An electric furnace retained, with a central heat pump installed above it: the Kahnawà:ke project
The basement was typical of what we see in Kahnawà:ke homes built between the late 1970s and early 1990s: exposed joists, a bare concrete floor, a Giant Super Cascade water heater in one corner, a makeshift laundry room behind a partition, and at the center of it all, a still perfectly functional Arcoaire Supreme electric furnace, with the original installer's yellowed sticker still stuck to the compartment door.
The homeowner called us for a simple reason: his winter electricity bill was no longer going down, and he had no central air conditioning. Two window air conditioners kept the bedrooms cool from June to September. His first idea was to tear everything out and start from scratch. We suggested the opposite, and that is what we installed: a GREE FLEXX R32 ULTRA central heat pump in an “add-on” configuration, installed directly above the existing electric furnace.
The starting point: a well-maintained electric furnace, but the only heating system
An electric furnace converts electricity into heat at an efficiency of roughly 1 to 1. One kilowatt-hour purchased provides one kilowatt-hour of heat. This is neither a malfunction nor a design flaw: it is the physical limit of electric resistance. As long as the home is heated exclusively this way, no thermostat adjustment, maintenance, or filter change will significantly reduce the bill.
A central heat pump, on the other hand, does not generate heat: it moves it. At -8°C, a unit like the FLEXX R32 still delivers around 3 units of heat for every 1 unit of electricity consumed. That is exactly where the savings are, and why the reasoning “my furnace works fine, so I don't need to change anything” misses the real issue.
Why an “add-on” rather than a complete replacement
The add-on approach consists of keeping the existing heating appliance and installing a coil in a casing within the air circuit, supplied by an outdoor heat pump unit. The electric furnace does not disappear: it becomes the backup heat, called on only when the heat pump is no longer sufficient.
The advantages in this specific case:
- The existing blower is reused. There is no need to pay for a second ventilation motor.
- The duct network remains in place. The plenums, drops, and grilles are retained, which considerably reduces sheet-metal work hours.
- The redundancy is real. If the heat pump fails on January 15, the electric elements take over completely. A home is never left without heat.
- The project cost drops significantly compared with a complete system replacement, for an almost identical energy-efficiency gain.
The drawback, and we always say this upfront: an add-on inherits the shortcomings of the existing system. If the ductwork is undersized, the heat pump will suffer. That is why we measure before selling, never afterward.
The selected unit: GREE FLEXX R32 ULTRA
We selected the GUD36W2/NhE-D(U) outdoor unit from the GREE FLEXX R32 ULTRA line, distributed by Dettson, with a rated capacity of 34,000 BTU/h for cooling and 34,000 BTU/h for heating, with modulation from 5,200 to 10,800 W depending on demand.
The figures that really matter
| Data | Value |
|---|---|
| SEER2 | 18 |
| HSPF2 | 10 |
| EER2 | 12 |
| COP | 3,2 |
| Heating range | -30 °C to 24 °C |
| Cooling range | -15 °C to 54 °C |
| Indoor section airflow | 3,176 ft³/min |
| Noise level (outdoor unit) | 61 dB(A) |
| Maximum piping length | 98.4 ft (30 m) |
| Maximum indoor/outdoor elevation difference | 49.2 ft (15 m) |
| Refrigerant | R32 (A2L), 2.9 lb factory charge |
| Certifications | ENERGY STAR, AHRI, Cold Climate |
| Warranty | 10 years + 2 years on the compressor and parts (with registration) |
Three elements of this specification sheet weighed heavily in the decision.
First: the heating range drops to -30 °C. That is not a decorative figure for the Montérégie region. Kahnawà:ke is exposed to winds from the St. Lawrence Seaway, and January nights at -27 °C with wind chill are not uncommon. A heat pump that cuts out at -15 °C leaves electric heating to work alone during the periods when it costs the most.
The second: the conventional 24 VAC thermostat. The FLEXX operates with a standard 24 VAC power supply and a conventional thermostat, with no proprietary controller required. For an add-on system that must coordinate a heat pump with existing electric heating elements, this compatibility makes all the difference in the control wiring.
The third: the included A2L leak-detection sensor and the factory-installed Wi-Fi. We will return to these below.
The installation process
First step — measuring before the quote. We measured the available airflow and the static pressure of the existing ductwork. A 3-ton heat pump requires approximately 350 to 400 CFM per ton, or 1,050 to 1,200 CFM. The Arcoaire furnace blower could achieve that, but only after changing the speed tap. In this type of assessment, one out of every two systems we inspect requires a correction.
Second step — the indoor cabinet. The coil was installed directly above the furnace, with a custom-fabricated sheet-metal transition, then the supply plenum was insulated and sealed with an aluminum vapor barrier on all sides. A coil that cools air in an unfinished basement produces condensation on its walls: if the cabinet is not properly insulated, the water eventually reaches the slab and the customer thinks there is a leak.
Third step — the refrigerant line. A 3/8-in. liquid line and a 3/4-in. gas line, brazed under nitrogen, routed along the structure with supports at the prescribed intervals, then continuously insulated all the way to the service valves. Vacuum evacuation, leak-tightness test, and refrigerant charge adjusted according to the actual length (0.32 oz per foot beyond the 24.6-ft factory charge).
Fourth step — the outdoor unit. Installed on a raised base. With less than 18 to 24 in. of ground clearance, accumulated snow and defrost water block airflow and freeze the bottom of the coil. This is the most common error we correct on installations done by others, throughout the South Shore, as well as in Laval and on the North Shore.
Fifth step — commissioning. Checking the settings, testing in heating mode, testing in cooling mode, running a forced defrost cycle, pairing Wi-Fi, and handing the commissioning report to the owner.
The three pitfalls of adding a heat pump to an electric furnace
1. Locking out the electric elements
This is the costliest and least visible trap. If the thermostat allows the electric elements to activate at the same time as the heat pump whenever it gets a little cold, the house will be comfortable, but the bill will barely decrease. The customer will have paid for a heat pump while continuing to heat with resistance elements.
The correct setup is to define a balance point—the outdoor temperature below which the heat pump alone is no longer sufficient—and prohibit auxiliary heat above that threshold. On this job, we established it and then adjusted it after three weeks of real-world data, rather than using a default value taken from a manual.
One point worth noting—and a nuance few contractors explain—is that a heat pump paired with an electric furnace does not qualify for Hydro-Québec's DT dual-energy rate, which targets a combination of electricity and fossil fuel. The savings come from the heat pump's efficiency, not the rate.
2. Airflow
A blower set up for electric heating is not necessarily set up for air conditioning. Too little air: the coil freezes. Too much air: dehumidification collapses, and the house becomes cold and clammy in July. Adjusting the blower speed is part of the installation, not an “extra.”
3. Condensate
Because the coil was above the furnace and the basement had no usable floor drain nearby, we installed a condensate pump with a trap and PVC piping. A warning we often repeat after being called to repair other people's damage: condensate must never be discharged into a water heater's drain pan. That pan is a last-resort safety device, not a drain. If a water heater subsequently leaks for its own reasons, it will find a pan that is already full, and the water damage will become a major loss.
R32 and A2L leak detection: what concretely changes
R32 is classified as A2L, meaning it is mildly flammable. There is nothing inherently concerning about that—the classification imposes specific installation, charge, and detection requirements, and the FLEXX comes with its leak-detection sensor included.
What changes in practice: brazing, leak testing, and refrigerant handling must be performed by technicians trained in A2L refrigerants, using the appropriate equipment. A contractor who treats an R32 installation exactly like an R410A installation has not updated their procedures. This is a question to ask before signing, whether in Kahnawà:ke, Montréal, Longueuil, or Châteauguay.
Rebates, local requirements, and cost-effectiveness: what we recommend to homeowners on the South Shore
LogisVert: the amount depends on capacity at -8 °C
Hydro-Québec's LogisVert program calculates the rebate for an air-source heat pump based on its heating capacity at -8 °C, not its nominal capacity, using a formula of approximately $120 per 1,000 BTU/h, up to a cap of about $6,700. General requirements: be a residential property owner and a Hydro-Québec customer, have the work performed by a contractor holding an RBQ licence, and install a unit listed on the approved list, ENERGY STAR certified and assigned an AHRI number.
The FLEXX R32 ULTRA checks all the boxes: ENERGY STAR, AHRI certification, and Cold Climate qualification. However, rebate amounts and criteria are reviewed periodically—we systematically verify the applicable amount for each application before putting it in writing in a quote, rather than announcing an appealing figure that never materializes.
The AHRI combination trap in an add-on configuration
Here is the technical issue that causes the most rebate applications for add-on installations to fail, and it deserves to be understood before the work begins.
An AHRI number does not certify a single unit: it certifies a combination—outdoor unit, indoor section, and, where applicable, heating equipment. When a coil is paired with a furnace from another brand, the installed combination may not match a published certified combination. The result: the rebate application is denied for a purely documentary reason, even though the system works perfectly.
We therefore verify that a certified combination exists before ordering the equipment, not afterward. It is a ten-minute check that protects several thousand dollars.
A framework that is not the same as that of a Quebec municipality
Working in Kahnawà:ke requires a level of precision that many contractors discover only on site. The Mohawk territory falls under the Mohawk Council of Kahnawà:ke for matters concerning building and land-use permits, rather than a Quebec municipal planning department. The procedures, forms, and contacts are not the same as those in Châteauguay or Sainte-Catherine, even though these cities are only a few minutes away by car.
Our approach is simple: we confirm the required permits with the relevant authorities before scheduling the installation date, and we never require the customer to handle a process themselves that we know better than they do.
The result, six months later
The system was commissioned in the spring. The owner spent the summer with quiet central air conditioning operating at 61 dB(A) outdoors, the window air conditioners were removed from the bedrooms, and consumption monitoring through the Wi-Fi app lets him see in real time when the electric backup engages. We will return in the fall to adjust the balance point based on the first complete heating season.
Our experience on the South Shore and elsewhere
At AirGreen, we regularly carry out this type of add-on conversion in Châteauguay, Delson, Saint-Constant, Sainte-Catherine, Saint-Isidore, Longueuil, and Boucherville, as well as in Montréal, Laval, and on the North Shore. Every electrically heated home with an existing duct network is a potential candidate—provided someone takes the time to measure before making a recommendation.
If your home is heated by an electric furnace and you are wondering whether an add-on is feasible for you, we take measurements, verify the AHRI combination, and provide you with a detailed quote in which every line is justified.