HVAC for a legal basement apartment in Ontario has three jobs: heat the suite, keep smoke and carbon monoxide from travelling between the two units, and pass the building permit inspection. In a house more than five years old, the Building Code lets one furnace serve both units if a duct-type smoke detector shuts the furnace down when it senses smoke. Our recommendation is to give the suite its own system, usually a ductless heat pump at $4,000–$7,000 installed, and leave the furnace to the main house.
Can one furnace heat both units? The existing-house rule
Yes, in an existing house, with a detector in the ductwork. Ontario's second-unit guide states that a house with a second unit may have a single furnace and a common system of air ducts, provided a special smoke detector is installed in the main supply or return duct. When it activates, it turns off the fuel supply and the electrical power to the furnace, so the blower stops pushing smoke from one unit into the other. The detector must meet the UL 268A standard.
Existing means more than five years old. A newer house is treated as new construction, where the Code's starting point is that air from one dwelling unit is not circulated into another, so plan on separate systems. The provincial guide was written to the 2012 Building Code and a new edition took effect on January 1, 2025, which means the plans examiner at your building department has the final word on the clause that applies.
Legal and comfortable are different tests. A shared furnace passes the first and often fails the second.
- One thermostat, normally upstairs, controls both households.
- Cooking smells, voices and television noise travel through the common ducts.
- Every duct that crosses the ceiling between the units is an opening in the fire separation.
- Heat cannot be metered or billed separately.
Fire separation around the furnace room and the ducts
The Code requires a 30-minute fire separation between the two units, and between each unit and any common area. The provincial guide gives one way to build it: 38 mm by 89 mm wood studs, 13 mm drywall on both sides and fibre insulation between the studs. Mechanical work touches that separation in four places.
- The furnace room: a furnace that serves both units sits in a space neither household controls alone. Expect the examiner to ask for it to be enclosed with rated walls, ceiling and door, and decide who has access to change the filter.
- Ducts through the rated ceiling: each supply and return that passes through is a penetration to seal and show on the permit drawings. Use steel duct and boots, and let the examiner rule on whether dampers are needed.
- Return air: do not cut a return grille into the furnace room wall. It defeats the enclosure and can pull flue gases from a natural-draft water heater into the ducts.
- Ceiling height: the guide lists 1.95 m as the ceiling height for a basement second unit, so every new bulkhead has to be planned against it.
Smoke and carbon monoxide alarms the second unit needs
Smoke alarms go on every level of the house, outside sleeping areas, in each bedroom of the second unit and in common areas shared by both households. Carbon monoxide alarms are required when the house has a fuel-burning appliance such as a gas furnace or water heater, or an attached garage. Since January 1, 2026 the Ontario Fire Code calls for a CO alarm on every storey, not only outside the bedrooms. Our guide to the carbon monoxide detector law in Ontario sets out the placement rules under the Hawkins-Gignac Act.
Two related items sit on the same permit. The suite's bathroom and kitchen each need ventilation, by an exhaust fan ducted outside or a window that opens. The electrical work in the second unit needs its own permit and an ESA inspection.
Independent options: a system the tenant controls
Free quotes, written pricing.
A separate system removes the shared air, gives the tenant a thermostat and lets you bill utilities by unit. These are the options, in the order we recommend them.
- Ductless cold-climate heat pump: heating and cooling from one wall head with its own control. A single zone runs $4,000–$7,000 installed, a cold-climate model adds roughly $500–$1,500, and the refrigerant lines leave through the rim joist, so nothing crosses the rated ceiling.
- Separate gas furnace with its own ducts: the right answer when the ceiling is open for renovation and the layout has several closed rooms that one wall head cannot reach. It needs a mechanical space, sidewall venting, a gas line and room for bulkheads.
- Electric baseboard: low cost to install and high cost to run, and it needs spare capacity in the panel. It works well as a bedroom supplement to a ductless head.
- Bedrooms with closed doors are the weak point of a single ductless head, so plan a small baseboard or a second head for them.
Our ductless mini-split installation page explains sizing and where the outdoor unit can go on a lot that now has two households using the yard.
When the suite leaves the furnace: cap the runs and rebalance
Taking the basement off the furnace changes the duct system upstairs. The basement supply runs are capped at the trunk, the basement return is closed, and the furnace now moves its air through fewer openings. Static pressure rises, so we measure it and add or enlarge returns on the main floor where the numbers call for it.
That is ductwork installation work, and it has to happen before the suite ceiling is drywalled. A furnace sized for the whole house may also be larger than the upper floors need alone, which shows up as short cycles. A two-stage furnace hides that; a single-stage one may need its blower speed and gas input checked.
Hot water for two households
Two households on one tank is the complaint landlords hear first. Two kitchens, two showers at 7 a.m. and often a second laundry empty a tank that was sized for one family.
- A larger or faster-recovering tank: a straight tank swap runs $1,200–$2,500 installed.
- A tankless water heater: $3,500–$6,500 installed, with no storage to run out, sized for the showers and taps that run at once.
- A separate electric tank for the suite: needs panel capacity, and lets a separately metered tenant pay for their own hot water.
- A rented water heater: read the contract before changing anything, because the tank belongs to the rental company.
Verdict: separate the systems unless the budget forbids it
- Choose a ductless heat pump for the suite if you are renovating anyway, want the tenant to control and pay for their own heat, or the house is under five years old.
- Choose the shared furnace with a duct smoke detector if the house is over five years old, the examiner accepts it, the basement ducts already exist and the budget is tight. Accept the thermostat compromise that comes with it.
- Choose a second furnace if the suite has three or more closed rooms and the ceiling is open.
- In every case, talk to the building department before buying equipment: the City of Burlington, Town of Oakville, Town of Milton, City of Hamilton, City of Mississauga and City of Brampton each review second-unit permits.
We design and install the mechanical side of second units and work from the permit drawings, not around them. For the short version of the shared-furnace question, read does a basement apartment need its own furnace; for larger conversions, see separate HVAC systems for duplexes and triplexes. To have the house looked at before you apply, book a site visit.
Frequently asked questions
Is a duct smoke detector the same as a regular smoke alarm?
No. A duct-type smoke detector is mounted in the main supply or return duct and wired to the furnace, so it cuts the fuel supply and electrical power when it senses smoke in the airstream. It does not sound an alarm in the rooms and does not replace the smoke alarms required in each unit. It should be tested at each furnace service.
Should the tenant have their own thermostat in a basement apartment?
For comfort and fewer disputes, yes. A basement runs cooler than the main floor in winter and needs almost no cooling in summer, so a single upstairs thermostat leaves the suite cold or the upstairs overheated. A separate system gives the tenant control. The landlord stays responsible for the minimum temperature set by the province or the municipal bylaw.
Can I reuse the existing basement ducts if I add a second furnace?
Sometimes. The existing branches are tied to the main trunk that feeds the upper floors, so they have to be cut away and connected to the new furnace's own plenum, with a dedicated return added for the suite. Whether that is worth doing depends on duct sizes and how much ceiling is open. A site visit answers it.
Who pays for heat when one furnace serves both units?
The gas and electricity account holder, which is normally the owner, because a shared furnace cannot be metered by unit. Most landlords in that situation include heat in the rent and price it accordingly. A separately metered suite with its own heat pump and electric water heater is the arrangement that lets a tenant pay their own utilities.