Callbacks on commercial HVAC jobs are one of the most expensive problems a contractor can face — and undersized rooftop units (RTUs) are one of the leading causes. Not because contractors are careless, but because the pressure to hit a lower equipment number on the quote is real, and the consequences don’t show up until the weather does.
In this post, we break down the 5 hidden costs of undersizing a commercial RTU, what proper sizing looks like in Ontario’s climate, and how to diagnose a problem before it turns into a warranty claim.
Why undersizing happens in the first place
Undersized RTUs rarely happen because of ignorance — they happen because of pressure. Here are the most common reasons contractors end up with equipment that’s too small for the job:
- Budget pressure at the quoting stage. A client pushes back on the price, and the path of least resistance is stepping down to a smaller unit. The upfront savings feel real. The downstream costs don’t.
- Rule-of-thumb sizing. Using square footage estimates instead of a proper Manual N or heat load calculation is still common on smaller commercial jobs. It works until it doesn’t.
- Ontario’s climate is unforgiving. We’re not in a mild climate. Design temperatures in Ontario range from −30°C in winter to +35°C with high humidity in summer. Equipment that might perform adequately in a more temperate region will struggle here — and undersized equipment will fail outright.
- Building use changes post-install. A retail space becomes a restaurant. An office gets a server room. The load the unit was sized for no longer reflects what the building actually demands.
The 5 hidden costs of an undersized RTU
The purchase price difference between a properly sized RTU and an undersized one might be a few thousand dollars. Here’s what that “saving” can actually cost over the life of the job.
1. Callbacks and diagnostic labour
When a commercial tenant calls because the building isn’t meeting temperature, someone has to go back. Under warranty, that’s on the contractor. A single callback — diagnostic time, travel, and follow-up — can easily run $500–$1,500. Two or three callbacks on a job and you’ve erased any margin the equipment savings generated.
2. Short-cycling damage and premature failure
An undersized unit runs at or near 100% capacity constantly, or worse, short-cycles — turning on and off in rapid succession trying to meet setpoint. Short-cycling is hard on compressors. A unit that should last 12–15 years can fail at 4–6 years when it’s been running this hard. The replacement cost falls on someone, and if there’s no documented load calculation on file, that conversation gets uncomfortable fast.
3. Higher energy bills for the tenant or building owner
An undersized unit running continuously draws more power than a properly sized unit cycling normally. The building owner notices this on their hydro bill. It becomes your problem when they connect the dots and start asking questions about the equipment spec.
4. Reputation damage
Commercial clients talk to each other. Property managers, building owners, and facility managers operate in networks. One unhappy commercial client who had to chase you on callbacks and then deal with high energy bills will tell others. In an industry where referrals are everything, this is a cost that never shows up on an invoice but absolutely affects the bottom line.
5. Mid-contract replacement liability
If the unit fails prematurely and there’s no load calculation on file to demonstrate the spec was appropriate for the building, you’re exposed. Who pays for the replacement? Who covers the business interruption while the building is without cooling? This is the cost that keeps contractors up at night — and it’s entirely avoidable.
What proper RTU sizing looks like in Ontario
The good news is that correct sizing isn’t complicated — it just requires doing the work upfront.
- Run a Manual N or ASHRAE load calculation. Not a square footage rule of thumb. A proper heat load calculation accounts for the building envelope, occupancy, internal heat gains, ventilation requirements, and orientation. It takes more time than a back-of-napkin estimate — and it protects you if anyone ever questions the spec.
- Use Ontario-specific design temperatures. ASHRAE design data varies significantly across the province. Toronto, Ottawa, and Windsor all have different 99% heating and 1% cooling design values. Make sure your calculation reflects the actual site location.
- Account for how the space is used. Restaurants, server rooms, gyms, and retail spaces all have different internal heat gain profiles. The occupancy type matters as much as the square footage.
- Document everything. Keep the load calculation on file. If a dispute arises two years later, a documented spec is the difference between a professional conversation and an expensive one.
5 signs the RTU on your current job might be undersized
If you’re troubleshooting an existing installation — or doing a pre-season check on a commercial building — here are the warning signs to watch for:
- The unit runs continuously without reaching setpoint on peak summer or winter days
- Supply air temperature is higher than expected (the unit is working too hard to keep up with the load)
- The compressor is short-cycling — on/off cycles under 5 minutes
- There are persistent hot or cold spots across the occupied space despite the system running
- Tenant complaints spike specifically in January and July — the two months when Ontario’s climate pushes equipment to its limits
If you’re seeing two or more of these signs on a job, a load recalculation is worth doing before anything else.
The right spec is the cheaper spec
The framing of “saving money” by stepping down to a smaller unit rarely holds up over a 5-year horizon. Callbacks, compressor replacements, energy complaints, and reputation damage all have real dollar values — they just don’t show up on the original quote.
Properly sized equipment, installed right the first time, is what protects your margin and your reputation on commercial jobs.
If you have a commercial RTU job coming up and want to talk through the spec, the Nordics team is here to help. Contact us before your next quote — we’ll make sure you’re looking at the right equipment for the building, the climate, and the budget.
Interested in becoming an Airquest dealer? Learn more about the Airquest Dealer Program here.
