Central Heat Pump ADD-ON ARCOAIRE HSH6 Premium | 3.5 Tons - 42,000 BTU (Coil EVD4X 17.5 in.) - R410A

A heat pump on the furnace, and ducts ready to keep up

The ADD-ON system adds a heat pump to an existing electric furnace: a coil is installed on the furnace and the outdoor unit heats and cools through the same ducts. The furnace's electric elements are only used as backup. The Central Heat Pump ADD-ON ARCOAIRE HSH6 Premium | 3.5 Tons - 42,000 BTU (Coil EVD4X 17.5 in.) - R410A pairs the HSH642GKA outdoor unit with the EVD4X42M17A coil.

In a central system, the equipment only does half the work. The other half belongs to the duct system: it carries warm or cool air to each room and brings it back to the coil. A well-chosen heat pump connected to an ill-suited duct system gives a house with uneven temperatures, noisy registers or a poorly cooled upper floor. This page explains what we look at in your ducts before proposing this pairing.

What air conditioning asks of the ducts

Many duct systems were designed for an electric furnace alone. Yet, for the same capacity, air conditioning and heat pump heating move more air than the furnace's heating elements, because the air comes out less hot, or cooler. A duct system sized tightly for the furnace can therefore become too narrow once the coil is added.

The signs are well known: whistling at the grilles, registers that blow hard near the furnace and weakly at the end of the run, a blower that struggles. We observe and measure the duct system during the visit; the quote specifies any corrections needed.

Return air, often the weak link

The blower can only push the air it receives. In many homes, supply registers are plentiful but return grilles are scarce, sometimes a single one on the main floor. The blower is then starved of air, airflow at the coil drops, and the heat pump's capacity drops with it.

Adding a return upstairs or enlarging an existing grille often fixes several things at once: better airflow, less noise, a more even temperature between levels. We tell you whether it would help in your home and where that return could run.

The upper floor that is too warm in summer

Cool air is heavier than warm air and tends to stay low, while heat from the roof builds up high. In a two-level house cooled from the basement, the upper floor is therefore often the hardest area. Three levers help: balancing the dampers to send more air upstairs in summer, a return upstairs, and adjusting the speed of the furnace blower.

The EVD4X42M17A coil also removes humidity from the air. A less humid house feels cooler at the same temperature, which helps the upper floor stay comfortable on warm nights.

The 17.5 in. coil and the plenum

The coil fits between the furnace's air outlet and the supply plenum, the sheet-metal box from which the main ducts branch out. Clearance is therefore needed above the furnace, and the existing plenum is often modified or rebuilt to accommodate the coil. The 17.5 in. format has to match the width of the furnace cabinet; if they differ, a sealed sheet-metal transition joins them.

The coil also adds resistance to the airflow. On a duct system that is already tight, it matters: one more reason to check the ducts before the installation rather than after. Finally, we seal the joints around the coil so that all of the air passes through it.

Ducts in unheated spaces

A duct that runs through a garage, a crawl space or an attic loses heat in winter and gains heat in summer. In cooling mode, its cold surface can also condense humidity from the surrounding air and drip. These sections must be well insulated and sealed. We locate them during the visit and tell you whether they need correcting.

System ratings

The pairing of the HSH642GKA outdoor unit and the EVD4X42M17A coil is not listed with SEER2 ratings in the AHRI certification directory. We confirm the ratings of the complete system before the quote; they also determine grant eligibility.

A system's ratings are established under standardized test conditions; in your home, the condition of the ducts affects what the system actually delivers. That is why we check the ducts at the same time as we confirm the ratings, and why this page shows none.

The 3.5-ton size and the HSH6 series

The 3.5-ton size often suits a good-sized two-storey house, a house on two levels with a finished basement, or a townhouse in Montreal whose duct system serves three levels. These examples are only a guide: insulation, sun exposure and airtightness change the load, which we establish with a room-by-room load calculation. In an ADD-ON, the furnace's electric elements cover the deep cold, which lets us size the heat pump on cooling and avoid the short cycles of an oversized unit.

The HSH642GKA unit belongs to the HSH6 series, one of Arcoaire's Premium series. The coil is the same as in the other series of this size; the outdoor unit makes the difference, and the ratings of the pairing are confirmed before the quote.

Winter with two heat sources

In cool weather, the heat pump heats alone, and air circulates through the ducts for longer, at a milder temperature than with the furnace's electric elements. When the cold deepens and the heat pump can no longer keep up, the thermostat brings those elements in as auxiliary heat; in deep cold, they carry most of the heating. The auxiliary heat threshold is set at commissioning according to the house and your preferences. During defrost cycles, the elements may come on briefly so that the air at the registers stays warm.

These longer cycles have a useful effect on distribution: air passes through the filter more often, and the temperature varies less from one room to the next.

LogisVert

This pairing does not carry an amount on Hydro-Québec's LogisVert list of October 19, 2026. If the grant matters for your project, we will point you to an eligible pairing of comparable size.

Installation on the existing furnace

The coil sits on the furnace's air outlet, and a compatible thermostat decides when the heat pump heats alone and when the electric furnace takes over. We check the condition of the furnace, the airflow of its blower and the duct sizes before the quote. The outdoor unit goes on a stand, above the snow line, on a dedicated circuit.

The insulated refrigerant line set connects the outdoor unit to the coil; it undergoes a nitrogen pressure test and a vacuum before the R-410A charge. At commissioning, we set the blower speed for cooling and heat pump heating, then check the air distribution at the main registers and the operation of the condensate drain.

Maintenance

Replace the system filter according to its format, keep the outdoor unit clear of snow and leaves, and plan annual HVAC maintenance of the coil, condensate drain and electrical connections. In winter, white vapour during defrost is normal.

Also keep the return grilles clear: furniture, a curtain or a rug blocking them reduces the airflow of the whole system. Dust the grilles from time to time, and leave the registers in unused rooms open, otherwise the duct system loses its balance.

AirGreen, for the life of the installation

A family business active for more than fifteen years in Montreal, Laval, Longueuil, the North Shore and the South Shore, we hold RBQ licences for ventilation, refrigeration and electrical work and carry $5,000,000 in liability insurance. The in-home assessment is free, the quote is in writing, and our financing options let you choose the right unit rather than the cheapest one.

The ventilation licence matters especially in an ADD-ON: work on the plenum, the returns and the ducts is part of the same job as the heat pump.

Request your free estimate for the Central Heat Pump ADD-ON ARCOAIRE HSH6 Premium | 3.5 Tons - 42,000 BTU (Coil EVD4X 17.5 in.) - R410A. We assess the home room by room and confirm the right size and your grant eligibility before signing.

Available while supplies last.

Warranty: 10 years on the compressor, 10 years on parts and 6 years on labor.