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Replace the furnace, or add a heat pump to it?
Your house already has a furnace and ducts: two paths lead to a central heat pump. You replace the furnace with a new indoor unit, with its own coil and backup elements. Or you keep the furnace and set a coil on top of it: that is the ADD-ON, and the furnace becomes the backup. To keep it concrete, this guide follows the MIDEA MO1SE-H36B-2A outdoor unit in both setups, each with its own AHRI certificate.
EVOX G3 · 36,000 BTU
MAUSE-H36B-2A / MO1SE-H36B-2A · AHRI 215471343
See the product page
EVOX G3 ADD-ON · 36,000 BTU
MAAHE-H36B-BA / MO1SE-H36B-2A · AHRI 216623524
See the product pageWhat the electricity costs, in dollars, winter and summer
AirGreen estimate for the same 36,000 BTU/h (3-ton) house in Montréal, on Hydro-Québec Rate D, taxes included. Both setups are calculated exactly the same way, with the same method as on all our comparison pages; only their certified efficiencies change. For the ADD-ON, the calculation counts the heat pump and the electric backup provided for in the HSPF2 standard: the switch to the furnace and its fuel are not modelled. Electric baseboards serve as the reference.
How this amount is calculated
- Heat to supply over the winter. A house whose heat loss reaches 36,000 BTU/h ( kW) at −23.1 °C, the Montréal design temperature: kW × 24 h × 4,167 degree-days ÷ (18 − (−23.1)) = kWh of heat for round-the-clock heating, all winter. Heating degree-days (18 °C base) and design temperature: Natural Resources Canada, HOT2000 climate data, Montréal Intl station. With fewer hours a day (“Heating” slider), the heat is reduced in the same proportion, for the heat pump as for the baseboards: this simplification ignores the recovery after a temperature setback.
- Heating electricity. Heat pump: heat × 3.412 ÷ HSPF2 Region V, the AHRI certificate value published by NEEP. Region V is the coldest in the standard, and its calculation includes the electric backup heat expected when the heat pump is no longer enough. Electric baseboards: 1 kWh of electricity for 1 kWh of heat.
- Cooling electricity. 36,000 BTU/h ÷ SEER2 × hours a day × 60 days ÷ 1,000: the method of Hydro-Québec’s air-conditioning calculator, with the seasonal SEER2 efficiency from the AHRI certificate. The number of hours is a scenario, adjustable above from 1 to 24 h. At 24 h, the calculation assumes rated capacity all day long: that is a ceiling, since a variable-speed unit slows down once the room reaches its set temperature.
- Price. Hydro-Québec Rate D in effect April 1, 2026: 7.065¢ per kWh up to 40 kWh a day, 11.142¢ beyond; the average price, 9.104¢, is the one Hydro-Québec’s calculators use. GST and QST included. Grid access charges, the same with or without a heat pump, are not counted.
AirGreen method v1.1, November 9, 2026. An estimate is a way to compare units under the same conditions. The actual bill depends on the house, its insulation, the settings and how it is used; this amount is not a promise. On Rate DT, the bill follows a different price grid, not calculated here.
The same basement, today and after the work
On the left, the furnace goes and a new indoor unit takes its place. On the right, it stays and gets a coil. Pick your furnace, then bring the outdoor temperature down: each basement shows which one heats the house, and what the certified heat pump can deliver at that temperature.
Illustrated cross-section: the basement, the ducts, the furnace and the MAUSE-H36B-2A indoor unit are diagrams (no official photo of this unit alone was found). The coil and the outdoor unit are the real photos; everything is at the same scale (widths and heights published by MIDEA; a 21 × 48 in furnace, as an example). The ADD-ON switchover with an electric furnace follows the EVOX coil service manual: S3 threshold adjustable from −30 to 8 °C, compressor locked out below the threshold, allowed again from the threshold + 2 °C; the manual gives no default value, the installer makes the choice. For the complete replacement, the electric elements are the indoor unit’s backup heat kit (up to 25 kW per MIDEA).
Your current installation, in four questions
The furnace type, its condition and the space above it weigh heavily on this choice. Tap the answer that matches your basement: you will see what it changes for each of the two paths. Neither path is ranked; what we measure on site counts for more than these four answers.
What stays, what gets added
The heat pump outside is the same in both examples. The difference lies in what goes with it in the basement, and in what takes over in extreme cold.
| For the homeowner | Complete replacement (example) | ADD-ON (example) |
|---|---|---|
| What stays in place | The ducts, if their condition allows | The furnace, its ducts, and its vent or chimney where applicable |
| What gets added in the basement | The MAUSE-H36B-2A indoor unit, in place of the furnace | The MAAHE-H36B-BA coil, set on the furnace |
| Backup heat in extreme cold | The indoor unit’s electric elements (kit up to 25 kW) | The existing furnace, electric or fuel-fired |
| Heat pump and backup at the same time | The elements supplement the heat pump when needed | No: with the coil’s factory setting, one source at a time; on Rate DT, never at the same time |
| Hydro-Québec Rate DT | Not covered: the house goes all-electric | Possible with a gas, oil, propane or pellet furnace, under conditions |
| The day the furnace gives out | Already settled: it has been replaced | It will need replacing; with oil, not by a fossil-fuel appliance |
| The control | 120N (X6W) wall control with Wi-Fi, or a 24 V thermostat from another brand | 120N (X6W) wall control with Wi-Fi, or a 24 V thermostat from another brand |
Sources: MIDEA submittal sheets for both combinations; EVOX coil service manual (SW2-4 switch: “only one” source at a time, default setting); Hydro-Québec, “Rate DT: how it works”; Government of Québec, news release of November 17, 2021 on heating oil.
What frames the ADD-ON, in figures
These benchmarks come from Hydro-Québec and the Government of Québec. They don’t say how much you will pay: they say what is allowed and how the system switches over.
Below −15 °C: the two setups, at the same temperatures
NEEP measures both combinations at the same temperatures, down to −30 °C (−22 °F). NEEP publishes nothing between −15 °C and −30 °C: at −20 and −25 °C, the table gives the range from the −15 °C value to the −30 °C value.
A new 58 in unit, or a 23 ½ in coil on the furnace
The units of both examples, at the same scale
Heights and widths published by MIDEA. Grid: 12 in. The MAUSE-H36B-2A indoor unit is drawn (diagram), the others are photos. With the ADD-ON, the coil adds to the height of the existing furnace: there has to be room up to the main duct.
| Installation | Complete replacement (example) | ADD-ON (example) |
|---|---|---|
| Maximum piping length | 246 ft | 246 ft |
| Precharged length | 25 ft+ 0.7 oz per foot beyond | 25 ft+ 0.7 oz per foot beyond |
| Maximum height difference | 98 ft | 98 ft |
| Outdoor unit W × D × H, weight | 38 9/16 × 16 3/8 × 38 3/8 in, 204 lb | 38 9/16 × 16 3/8 × 38 3/8 in, 204 lb |
| Indoor unit, weight | MAUSE unit 17 1/2 × 21 1/2 × 58 1/8 in, 149 lb | MAAHE coil 17 1/2 × 21 × 23 1/2 in, 59.5 lbon the existing furnace |
| Electrical protectionPer the manufacturer | Outdoor unit: MCA 29 A, MOP 30 A; indoor unit: MCA 6 A, MOP 15 A, without the backup kit | Outdoor unit: MCA 29 A, MOP 30 A; the furnace keeps its own supply |
| Stated heating / cooling range | −30 to 24 °C / −30 to 50 °C | −30 to 24 °C / −30 to 50 °C |
Sources: MIDEA, submittal sheets MAUSE-H36B-2A / MO1SE-H36B-2A and MAAHE-H36B-BA / MO1SE-H36B-2A; EVOX Extreme Heat installation manual, rev. 5 (Oct. 23, 2025) for the maximum length of the complete replacement. The outdoor unit’s sound level is printed as 63 dB(A) on the first sheet and 64 dB(A) on the second.
Before you choose
Who decides the switch to fuel?
On Rate DT, an automatic switching device does, connected to an outdoor temperature probe that Hydro-Québec supplies and installs, usually built into the meter. Hydro-Québec advises against manual switching, unless one of the two sources is faulty.
Can I keep an oil furnace with an ADD-ON?
An existing oil furnace can serve as backup, but the regulation sets limits. According to the Government of Québec news release, since December 31, 2023 it has been prohibited to install an oil appliance in an existing building or to replace one with a fossil-fuel appliance. The regulation will also prohibit repairing oil heating appliances more than 20 years old; the news release does not say from when. Check your furnace’s age before choosing this path.
Must the fuel be able to heat the house on its own?
On Rate DT, yes: Hydro-Québec requires the fuel to supply the needed heat on its own in extreme cold, and does not allow both sources to be used at the same time.
Why do the certified efficiencies differ, with the same outdoor unit?
Because an AHRI certificate covers a complete combination: the outdoor unit and the indoor unit tested together. On one side, the MAUSE-H36B-2A indoor unit; on the other, the MAAHE-H36B-BA coil. The values of one do not carry over to the other.
Are both eligible for rebates?
NEEP lists both combinations as cold-climate heat pumps. The complete combination also carries the ENERGY STAR designation; the ADD-ON combination does not carry it in the NEEP database consulted. Eligibility depends on the official list in effect on the purchase date and on the combination installed; this page promises no amount.
Are my ducts suitable?
In both cases, the existing ducts can be reused if their size and condition allow. With the ADD-ON, there also has to be enough height between the top of the furnace and the main duct for the coil (23 ½ in for the example). These points are checked during the estimator’s free visit.
The AHRI and NEEP values of both examples
| Certified value | EVOX G3 (complete replacement) | EVOX G3 ADD-ON |
|---|---|---|
| Cooling | ||
| Rated capacity at 35 °C | 36,000 BTU/h | 36,000 BTU/h |
| SEER2 | 18.0 | 15.8 |
| EER2 | 12.0 | 10.7 |
| Heating, same conditions | ||
| HSPF2 Region IV / V | 10.0 / 8.2 | 9.4 / 7.9 |
| 8.3 °C: min. / rated / max.BTU/h | 11,400 / 37,000 / 47,000 | 11,900 / 38,000 / 46,000 |
| 8.3 °C: COP at min., rated, max. | 4.45 / 3.50 / 3.27 | 4.25 / 3.30 / 2.97 |
| −8.3 °C: min. / rated / max.BTU/h | 11,000 / 33,000 / 41,500 | 9,300 / 29,200 / 39,000 |
| −8.3 °C: COP at min., rated, max. | 2.58 / 2.50 / 2.21 | 2.60 / 2.50 / 2.18 |
| −15 °C: min. / rated / max.BTU/h | 8,900 / 37,400 / 37,400 | 10,400 / 36,000 / 36,000 |
| −15 °C: COP at min., rated, max. | 2.16 / 1.90 / 1.90 | 1.77 / 1.90 / 1.90 |
| −30 °C: capacity min. / max.BTU/h | 4,700 / 27,600 | 5,500 / 28,600 |
| −30 °C: COP at min. and max. | 1.28 / 1.56 | 1.25 / 1.61 |
| Listings and refrigerant | ||
| ENERGY STAR / cold climate (NEEP) | Yes / Yes | Not listed / Yes |
| AHRI certificate · NEEP record | 215471343 · 387396 | 216623524 · 381863 |
| RefrigerantGWP per the IPCC (AR4); ASHRAE 34 class | R-454B · GWP 466 · A2L | R-454B · GWP 466 · A2L |
higher marks the higher value of a given measure, under the same conditions. It is not a recommendation. Capacities and COPs at each test point are not highlighted: which one is higher changes from point to point.
Both setups are on the AirGreen website.
Replace or add: we start with your furnace. Bring its type and age, an idea of the space above it, and your electrical service.
Where each figure comes from
Data checked on November 9, 2026. This guide explains two ways to install a central heat pump; it designates no option and no brand. The models cited serve as examples, sold by AirGreen. Performance: AHRI certificates and the NEEP database. Dimensions, piping, ranges and switchover logic: per the manufacturer. Rate DT and heating-oil rules: official pages of Hydro-Québec and the Government of Québec. When a source contradicts the AHRI certificate, the certificate prevails.
Trademarks. MIDEA and EVOX are trademarks of their respective owners. Logos are used only to identify the products shown. AirGreen is a dealer for the brand; this page is not produced, sponsored or endorsed by the manufacturer.
Photos: AirGreen and the manufacturer, for identification.
Does a figure look wrong to you? Write to us: every reported figure is checked again.