The furnace stays in place while the heat pump takes over: anatomy of a project in Acton Vale
Along Route 116, between Saint-Hyacinthe and Drummondville, homes from the 1970s and 1980s all share one feature: the ductwork is already there, made of sheet metal, properly sized, and often in better condition than the unit blowing air through it. In Acton Vale, a homeowner called us with a specific request. His forced-air electric furnace was still working perfectly. He didn't want to throw it away. He simply wanted to stop heating 100% of his home with electric resistance and finally have central air conditioning in the summer.
That is exactly the purpose of a central “add-on” heat pump. And this is the kind of project where the difference between a proper installation and a mediocre one isn't visible in the photo: it's visible on the January Hydro-Québec bill.
The starting point: a good duct network, but the wrong heating method
Before proposing anything, we inspected the unfinished basement. We weren't looking for the unit, but for everything around it:
- The condition of the duct network: sections, leaks at the joints, undersized return air, and dampers blocked by a renovated floor.
- The available electrical capacity at the panel, because a central heat pump never arrives alone at an already loaded panel.
- The available ceiling height in the mechanical room, which alone determines the indoor unit's configuration.
- The home's load calculation, room by room, rather than a rule of thumb based on square footage.
The verdict was favorable: a sound duct network, adequate return air after a slight enlargement, and a calculated load pointing to a 34,000 BTU capacity. The constraint, however, was geometric. The basement ceiling did not leave enough clearance for an upright indoor unit. The unit would have to be laid horizontally.
Why the GREE FLEXX R32 in an “add-on” configuration
The FLEXX from GREE is an inverter-technology central system designed for connection to 24 VAC with a conventional thermostat and—this is the decisive point here—for horizontal (side-discharge) airflow. A unit designed from the outset to be laid horizontally is not the same as a vertical unit turned on its side in the hope that the drain will cooperate.
The pairing selected for Acton Vale:
- Outdoor unit: GUD36W2/NhE-D(U)
- Indoor unit (air handler): GUD36AH2/G-D(U)
- Nominal capacity: 34,000 BTU for both cooling and heating
- Actual modulation: 18,000 to 37,000 BTU, which matters more than the nominal figure
- SEER2: 18 — HSPF2: 10 — EER2: 12
- COP: 3.2
- AHRI number: 217120760, ENERGY STAR certified
- R32 refrigerant, factory charge of 2.9 lb (102.3 oz)
- Heating range: -30 °C to 24 °C — Cooling range: -15 °C to 54 °C
- factory-installed Wi-Fi, leak detection sensor included
- Power supply 208-230 V : outdoor unit MCA 27.7 A / MOCP 30 A, indoor unit MCA 5.3 A / MOCP 15 A
The modulation range of 18,000 to 37,000 BTU is what makes the FLEXX suitable as an add-on. A home in Acton Vale does not require 34,000 BTU for 90% of the winter. It requires 12,000, 15,000, or 20,000. An inverter compressor that can ramp down to 18,000 BTU runs for long periods at low speed instead of starting and stopping every eight minutes. The concrete result: fewer temperature differences between rooms, less startup noise in the ducts, and seasonal efficiency that finally resembles the figure printed on the specification sheet.
A Unit Laid on Blocks: What the Horizontal Configuration Really Requires
The indoor unit was installed horizontally, resting on concrete blocks a few centimeters from the ceiling. Three non-negotiable requirements in this position:
- Slope toward the drain. A horizontal unit must be tilted slightly toward its condensate outlet. A perfectly level unit retains water in its pan. In summer, a pan that retains water becomes a source of odors and, eventually, water damage to the floor.
- Service clearance. Access to the control box, motor, and indoor filters must remain possible without removing the plenum. We systematically leave the necessary space, even when that means moving the unit back a few inches.
- Air transition. The connecting plenum was custom-built, insulated, and sealed with aluminum tape. An overly abrupt transition at the outlet of a horizontal unit creates an invisible pressure drop that reduces airflow and, with it, heating capacity in cold weather.
On this job, the unit rests on blocks rather than being suspended. The basement floor was level and dry, and supporting the load with blocks prevents vibration from being transmitted to the floor structure—a point that occupants on the ground floor notice immediately.
The detail almost no one checks: the A2L leak detection sensor
An orange bilingual label on the unit’s panel states that if the unit discharges air from the right side, the refrigerant sensor must be placed in the location shown in the figure. This label is new to the Quebec landscape, and it concretely changes installation work.
R32 is an A2L-class refrigerant: mildly flammable. Central units that use it now include a factory-installed leak detection sensor, with a mitigation mode that starts the blower to disperse the refrigerant if a threshold is reached. The sensor is effective only when it is on the correct side of the airflow.
What this means on a real job site:
- The sensor position depends on the selected discharge direction. On a horizontal unit, this is not the factory-default direction. The sensor must be repositioned according to the figure, and its position must be checked before closing the panel.
- The mitigation mode must be able to start the blower at any time, which means the indoor unit’s power cannot be shut off during the off-season.
- No ignition source may be added beneath the unit afterward. A rural basement will often end up accommodating a water heater, compressor, or generator. We discuss this with the customer at the time of installation, not afterward.
We are already seeing A2L units installed by others in service where the sensor was left in its factory position on a unit installed horizontally. The unit operates. The protection does not.
Connecting two units to a single 24 V thermostat
This is the heart of a successful add-on installation, and it is where the majority of sloppy installations give themselves away.
The existing electric furnace was retained, but its blower was locked out. Two blowers in series in the same duct system do not add up; they interfere with each other. In the configuration selected, the FLEXX blower moves the air at all times, and the furnace’s electric elements now serve only as supplemental heat, staged by the conventional 24 VAC thermostat.
The sequence is simple to describe but demanding to wire correctly:
- When there is a call for heat, the heat pump starts up and modulates on its own.
- Below the balance point, when the heat pump's capacity is no longer sufficient for the load, the first stage of electric resistance heat is added.
- The second stage comes on only in the event of a significant temperature drop or recovery after a nighttime setback.
- If the heat pump fails, emergency mode switches directly to the electric resistance heaters without letting the house cool down.
The classic mistake is to let the electric resistance heaters start at the same time as the heat pump. The house is warm, the customer is happy, and they pay exactly the same rate as before despite having invested in a heat pump. We measure and lock in this delay during commissioning, and verify it with an ammeter rather than relying on a thermostat displaying “heating.”
The balance point: where the bill is determined
The balance point is the outdoor temperature at which the heat pump's capacity exactly matches the home's heat loss. Above it, the heat pump is sufficient. Below it, supplemental heat operates.
To determine it, we cross-reference three data points: the home's calculated heating load, the certified capacity curve of the GUD36W2/NhE-D(U) at low temperatures, and the applicable electricity rate. Since the FLEXX is approved for heating operation down to -30 °C, supplemental heat comes on much later than with a previous generation of central systems. In practical terms, for most of the winter in the Acton RCM, the heat pump remains the primary heating mode, and the electric resistance heaters are used only during cold snaps.
We record the selected balance point on the commissioning sheet provided to the customer. It is a figure a future technician should be able to read, not guess.
Refrigerant line, charge, and commissioning
The manufacturer's data sets the parameters for the work:
- 3/8 in. liquid line, 3/4 in. gas line
- Precharged length: 24.6 ft (7.5 m)
- Maximum length between units: 98.4 ft (30 m)
- Maximum elevation difference: 49.2 ft (15 m)
- Additional charge: 0.32 oz/ft (30 g/m) beyond the precharged length
On this project, the refrigerant line was routed through the foundation wall to the outdoor slab. Every foot beyond 24.6 ft was measured, recorded, and converted into additional charge. An R32 system operating with an approximate charge loses efficiency long before it breaks down, and the owner has no way to notice.
Commissioning always includes: evacuation with a micron reading, leak testing, weighing the additional charge, recording pressures and superheat, measuring the actual airflow in the ductwork, verifying startup of A2L mitigation mode, and configuring the built-in Wi-Fi.
Condensate: the small black pipe that causes major damage
The black corrugated hose beneath the unit is probably the least expensive component of the system and the one that generates the most service calls. With a horizontal unit, there is no room for error: a trap at the proper height, a continuous slope, no low points where water can stagnate, and a route that does not pass through a cold area where it could freeze.
We route the condensate to an existing drain or to a condensate pump equipped with a safety switch that cuts off the cooling call if the water level rises too high. Without this switch, a pump that stops working on a Thursday in July announces itself with a puddle beneath the unit by Sunday.
Mistakes to avoid with an “add-on” central heat pump
- Reusing a duct network without measuring it. A 34,000 BTU unit requires airflow that many original duct systems cannot provide. The shortfall only becomes apparent in January.
- Keeping the old thermostat. A thermostat that does not properly manage staging of the supplemental heat defeats the intended savings.
- Leaving both fans active. A fan that has not been locked out in the existing unit creates recirculation and noise.
- Installing the outdoor unit too low. In Acton Vale, snow on the ground and snow sliding off a roof are unforgiving. A raised base and sufficient clearance beneath the unit are essential for defrosting.
- Neglecting the position of the A2L sensor on a horizontal unit.
- Failing to document the added refrigerant charge. Without this information, any future diagnosis starts blind.
Incentives: what is realistic and what needs to be verified
A certified ENERGY STAR central system with a valid AHRI number, such as the GUD36W2/NhE-D(U) + GUD36AH2/G-D(U) combination, qualifies for Quebec heat pump incentive programs. Hydro-Québec's LogisVert program generally calculates the incentive based on the certified capacity at -8 °C, which explains why two units with the same “tonnage” may not qualify for the same amount. The Rénoclimat program may be added when energy-efficiency work accompanies the installation, while Chauffez vert targets the replacement of an oil- or propane-fired system.
Two points we validate before signing, and that cause many applications submitted elsewhere to fail:
- The contractor must be registered with the program and hold a valid RBQ licence. Registration is not automatic.
- The exact model must appear on the list of eligible equipment, and the AHRI number must match the pair actually installed, not just the outdoor unit.
Amounts and conditions change from year to year. At AirGreen, we verify the eligibility of the selected combination before purchase and provide the documentation required for the application.
What we learned from the Acton Vale project
This project was nothing spectacular. An ordinary house, an unfinished basement, a low ceiling, an existing ductwork network, and a homeowner who wanted to keep what was still working. This is precisely the type of project where a well-executed HVAC installation delivers the greatest value: the heat pump handles most of the high-efficiency heating, the existing system becomes insurance rather than a monthly expense, and central air conditioning comes as a bonus in a network that has already been paid for.
Our coverage in the region
At AirGreen, we install and service central systems, wall-mounted heat pumps, air conditioners, VRF systems, and air exchangers in Montreal, Laval, Longueuil, on the North Shore and South Shore, as well as in the Montérégie RCMs—from Acton Vale to Saint-Hyacinthe and from Beloeil to Châteauguay. Our teams work just as well in the unfinished basements of 1970s bungalows as in the mechanical rooms of multi-unit buildings.
Our clients in Acton Vale, Roxton Falls, and Sainte-Christine have one thing in common with those in Le Plateau and Brossard: they want a properly selected, properly installed, and properly commissioned system, along with a phone number that answers when something goes wrong.
Talk to someone who has done this kind of project before
If you have a furnace that is still working and an existing ductwork network, a central add-on heat pump is probably the most cost-effective upgrade you can make. The load calculation, balance point, auxiliary heat staging, and A2L sensor location must, however, be handled by people who deal with them every day.
Contact AirGreen for an assessment of your central system, an actual load calculation, and a detailed quote for a GREE FLEXX R32 suited to your home. We respond quickly, measure before making recommendations, and document everything we install.