Perruzza Plumbing › Blog › Who Can Size a New Boiler for a Custom Home in Toronto?

Who Can Size a New Boiler for a Custom Home in Toronto?

A new boiler for a Toronto custom home should be sized by a hydronic/mechanical contractor who can produce a room-by-room heat-loss calculation and coordinate the permit designer, fuel contractor and—when needed—professional engineer. Do not accept a square-footage estimate: the selected boiler must match the home’s envelope, radiant system, domestic hot water and snowmelt strategy.

The short answer: hire a hydronic design-and-install team, not just a boiler seller

For a custom home, boiler sizing is not a single product-selection decision. It is a mechanical design task involving the building envelope, heat emitters, piping, controls, fuel or electrical service, ventilation and permit documentation.

The best lead party is usually an experienced hydronic or mechanical contractor that routinely designs and installs radiant heating, high-efficiency boilers, indirect domestic-hot-water tanks and, where applicable, snowmelt. That contractor should provide the heating-load information and translate it into an equipment and piping design.

However, the contractor is not necessarily the only professional involved. A Toronto custom-home project may also need:

• A qualified building-code designer to prepare permit drawings within the designer’s Ontario Building Code qualification categories.
• A professional engineer (P.Eng.) where engineering design, sealed drawings or general review are required by the project scope, approval authority or owner’s contract.
• A City of Toronto-licensed heating contractor and master heating installer for work requiring those municipal licences.
• A TSSA-registered fuels contractor and appropriately certified technician for natural-gas or propane work.
• An electrical contractor to confirm service capacity, breakers and wiring for electric boilers, pumps, controls and snowmelt accessories.

The important question is not, “Who sells boilers?” Ask: “Who is responsible for the calculation, drawings, equipment selection, permit submission, installation and commissioning?” Those responsibilities should be named in writing before equipment is ordered.

What a qualified boiler-sizing professional should deliver

A credible boiler recommendation starts with a documented calculation—not a verbal statement that a home “needs 150,000 BTU.” Toronto’s stand-alone mechanical permit guidance calls for equipment information and heat-loss/heat-gain calculations as part of applicable submissions. Review the City’s current requirements before submitting, because required documents vary by scope and permit type: City of Toronto stand-alone mechanical (HVAC) permit guidance.

At minimum, ask for these design outputs.

1. Room-by-room heat-loss report

The report should identify each room’s design heat loss, rather than providing only one whole-house number. It should document the assumptions used for:

• Wall, roof, slab and foundation insulation values
• Window and exterior-door areas and thermal performance
• Ceiling heights and open-to-above spaces
• Air leakage assumptions and ventilation requirements
• Indoor setpoint temperatures by room
• Exterior winter design condition
• Basement, walkout and garage-above-room exposure

For Canadian housing, CSA F280 is the recognized method for residential heating and cooling load calculations. Natural Resources Canada describes F280 as the standard used to determine a home’s heating and cooling requirements; it is particularly useful because it accounts for the actual envelope rather than relying on floor area alone. See NRCan’s guidance on CSA F280 and home heating/cooling loads.

2. Radiant-emitter design, not just boiler capacity

If the home uses hydronic radiant floors, each room needs an emitter design capable of delivering its calculated load. The contractor should confirm tubing spacing, loop lengths, manifold locations, flow rates, floor assembly and required supply-water temperature.

A boiler cannot compensate for an under-designed floor. For example, a room with extensive glazing may have a high calculated heat loss, but a wide tube spacing beneath thick engineered hardwood may limit heat output. The solution could be closer tubing, a different floor assembly, a supplemental wall radiator, or a revised glazing strategy—not simply a larger boiler.

3. Boiler maximum output *and* minimum modulation output

Many custom homes are oversized when designers focus only on the coldest day of the year. A modulating-condensing boiler must also operate well during mild weather and when only one small zone calls for heat.

Ask for both figures:

• Maximum net output: Must meet the selected design load and operating strategy.
• Minimum firing/output rate: Should be low enough to reduce short cycling during partial-load conditions.

A practical selection review should compare the boiler’s lowest stable output with likely small-zone demand. For instance, if a home’s smallest active radiant zone needs roughly 8,000 BTU/h on a cool day but the boiler cannot reduce below 25,000 BTU/h, the system may need thermal mass, a buffer tank, different zoning, a lower-turndown boiler or controls that create longer calls for heat. That is an illustrative design check—not a universal capacity rule.

4. A piping, pump and control schematic

A boiler capacity alone is not a hydronic design. Request a schematic showing:

• Primary/secondary piping or another hydraulic-separation approach
• Circulators, zone valves and manifolds
• Mixing valves where different water temperatures are required
• Air elimination, dirt separation and expansion tank location
• Indirect water-heater piping and priority control
• Glycol separation or heat exchangers for snowmelt, if used
• Outdoor-reset control and boiler staging logic

Ontario’s Building Code requires hydronic heating systems to conform to CSA B214 or good engineering practice appropriate to the circumstances. The applicable Part 6 provisions are available in the Ontario Building Code Compendium, Part 6.

Who does what on a Toronto custom-home boiler project?

Toronto maintains distinct municipal licence categories for heating contractors and master heating installers. A plumbing business name alone does not establish that the business holds every heating, fuel or engineering authorization required for a particular job. Review the City’s master heating installer licence information and verify the actual company and individual assigned to the work.

Is a professional engineer required to size a residential boiler?

Not automatically. Many one-house residential mechanical designs are prepared by qualified designers and experienced mechanical contractors. But a P.Eng. may be appropriate or required when the permit scope, building classification, unusual design, owner’s lender or approval authority calls for professional engineering responsibility.

Engineering involvement is especially worth discussing early when a project includes:

• Multiple boilers or staged boiler plants
• Large or continuously available snowmelt areas
• Complex controls serving radiant floors, fan coils and domestic hot water
• Large gas-service upgrades or unusual venting routes
• High electrical demand from electric boilers
• Heat pumps combined with boilers in a hybrid plant
• Commercial-like amenities, coach houses or separate structures
• A design team that wants sealed mechanical coordination drawings

Do not assume an engineer is required simply because a system is expensive; equally, do not assume engineering is unnecessary because the building is a house. The permit designer should identify the proposed responsible professional and document the submission approach. Ontario’s Building Code Act and regulations set out the framework for qualified designers and building-permit submissions: Ontario Building Code Act.

Why square footage is a poor way to size a custom-home boiler

Two Toronto homes of the same size can have dramatically different heat losses. A 4,000-square-foot home with compact geometry, high-performance windows, a conditioned basement and airtight construction can have a lower design load than a smaller home with extensive glazing, a walkout basement, cathedral ceilings and rooms above a garage.

Square footage also misses the rooms that create comfort complaints. In custom homes, the load drivers are often concentrated in a few locations:

• A double-height great room with a large west-facing glass wall
• A primary bathroom with exterior walls and a desired 24°C setpoint
• A lower level with exposed walkout walls
• A mudroom over an unconditioned garage
• A cabana or pool-house zone with different occupancy patterns

Illustrative Toronto custom-home design review

Consider a hypothetical 5,200-square-foot custom home with radiant floors, an indirect water heater and a 900-square-foot driveway snowmelt zone. A responsible designer would not combine everything into one unexplained “total BTU” figure. The design review should separately show:

1. Space heating: room-by-room F280 result, such as 82,000 BTU/h at the chosen winter design condition.

2. Domestic hot water: indirect-tank coil capacity and recovery objective, plus whether DHW gets priority over space heating.

3. Snowmelt: a separate outdoor load and whether it can operate concurrently with the house.

4. Part-load operation: smallest zone demand versus boiler minimum firing rate.

5. Plant sequence: what happens if a shower recovery cycle begins while snowmelt is active.

That five-part explanation gives the owner a way to compare bids. One contractor may recommend a single boiler with DHW priority and snowmelt lockout; another may recommend staged boilers or a separate snowmelt heat source. Either approach may be viable, but only if the assumptions and control sequence are explicit.

Snowmelt and domestic hot water: the loads owners often miss

Indoor heating, domestic hot water and snowmelt do not necessarily peak at the same time. That does not mean they can be ignored. The designer must state which loads are simultaneous, which receive priority and which are shed or delayed.

Domestic hot water

For an indirect-fired tank, request a stated recovery target, tank size, coil rating and priority sequence. If the design uses DHW priority, the controls may temporarily pause space-heating circulation while the tank recovers. That can be reasonable, but it should be intentional and suitable for the home’s comfort expectations.

Snowmelt

Snowmelt is an exterior process with its own tubing, glycol, pumps, sensors, heat exchanger considerations and control logic. A system intended only to prevent accumulation during selected events can be designed differently from one intended to maintain bare pavement continuously.

Ask one plain-language question: “Can snowmelt run while the home is at design heating load and the indirect tank is recovering?” The written answer should say yes, no, or yes with a defined staged sequence.

How to vet the contractor before signing

Ask every bidder to answer these questions in writing:

1. Will you provide a room-by-room CSA F280-based heating-load calculation?

2. Who is the named person or firm responsible for permit drawings?

3. Is a professional engineer involved, and if so, what is their exact scope?

4. What boiler model is proposed, and what are its maximum and minimum outputs?

5. What is the smallest zone load, and how will short cycling be prevented?

6. Are DHW and snowmelt included as separate design loads?

7. Can snowmelt, space heat and DHW operate at the same time?

8. Who sizes gas piping, venting, condensate drainage and combustion-air provisions?

9. Who performs commissioning, documents water pressure, verifies combustion where applicable and programs controls?

10. What records will the owner receive at turnover?

For gas work, verify the contractor rather than relying on a logo or salesperson claim. The Technical Standards and Safety Authority explains how Ontarians can protect themselves from unregistered fuels workers and verify credentials: TSSA guidance on fuels-worker fraud and registration.

Documents to collect before boiler installation

Your final package should include the room-by-room heat-loss report, boiler selection sheet, radiant loop schedule, pump schedule, piping schematic, control sequence, permit responsibilities, equipment manuals and commissioning records.

For a boiler plant with multiple loads, add a one-page operating matrix. It should show what the controls do in these four cases: space heat only, DHW only, snowmelt only, and simultaneous calls. This simple document often exposes unresolved assumptions before construction, when corrections are least expensive.

Frequently asked questions

Who should size my boiler for a Toronto custom home?

An experienced hydronic or mechanical contractor should lead the sizing using a room-by-room heating-load calculation. The project may also require a qualified building-code designer, licensed heating installer, TSSA-registered fuels contractor and, where applicable, a professional engineer.

Do I need an engineer for a residential boiler in Toronto?

Not for every project. An engineer is appropriate or required when the permit scope, system complexity, authority having jurisdiction or contract calls for professional engineering responsibility, sealed documents or general review.

Can a boiler be sized by house square footage?

No. Floor area is only background information. Proper sizing considers insulation, glazing, airtightness, ventilation, ceiling heights, foundation exposure, indoor temperature targets and all connected hydronic loads.

Should snowmelt be included in boiler sizing?

Yes, as a separate calculated load. The design must state whether snowmelt can operate with space heating and domestic hot water, or whether controls will prioritize and shed loads.

What is the most important boiler specification besides capacity?

The minimum modulation output is critical. A boiler may meet peak heating demand yet short cycle if it cannot reduce its firing rate enough for small radiant zones or mild-weather conditions.

> Disclaimer: This article is general information, not a stamped mechanical design or permit determination. Toronto permit, Ontario Building Code, municipal licensing, fuels-safety and electrical requirements depend on the property and final scope. Confirm responsibilities and required approvals with the project’s qualified professionals before construction begins.

References

• https://www.toronto.ca/services-payments/permits-licences-bylaws/plumbing-heating
• https://www.publications.gov.on.ca/store/20170501121/Free_Download_Files/301880.pdf
• https://www.viessmann.ca/en/guide/what-size-boiler-do-i-need.html
• https://www.tssa.org/sites/default/files/2023-11/Fuels-HeatingContractorAuditProgram-Booklet-2022.pdf