in Montreal, Laval, Longueuil, the North Shore, and the South Shore, and the most demanding projects are almost never the most spectacular. They are the ones where six subsystems must coexist within three square meters.
The starting point: an electrically heated home with existing ductwork
The Saint-Césaire customer had been living with a classic electric furnace: a vertical cabinet, resistance elements, a fan, and a Hydro-Québec bill that was perfectly predictable in January. The air distribution network was sound, the plenums were properly sized, and the finished basement provided acceptable clearance in front of the unit.
Their request was simple: keep the comfort of a central forced-air system, add air conditioning for Montérégie's summers, and stop paying for 100% resistance heating from October to April.
This is exactly the type of application where the GREE FLEXX R32 makes sense. The indoor unit replaces the furnace in the same space, connects to the same plenums, retains a conventional 24 VAC thermostat, and receives an integrated electric heating kit. From the living room, nothing changes. From the electric meter's perspective, almost everything changes.
The installed equipment and its certified data
The selected system is a 3-ton GREE FLEXX R32:
- Indoor unit (air handler cabinet): GUD36AH2/G-D(U)
- Outdoor unit: GUD36W2/NhE-D(U)
- AHRI number : 217120760
- Cooling capacity: 34,000 BTU/h nominal, modulation from 18,000 to 37,000 BTU/h
- Heating capacity: 34,000 BTU/h nominal, modulation from 18,000 to 37,000 BTU/h
- TRÉS2 (SEER2) : 18
- CPSC2 (HSPF2) : 10
- TRÉ2 (EER2) : 12
- COP : 3,2
- Refrigerant: R32, factory charge of 2.9 kg
- Heating range: -30 °C to 24 °C
- Cooling range: -15 °C to 54 °C
- Power supply: 208–230 V
- ENERGY STAR certification, including an A2L leak-detection sensor and a factory-installed Wi-Fi module
Two figures deserve emphasis because they govern actual comfort in a Saint-Césaire home. The first is the modulation range: 18,000 to 37,000 BTU/h. An inverter-driven compressor that can reduce its output to half its nominal capacity does not short-cycle in April or October, resulting in much smaller temperature differences between the basement and the upper floor. The second is the CPSC2 of 10: this is the seasonal heating index, and it—not the TRÉS2—determines what the customer will actually save in a climate where the heating season lasts seven months.
Why people still refer to it as an “electric furnace” after installation
This is the most important technical distinction in the project, and the one most often poorly explained to customers.
The installed cabinet is not a furnace in the strict sense. It is an air handler containing the system's indoor coil, blower, and an electric heating kit that serves as the former furnace. The two visible breaker blocks at the upper left of the unit supply these resistance elements.
From the building's perspective, the result is a dual-source heating system.
The exact role of the electric elements
In winter, the heat pump provides heat as long as it is capable of doing so and as long as it is economically advantageous. The electric elements intervene in only three cases:
- Below the balance point — the outdoor temperature at which the heat pump's capacity intersects the home's heat loss. On this project, with a nominal capacity of 34,000 BTU/h and a modestly sized home, this point is fairly low on the temperature scale, leaving the heat pump in sole control for the vast majority of the winter.
- During defrost — the reverse cycle momentarily cools the supply air, and some of the elements compensate so no one feels a cold draft coming from the vents.
- In emergency heat — if the outdoor unit is out of service, the house remains fully heated by the elements, exactly as before.
Wiring for a conventional 24 VAC thermostat
The FLEXX accepts a conventional 24 VAC thermostat, which is a real advantage: no proprietary controls, no dependence on an app, and replacement in ten years with any heat-pump thermostat on the market.
It still has to be configured correctly. Three settings must be made during commissioning, and these are the ones we most often see overlooked:
- The thermostat configuration in heat pump with electric auxiliary heat mode, rather than conventional two-stage heating mode.
- The reversing-valve polarity (O or B, depending on the manufacturer). An error here results in a system that cools when heating is requested.
- The auxiliary lockout and time delay, so that the elements do not engage as soon as the thermostat is raised by two degrees. This is the setting that makes the difference between an electricity bill that goes down and one that does not.
The electrical side: what the heating kit changes in the panel
The air handler alone has very modest power consumption: MCA of 5.3 A and MOCP of 15 A for the 3-ton model. However, this figure is only part of the story, because the electric heating kit is sized and powered separately.
The panel load calculation must therefore account for the air handler, the heating kit, and the outdoor unit, whose MCA is 27.7 A and MOCP is 30 A on this model. On a replacement project like the one in Saint-Césaire, the good news is that the old electric furnace left behind a circuit already sized for full resistive heating. The bad news, in older homes in the area, is that space sometimes has to be added to the panel for the outdoor-unit circuit.
We systematically verify three things before ordering the equipment: the panel capacity, the available breaker space, and the rating of the existing conductors. A grant-eligible central system that cannot be powered is a project that comes to a halt halfway through.
The air-distribution connection: the filter base and static pressure
Under the unit, there is a galvanized sheet-metal filter base marked “Air Flow” with a marker, with an upward-pointing arrow. This detail is not decorative. It indicates the direction of airflow and reminds anyone who opens this cabinet eight years from now which way the filter should be slid in.
The decisive technical factor here is external static pressure. The air handler cabinet is designed to deliver its rated airflow at approximately 125 Pa, or 0.5 in. of water column. A ductwork system originally designed for an electric furnace generally handles this constraint well because the furnace itself offered little resistance. But once a high-density pleated filter, a bypass humidifier, and a poorly flared transition are added, this value can be exceeded without realizing it.
We measure static pressure during commissioning, on both sides of the unit. It is a ten-minute procedure that immediately reveals whether the duct system is restricting the blower. On this job, it guided the choice of thickness and density of the inner filter: a thicker media, and therefore a larger surface area, means less resistance at the same airflow while maintaining good capture.
The bypass humidifier: where most installations go wrong
The white unit mounted on the return plenum is a Honeywell Home bypass humidifier, connected to the supply plenum by the insulated flexible duct running above the unit. The red PEX line rising along the wall is its water supply.
A humidifier paired with a central heat pump requires special attention for one simple physical reason: air delivered by a heat pump is significantly cooler than air from a furnace. An evaporative humidifier that relied on 50 °C air to vaporize its water is instead supplied with air at 32 or 35 °C. Its performance drops.
What we do to ensure the system actually works:
- Orient the bypass in the right direction: air must be drawn from the supply plenum and returned to the return plenum, never the other way around. A reversed connection sends cold air through the humidifier, and it produces almost nothing.
- Provide a proper water supply: a compliant shutoff valve with a dedicated fitting, not a saddle valve pierced into a copper pipe. A saddle valve is a leak waiting to happen in a finished basement.
- Close the bypass damper in summer. If left open, it short-circuits some of the conditioned air directly back to the return, reducing cooling capacity and increasing energy use. This is the only seasonal maintenance action required from the customer, and we leave a written note about it.
- Adjust the humidistat according to the outdoor temperature. At -25 °C, an indoor humidity level of 45% causes condensation to run down the windows and eventually damages the frames. We lower the setting to between 30% and 35% during severe cold.
- Replace the evaporative pad every year. Water in the Montérégie region is mineral-rich, and a scaled-up pad barely humidifies at all.
Condensate, slope, and cleaning openings
The PVC condensate connection exits from the bottom of the unit with a trap and a red cleaning plug. During the cooling season, the indoor coil produces a considerable amount of water, making this drain the most critical component in the entire system: when it becomes blocked, it causes water damage.
Our rules on this matter are firm:
- A trap sized according to the fan pressure; otherwise, it may empty or back up.
- A continuous slope toward the discharge point, with no concealed back-pitch behind a stud.
- Accessible cleaning openings, not buried behind the cabinet.
- Discharge into a suitable drain, never into a collection pan shared with a water heater.
The humidifier also has a purge drain, and it operates in winter, that is, outside the condensation season. Both drains must be checked at different times of the year.
The A2L leak detection sensor in an enclosed mechanical room
R32 is an A2L-class refrigerant and is slightly flammable. The FLEXX has a factory-installed A2L leak detection sensor and the associated safety logic: when a leak is detected, the indoor fan starts to dilute and disperse the refrigerant.
In a basement in Saint-Césaire where the unit is installed in a relatively enclosed room, two checks are essential. First, calculate the minimum room volume based on the refrigerant charge, as indicated in the notice affixed to the cabinet. Then ensure that the fan can actually circulate the air: a sensor controlling a fan whose grilles are blocked by storage boxes is useless. We always keep the service clearances unobstructed and explain this to the customer before leaving.
Mistakes to avoid with this type of conversion
- Keeping the old thermostat without reconfiguring it. A thermostat set to furnace mode will run the electric elements continuously and eliminate the heat pump's economic advantage.
- Neglecting the building load calculation. Simply reusing the old furnace's capacity almost always leads to oversizing, which causes short cycling and, consequently, poor humidity control in summer.
- Installing an overly restrictive filter because it promises better air quality. Without measuring static pressure, this guarantees a loss of airflow.
- Leaving the humidifier bypass damper open all year.
- Forgetting to weigh the refrigerant charge when the line length exceeds the 7.5 m factory charge length. The additional charge for this model is 30 g per additional metre, and the maximum distance between the units is 30 m with a maximum elevation difference of 15 m.
- Failing to document commissioning. Without initial readings, no technician will be able to diagnose a deviation three years from now.
Rebates and eligibility
The FLEXX is ENERGY STAR certified and has a specific AHRI number for each configuration: 217120760 for the unit installed in Saint-Césaire. These two elements are what government programs and distributors require to validate eligibility for applicable rebates.
Program criteria and amounts change depending on the period and level of government. We verify the eligibility of the exact configuration before signing, never afterward, and provide the customer with the corresponding AHRI certificate along with the invoice. This is the only way to avoid having an application rejected because a model number does not match the installed pair.
What this Montérégie project illustrates
Converting an electric furnace to a central heat pump system is not an appliance swap. It is the coordination of a ductwork network, an electrical panel, a thermostat, a humidifier, a condensate drain, and control logic for two heat sources. Each of these six elements can, on its own, undermine the system's performance.
At AirGreen, we provide HVAC installation and HVAC maintenance for central systems in Saint-Césaire, throughout the Rouville RCM and Montérégie, as well as in Montréal, Laval, Longueuil, on the North Shore and the South Shore. Our technicians measure static pressure, weigh the refrigerant charge, configure the auxiliary heat lockout, and provide written commissioning readings.
For an assessment of your home and a detailed quote for a GREE FLEXX R32 central system, contact our team. We come to your home, take measurements, and explain exactly what will be done before anything is disconnected.