Central Heat Pump ADD-ON ARCOAIRE HSH4 Premium | 3.5 Tons - 42,000 BTU (Coil EVD4X 17.5 in.) - R410A
Two heat sources, one thermostat that picks the right one
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 is only used as backup, through its electric elements. The Central Heat Pump ADD-ON ARCOAIRE HSH4 Premium | 3.5 Tons - 42,000 BTU (Coil EVD4X 17.5 in.) - R410A pairs the HSH442GKA outdoor unit with the EVD4X42M17A coil.
An ADD-ON system is only as good as the way its two heat sources share the winter. This page therefore focuses on what happens between the heat pump and the furnace's electric elements: which one heats, when, and how we set that decision for your home.
Why keep the furnace
A central heat pump heats very efficiently in mild and cool weather, but its capacity drops as the outdoor air gets colder, while the needs of the house rise. The existing electric furnace fills that gap with its heating elements. It is already installed, already connected to the ducts, and it remains available for the days when the cold exceeds what the heat pump can supply alone.
This sharing also lets us size the heat pump on the cooling load rather than on the coldest day of the year. The unit stays well suited to summer, whereas an oversized unit would cool in short cycles and remove humidity poorly.
The thermostat, the brain of the system
The heat pump thermostat replaces the furnace thermostat. It controls the heat pump, the furnace's electric elements and the blower. Usually connected to an outdoor sensor, it knows the outside temperature and decides, at each call for heat, which source to start. In summer, it controls the cooling with the same device.
The furnace's elements become auxiliary heat: the thermostat brings them in when the heat pump can no longer hold the set point, and leaves them off the rest of the time. With an outdoor sensor, it can also stop the heat pump below a chosen temperature and let the elements heat alone. Both settings are set according to your house.
Your home's balance point
Every house has its balance point: the outdoor temperature at which the heat supplied by the heat pump equals the heat the house loses. Above that point, the heat pump is enough. Below it, the furnace's elements have to add their heat. A well-insulated house has a lower balance point than a house that loses a lot of heat, because it needs less energy at the same outdoor temperature.
This point appears on no spec sheet: it depends on your house as much as on the equipment. We estimate it from the load calculation and the confirmed ratings of the pairing, then use it to set the thermostat.
Comfort or consumption: two ways to set auxiliary heat
The thermostat setting reflects your priorities. If you want the heat pump to heat as long as possible, we set auxiliary heat close to the balance point: the furnace's elements only step in when the heat pump truly can no longer keep up. If you prefer warmer air at the registers from the first real cold days, we let auxiliary heat come in earlier.
With an electric furnace, the first setting is almost always the more frugal one: the elements turn electricity into heat without capturing anything outdoors, whereas the heat pump moves heat from the outdoor air. We explain the effect of each setting during commissioning, and you can change it later as you get to know the system.
Defrost, without a cold draft
In cold, humid weather, frost forms on the outdoor unit's coil. The heat pump clears it with short defrost cycles, during which a plume of white vapour comes out of the unit; this is normal. During these cycles, the control can briefly run the furnace's elements so that the air coming out of the registers stays warm. We check this behaviour at commissioning.
What the heat pump changes for the electric furnace
In a home heated by an electric furnace, the heating elements used to provide all of the heating. With the heat pump, they become a supplement: the heat pump covers the larger part of the season, and the elements are only used when the cold deepens or during defrost. The furnace runs fewer hours, but it must stay in good condition, ready to take over.
On the electrical panel, nothing is removed: the furnace's elements stay connected for the coldest nights, and the outdoor unit is added on its own circuit. That is why the panel's capacity is part of the visit, just like the furnace and the ducts.
The 3.5-ton size and the 17.5 in. coil
The 3.5-ton size often serves a large house on two levels, a two-storey home with a finished basement or a bungalow with a lot of glazing in Longueuil or on the North Shore. These examples are only a guide: insulation, window age and airtightness change the actual load, and a room-by-room load calculation settles it before the quote.
The EVD4X42M17A coil is a 17.5 in. format; it has to match the width of the furnace cabinet, with a sheet-metal transition when needed. In summer, the same coil cools and dries the air of the whole house, and condensate drains to the floor drain or a pump. The name Premium indicates the rank of the HSH442GKA unit in the Arcoaire lineup; it does not replace the ratings of the pairing.
System ratings
The pairing of the HSH442GKA 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.
These ratings are also used to estimate the house's balance point and to set the thermostat's auxiliary heat. That is one more reason to confirm them in writing rather than rely on an approximate value; this page therefore shows none.
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 furnace's elements take 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.
Control wiring and commissioning
A heat pump thermostat needs more conductors than an ordinary furnace thermostat. The existing wire is sometimes enough; otherwise, we run a new cable between the thermostat and the furnace. A second cable connects the furnace to the outdoor unit, and the outdoor sensor, when one is planned, is installed out of direct sun and away from the air discharged by the unit.
On the power side, the outdoor unit is fed by a dedicated circuit and an outdoor disconnect; we check before the quote that the panel can carry the unit in addition to the furnace's elements. The unit sits on a stand above the snow line, away from roof drip and bedroom windows.
At commissioning, we perform the nitrogen pressure test and the vacuum on the refrigerant line set, adjust the R-410A charge and the airflow, then test each mode: cooling, heat pump heating, auxiliary heat from the furnace's elements and defrost.
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 outdoor sensor clear: a sensor buried in snow or exposed to a warm air outlet distorts the thermostat's decision. Also have the electric furnace, its elements and its blower checked during the annual maintenance.
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.
Request your free estimate for the Central Heat Pump ADD-ON ARCOAIRE HSH4 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.