The differential pressure gauge before the cheque book: why a successful replacement begins with measuring static pressure
At Saint-Robert, the house overlooked flat fields, a short distance from Lavallière Bay and Lake Saint-Pierre. The central system in place was about fifteen years old. It was still working, but it was expensive to operate and could no longer dehumidify the house in July.
The homeowners called us in June, not January. It was the best decision they made in this case—because a planned replacement allows you to do something an emergency replacement never does: measure the existing ductwork before choosing the equipment.
The installed system is a 3-ton GREE FLEXX R32 central heat pump, with the GUD36W2/NhE-D(U) outdoor unit paired with the GUD36AH2/G-D(U) air handler, mounted on brackets anchored into the foundation against a vinyl-clad wall.
The installed pairing
- Outdoor unit: GUD36W2/NhE-D(U) — single fan, 1/6 HP motor
- Air handler: GUD36AH2/G-D(U)
- Rated capacity: 34,000 BTU for both cooling and heating
- Modulation: 18,000 to 37,000 BTU
- 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 down to -30 °C, cooling from -15 °C to 54 °C
- 208-230 V : outdoor MCA 27.7 A / MOCP 30 A, indoor MCA 5.3 A / MOCP 15 A
- Indoor unit: 21 1/4 x 48 1/5 x 21 1/4 in., 163.1 lb (74 kg)
- Factory-installed Wi-Fi, A2L leak detection probe included
Removing the old system: what is regulated by law
A replacement begins with dismantling, and this step is not left to the contractor’s discretion.
The refrigerant contained in the old system—R-22 or R-410A, depending on the age of the unit—must be recovered into an approved cylinder using recovery equipment by a technician holding the required environmental certification. Venting it into the atmosphere is illegal in Quebec, yet it is still seen on rushed job sites where no one is watching.
We collect it, weigh what was recovered, and transport the bottle to a return point. The recovered quantity is recorded in the file. The owner will never see this line on their electricity bill, but it is part of what they are buying when they choose one installer over another.
The copper in the old refrigerant line is also removed. A line that was used for an R-22 system with mineral oil cannot be reused as-is for a circuit R32 operating with POE oil: incompatible oil residues circulate and attack the new compressor. On this project, the line was completely replaced.
The measurement almost no one takes: external static pressure
This is the heart of the project, and it is the least understood aspect of all residential central air conditioning.
A central heat pump does not produce its advertised BTUs in a vacuum. It produces them at a given airflow, which is itself achieved at a given static pressure. If the ductwork offers more resistance than expected, the blower cannot deliver the airflow, the coil does not receive enough air, and the actual capacity drops—in both heating and cooling.
Even before finalizing the quote, we connect a differential pressure gauge to the existing system and measure the external static pressure on both sides of the equipment. What we regularly find in homes on the South Shore and in Montérégie:
- Clearly undersized return-air openings, often just one and often too small.
- Thick, high-filtration indoor filters installed by the homeowner with the best intentions, which alone double the system's resistance.
- Branches capped off during a renovation and never rebalanced.
- Tight elbows and crushed sections behind a suspended ceiling.
In Saint-Robert, the measurements led to two corrections before installation: enlarging the return-air opening and replacing the filter cabinet with a model offering a much larger media surface area at equivalent filtration efficiency.
Why filter surface area changes everything
A finer filter is not necessarily more restrictive. What matters is the media surface area available to the airflow. A 4-inch-thick pleated filter with a large developed surface area can filter more finely than a 1-inch flat filter while creating less resistance.
This is the most cost-effective improvement a homeowner can make to an existing system: it improves air quality, reduces static pressure, increases actual airflow, and extends the interval between replacements. We systematically include it in replacement quotes rather than leaving it as an option.
Latent load: what the summers in Saint-Robert impose
The house is located in a low, flat area near Lavallière Bay and Lake Saint-Pierre. In summer, the air is heavy. The house's load is therefore not just a matter of heat—it is largely a matter of humidity.
A central system dries the air by cooling it below its dew point across the indoor coil. For this to work, the system needs sufficient run time. An oversized unit reaches the temperature setpoint in fifteen minutes, shuts off, and leaves the humidity behind. The house is cool and clammy: the worst of both worlds.
That is exactly why we refused to “go up one size for safety.” The modulation range of the 34,000 BTU system — from 18,000 to 37,000 BTU — allows the unit to run for long periods at low output during humid July days instead of cycling on and off. The result is measured with a hygrometer, not just the thermostat.
We also explain what a heat pump does not do: it does not replace a dehumidifier in an unconditioned basement, and it does not correct a source of structural moisture. In a low-lying home, we always inspect the perimeter drainage, the crawl-space vapor barrier, and the dryer venting before concluding that the HVAC system will solve everything.
Bracket mounting against a vinyl-clad wall
The outdoor unit is suspended on a galvanized steel support, with vibration-isolating pads between the cabinet and the crossbars, against a vinyl-clad wall and above a paver walkway.
The key technical point: vinyl siding carries no load. It is stapled in place, floats to allow for expansion, and forms part of the wall's drainage plane. A bracket bolted into vinyl siding will sag, tear the siding, and create a path for water to enter behind the cladding.
On this job, the brackets were anchored into the concrete foundation wall, below the siding line. This is the strongest and most durable solution:
- The load — 187.5 lb for the unit, plus winter ice — is supported by solid concrete.
- No penetrations are made through the wall's drainage plane.
- Vibrations are transmitted to the foundation rather than the floor framing, making them much less audible indoors.
Every penetration is sealed. The bundle of piping and wiring entering the house is fitted with a drip loop, and the disconnect box is mounted within easy reach.
The clearance beneath the unit and the paver walkway
Installing a low-mounted unit above a traffic surface requires special care. Defrost water will fall onto the pavers all winter. We therefore plan either to divert the drainage or install a drainage bed beneath the unit, and we discuss this with the client before drilling.
Clearance beneath the cabinet must also remain sufficient to prevent accumulated snow from reaching the coil. A height that seems generous in June is no longer so in February.
Commissioning during the cooling season: the readings are finally clear
An installation carried out in summer offers an advantage that emergency winter jobs do not: performance readings in cooling mode are usable.
Our commissioning sequence on this job:
- Dry nitrogen leak test on the new line, with timed hold and reading.
- Deep evacuation with a micron gauge, followed by an isolated pump rise test.
- Weighed charge: actual line length, minus the 24.6 ft (7.5 m) precharged length, plus 0.32 oz per foot (30 g/m). 3/8-inch liquid line, 3/4-inch gas line, maximum length 98.4 ft (30 m), maximum elevation difference 49.2 ft (15 m).
- Measurement of actual airflow and static pressure after duct system modifications.
- Recording the temperature difference between return and supply air, compared with the expected target for the day's ambient humidity.
- Verification of the A2L detection sensor's operation and mitigation mode.
- 24 VAC thermostat configuration and pairing of the factory-installed Wi-Fi.
The recorded readings are documented on a form given to the homeowner. They provide the baseline reference: in five years, a technician will know whether or not the system has drifted.
Mistakes to avoid when replacing a central system
- Choose the wire gauge based on the old unit. The old system may have been oversized from the outset. Repeating the mistake for another fifteen years makes no sense.
- Reuse the refrigerant line from an R-22 system. Mineral oil residue shortens the life of the new compressor.
- Ignore static pressure because “the ducts are already there.”
- Install a 1-inch high-filtration filter on an already restricted duct system.
- Oversize the system for peace of mind, which ruins summer dehumidification.
- Skip refrigerant recovery from the old unit.
- Anchor a support bracket in vinyl siding rather than in the structure or foundation.
- Do not keep the commissioning record. Without a baseline reference, any future diagnosis starts from scratch.
Grants: the special case of replacement
The GUD36W2/NhE-D(U) + GUD36AH2/G-D(U) pairing, certified ENERGY STAR under AHRI number 217120760, qualifies for Quebec assistance programs for heat pumps.
Replacement cases require particular attention: programs distinguish between adding an efficient appliance where there was none, replacing a fossil-fuel system, and replacing an existing electric appliance. The amounts and conditions are not the same, and an application filed under the wrong category is rejected.
Three items we validate before ordering:
- Does the exact pair appear on the list of eligible equipment, with the correct AHRI number.
- Which work category applies based on what is being removed.
- What is the submission deadline from the invoice date.
The rules change every year. We confirm eligibility in writing before the equipment is ordered, not once the installation is complete.
What Saint-Robert illustrates better than any sales pitch
Two contractors can offer the same GREE FLEXX R32, at the same price, with the same manufacturer's warranty. One will have measured static pressure, enlarged the return air, redone the refrigerant line, recovered the old refrigerant, weighed the charge, and provided a measurement report. The other will have simply swapped one box for another.
The second system will work. It will consume more, dehumidify less effectively in July, lose capacity at -20 °C, and no one will understand why—because the measurement that would have explained it was never taken.
Our presence in Montérégie and Greater Montreal
At AirGreen, we design, install, and maintain central heat pumps, ductless heat pumps, air conditioners, VRF systems, and air exchangers in Montreal, Laval, Longueuil, on the North Shore, on the South Shore, and throughout Montérégie, including Saint-Robert, Saint-Ours, Sainte-Victoire-de-Sorel, Yamaska, and the Sorel-Tracy area.
Our teams work on both lowland homes with existing ductwork and urban multi-unit buildings. Load calculations, ductwork measurements, and documented commissioning are part of the work in both cases.
Have your system assessed before it forces your hand
If your central system is more than twelve years old, your home stays humid in July despite the air conditioning, or your summer bills have increased without any change in your habits, it is worth having an assessment done now rather than during a heat wave.
Contact AirGreen for a load calculation, a static pressure measurement of your ductwork, and a detailed quote for a GREE FLEXX R32 system. You will know what is actually limiting your current installation before choosing its replacement.