Commercial HVAC Retrofit & Rooftop Unit Replacement in Ontario

Replacing a commercial rooftop unit is rarely just a purchase order and a crane date. Capacity, ventilation, roof curbs, structure, electrical supply, controls, permits and downtime all meet at one piece of equipment—and each interface can change the project.

When an RTU Replacement Becomes an Engineering Project

A failed or end-of-life rooftop unit creates pressure to move quickly. The temptation is to match the old nameplate, request a supplier quote and schedule a crane. That approach may work only when the existing design basis is understood and every important interface remains compatible.

A replacement becomes an engineering retrofit when the team must decide what the building needs—not simply what was installed before. Common triggers include persistent comfort complaints, occupancy changes, added exhaust, a different fuel or heating source, revised ventilation, altered ductwork, a new curb adapter, increased weight, changed electrical characteristics, a controls upgrade, or missing record drawings.

For building owners and property managers, the practical objective is continuity: the new system should fit, start, pass inspection, maintain required ventilation and deliver the intended conditions without creating a second project after installation.

The Like-for-Like Test

“Same tonnage” is not a complete replacement criterion. Nominal cooling capacity is only one line in a much larger comparison. Two units with the same catalogue tonnage can deliver different sensible capacity, moisture removal, heating output, outdoor air, fan performance and electrical requirements at project conditions.

A credible like-for-like review compares the complete operating and installation requirements—not only manufacturer, age or nominal tonnage.

Ontario Permit and Professional Review Context

Permit requirements should be confirmed with the authority having jurisdiction for the actual address and scope. A commercial replacement may be processed as stand-alone mechanical work or as part of a broader building permit. Other approvals can apply where work affects structure, gas, electrical systems, fire alarm interfaces, zoning, heritage or other regulated elements.

The City of Toronto's current Stand-alone Mechanical (HVAC) guide, for example, identifies mechanical system layouts, equipment type/location/size, heat-loss and heat-gain calculations, duct design calculations and ventilation calculations among the required documentation. Other municipalities administer their own forms, checklists and intake processes.

Where professional design or general review is required, the project team should establish those responsibilities at the beginning. General review is a construction-phase process related to general conformity with the documents that formed the basis of permit; it is not a substitute for developing the design. See our broader guide to mechanical permit drawings in Ontario.

Existing-Condition Assessment: The Highest-Value First Step

A retrofit design starts by establishing what exists and what has changed since the original installation. Old drawings are useful, but field conditions and current operation govern.

If the existing unit failed prematurely, replacing it without investigating the cause can repeat the failure. Airside restrictions, dirty coils, poor control sequences, inadequate combustion, unstable power, refrigerant issues, short cycling or an incorrect original selection should be considered where evidence points in that direction.

Loads, Airflow and Ventilation

Existing equipment capacity is a useful reference, not proof of the current building load. Envelope changes, tenant density, plug loads, operating hours, exhaust systems and space use may have changed. Conversely, an older unit may have been oversized from the beginning.

A project-specific heating and cooling load calculation helps establish the required capacity and zoning. The selection then checks sensible and latent performance, fan duty, ventilation, part-load behaviour and manufacturer data at project conditions.

Outdoor air is a system requirement

Ventilation should not be assumed from the old damper position. The design basis should consider current occupancy, applicable code and referenced standards, exhaust replacement, pressurization and how outdoor air is controlled at full and part load. If the new unit uses a different fan or control strategy, minimum outdoor-air performance may need a new sequence and balancing approach.

An economizer, demand-controlled ventilation, heat recovery or dedicated outdoor-air system can materially change both energy and control requirements. Those features should be selected because they fit the project—not simply because they appear in a standard equipment option list.

Roof, Electrical and Controls Coordination

Roof, curb and structure

The new unit may require a curb adapter, revised openings, rails, dunnage or a new support arrangement. Weight and centre of gravity can change. Snow accumulation, drainage, roofing warranty, service access and fall-protection conditions also affect the installation. Structural review should be obtained where the change or available information warrants it.

Electrical service and equipment

Coordinate the final mechanical selection with the electrical design before release. Voltage and phase are only the start: verify MCA, MOCP, disconnecting means, feeder and protection, electric heat stages, controls power, service capacity and any standby-power requirements. Our HVAC electrical coordination guide explains the common interfaces in more detail.

Controls, life safety and commissioning

Document the required operating sequence. Confirm occupied and unoccupied modes, heating/cooling stages, economizer control, freeze protection, condensate safeties, smoke shutdown, fire alarm interface, alarms, BAS points and restart behaviour. Commissioning should verify operation—not merely that the fan and compressors start.

What a Retrofit Design Package Should Include

The required deliverables depend on project stage and permit path. A coordinated commercial RTU replacement package commonly addresses:

• Existing and proposed equipment information, locations and areas served.
• Basis of design, load summary and ventilation assumptions.
• Roof plan, curb/opening requirements and applicable structural coordination notes.
• Duct transitions, airflow, dampers, accessories and condensate requirements.
• Equipment schedule with project-specific performance and electrical data.
• Gas, hydronic or other utility interfaces where applicable.
• Controls sequence, BAS points and life-safety interlocks.
• Electrical feeder, disconnect, protection and control-power coordination.
• Permit forms, supporting calculations and professional documentation appropriate to the scope.
• Construction notes, equipment substitution criteria and commissioning requirements.

A permit set and a tender/construction package do not always serve the same purpose. If the owner expects competitive pricing and fewer assumptions, the documents should define the construction scope, interfaces and acceptable substitutions—not only the minimum information needed for review.

A Lower-Risk RTU Replacement Workflow

1. Define the problem and operating constraints. Confirm failure urgency, areas served, comfort issues, business hours, downtime limits, budget stage and desired construction window.
2. Survey the existing installation. Gather nameplates, field dimensions, roof/curb conditions, power, controls, ductwork, service history and available drawings.
3. Confirm the design and permit basis. Establish loads, ventilation, applicable approvals, professional responsibilities and any structural or electrical studies.
4. Select around real interfaces. Compare performance, airflow, physical fit, weight, power, fuel, controls, serviceability and procurement constraints.
5. Issue coordinated documents. Align mechanical, structural, electrical, controls, roofing and crane requirements for permit, pricing and construction.
6. Review submittals before release. Verify the proposed unit and accessories against the design, and resolve substitutions before fabrication or shipment.
7. Plan the outage and commissioning. Sequence isolation, removal, curb/roof work, lifting, connections, start-up, balancing, controls verification and closeout.

What to send an engineer for an initial review

A useful first package can be simple: building address and use, photos of the existing unit and nameplate, the areas it serves, available mechanical/roof/electrical drawings, known comfort or maintenance issues, proposed replacement information and the desired schedule. ETEM can use that information to identify the missing inputs and define an appropriate design scope.

Technical and Regulatory References

1. City of Toronto — Stand-alone Mechanical (HVAC)Current municipal guide identifying submission documentation for stand-alone mechanical permit work, including system layouts, equipment information and supporting calculations.
2. Government of Ontario — Ontario's Building CodeOfficial access point for the current Ontario Building Code and compendium information.
3. Building Code Act, 1992Provincial statute establishing Ontario's building-permit and code framework.
4. Professional Engineers Ontario — General Review of Construction GuidelineProfessional guidance on general review and the distinction between design responsibility and construction-phase review.
5. ASHRAE — Read-Only StandardsOfficial access to read-only versions of standards relevant to ventilation, thermal conditions, load calculations and energy performance.

Frequently Asked Questions