Most rooftop units (RTUs) are replaced either in a hurry, after a failure on a hot day, or as part of a capital plan with time to think. The hurried replacement almost always means a like-for-like swap: same tonnage, same curb, same controls. That can be the right answer, but often it locks in problems that a little checking would have solved.
Replacement is a rare opportunity. The roof is open, a crane is on site, and the next chance to change anything may be 15 or 20 years away. Spending a few hours on the checks below, ideally before the old unit fails, pays off in comfort, energy and avoided surprises.
1. Confirm why the unit is being replaced
Start by understanding the old unit's history:
- Age and condition: compressors, heat exchanger, coils, cabinet and curb
- Service records: recurring failures, refrigerant leaks, cracked heat exchangers
- Complaints: hot or cold zones, humidity, noise, drafts
- Refrigerant type: R-22 units are increasingly expensive to service, and R-410A service costs are expected to rise as HFC supply declines
If the unit is failing early, find out why. Short cycling from oversizing, poor airflow, low voltage or bad drainage will affect the new unit too.
2. Check the load and the space use
Buildings change. Lighting has moved to LED, tenants have changed, walls have moved and windows may have been replaced. A unit sized 20 years ago may no longer match the space.
- Review current use and occupancy of the zones served.
- Do a load calculation, or at least a simplified check, for cooling and heating.
- Compare with the old unit's capacity and with any complaints. A unit that never kept up may be undersized; one that short cycles may be oversized.
- Confirm the ventilation requirement for the current use under ASHRAE 62.1 or the code-referenced standard.
Right-sizing often reduces capacity, which lowers first cost and improves humidity control.
3. Measure the existing airflow
Before the old unit is removed, measure its supply airflow, outdoor airflow and external static pressure. These numbers tell you:
- Whether the duct system can carry the design airflow for the new unit
- Whether the old fan was struggling against high static pressure
- What fan selection the new unit needs
Many RTUs operate well below design airflow because of undersized ducts, crushed flex duct or dirty coils. A new unit with the same fan will perform no better. An air balancing contractor can take these readings quickly.
4. Curb and roof structure
| Check | Why it matters |
|---|---|
| Curb dimensions | New units rarely match old footprints exactly; an adapter curb may be needed |
| Supply and return openings | Must align with existing duct connections or require transitions |
| Curb condition and height | Rotted or leaking curbs should be replaced; height must suit snow depth and roofing |
| Unit weight | New units may be heavier, especially with heat pumps, energy recovery or added options |
| Roof structure | A structural engineer may need to confirm capacity, especially for heavier units or new locations |
| Roofing | Coordinate with the roofer and roof warranty; re-flashing is usually needed |
Adapter curbs are common and work well when properly sealed and insulated. A poorly built adapter is a frequent source of air leakage and water entry.
5. Electrical service
- Confirm voltage, phase and available capacity. Heat pump or electric heat units can need much more capacity than gas-heated units.
- Check the disconnect and conductor sizing against the new unit's minimum circuit ampacity and maximum overcurrent protection.
- Look at power quality: low voltage and phase imbalance shorten compressor life.
- Electrical work must follow the Canadian Electrical Code and provincial requirements, with permits as required.
6. Gas supply and venting
For gas-fired units, check gas pipe sizing and pressure for the new input rating, including other appliances on the same line. Confirm that flue terminations meet clearance requirements to intakes, openings and property lines. Gas work must comply with CSA B149.1 and be done by a licensed gas fitter.
7. Heating source decision
Replacement is the natural time to consider electrification. Options include a heat pump RTU with gas or electric backup, a dual fuel unit, or a cold climate heat pump unit. Check:
- Heat pump capacity at the local winter design temperature
- Electrical capacity for heat pump and backup heat
- Utility incentive programs, which vary by province and change over time
Our guide on heat pump retrofits in commercial buildings covers this in more depth.
8. Ventilation and economizer
- Confirm the new unit can deliver required outdoor air, ideally with an outdoor airflow measuring station or a verified minimum position.
- Check whether the energy code in force requires an economizer for the new unit size, and specify a reliable damper and controller.
- Consider demand-controlled ventilation for variable-occupancy spaces.
- Consider energy recovery where outdoor air volumes are high.
Economizer faults are one of the most common RTU problems. See economizers: types, controls and common failures.
9. Controls and integration
- Will the new unit connect to the existing building automation system? Check protocol, often BACnet, and points available.
- Are thermostats in the right location and calibrated?
- Is scheduling in place, so the unit does not run 24/7?
- Does the energy code require fan speed control, economizer fault detection or other control features for this size of unit?
- Are smoke detectors and fire alarm interlocks required and reconnected?
10. Codes, permits and documentation
A replacement may trigger code requirements. Depending on the province and municipality:
- Mechanical, gas and electrical permits are typically required
- An engineer's seal may be required for larger units, structural changes or changes in system type
- The current energy code may apply to replacement equipment
- Refrigerant handling must follow provincial regulations
Our guide on mechanical permits and engineering seals explains when an engineer is required.
After installation
- Start up the unit following the manufacturer's procedure, including refrigerant charge verification.
- Balance supply and outdoor airflow to design values.
- Verify heating, cooling, economizer and ventilation sequences.
- Check for air leaks at the curb and duct connections.
- Update drawings, O&M information and the asset register.
Frequently asked questions
Can I just replace the unit with the same tonnage?
You can, and sometimes it is the right answer. But load, use and codes may have changed. A short review often shows a smaller unit, different heating source or better controls would perform better.
How long does a rooftop replacement take?
The crane lift itself can take hours. Planning, permits, structural review, adapter curbs and controls integration often take weeks. That is why planning replacements before failure is so valuable.
Should I replace several units at once?
Grouping replacements can reduce crane and mobilization costs and allows consistent controls. A capital plan helps prioritize units by condition and risk.
Key takeaways
- Replacement is a rare chance to fix sizing, airflow, controls and heating source; do not default to like-for-like without checking.
- Measure existing airflow and static pressure before removing the old unit.
- Check curb fit, roof structure, electrical capacity and gas supply early.
- Consider heat pump options, economizer reliability and ventilation control.
- Confirm permits and code requirements, then balance and verify after installation.
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