A commercial rooftop unit is usually due for replacement when it's around 15 years old or older, needs major repairs more and more often, can't keep the space comfortable, or costs noticeably more to run than it used to. One of those signs alone may just mean a repair. Two or three together usually mean it's time to start planning a replacement, before the unit picks the date for you in the middle of July.
Sign 1: The unit is 15 years old or more
Age is the first thing to check, because it frames every repair decision. A white paper on the U.S. Department of Energy-backed Advanced RTU Campaign puts the useful life of a rooftop unit at 15 to 20 years, while noting that units can fail sooner or last longer. The Department of Energy also points out that rooftop units over 15 years old often provide reduced service and waste a good deal of energy and money.
The manufacture date is on the unit's nameplate. If you have several rooftop units, list each one with its age, tonnage and repair history. That list becomes your replacement plan.
Sign 2: Repairs are getting bigger and closer together
Filters, belts and the occasional capacitor are normal upkeep. The warning signs are the expensive repairs: a failed compressor, a refrigerant leak that keeps coming back, a condenser coil that's corroded through, or a cracked heat exchanger on a gas-fired unit. A cracked heat exchanger is a safety problem, not just a cost problem, and should be dealt with right away.
When a unit has needed more than one major repair in the past couple of years, it's worth pricing a replacement before approving the next one.
Sign 3: People are complaining about comfort
Rooms that never quite cool down, hot and cold spots, clammy air in summer, or a unit that runs constantly and still falls behind on the hottest days all point to a unit that has lost capacity. Sometimes the cause is fixable, like a dirty coil, a low charge or a bad economizer. On an old unit, it's often a sign the equipment is wearing out. It can also mean the unit was never the right size for how the space is used today.
Sign 4: Energy bills are creeping up
Newer rooftop units, especially ones with variable-speed fans and better controls, use much less energy than units from 15 or 20 years ago. The Department of Energy's account of Walgreens' program describes one store where replacing rooftop units cut their energy use by more than half. Your results depend on the old unit, the new one and the building, but the point holds: an old unit costs you every month, not just when it breaks.
Sign 5: The refrigerant question
The refrigerant rules have changed, and it affects how you think about an old unit. Under the EPA's Technology Transitions rules, new residential and light commercial air conditioning and heat pump systems, including unitary systems, have to use a refrigerant with a global warming potential of 700 or less. In 2026, the EPA removed the installation deadline for systems whose components were all made or imported before January 1, 2025, so that older stock can still be installed while it lasts.
For an existing unit, the key point is that the rules don't stop you from repairing it. The EPA's 2026 rule notes that the earlier rule did not prohibit the sale of components needed to repair existing systems. But when an old unit needs a major refrigerant-side repair, you're putting new money into equipment built for the old refrigerant. That's one more reason to compare the repair against a replacement.
Sign 6: The roof is telling you something
Rust through the cabinet, a curb that's rotting or leaking, or water stains on the ceiling under the unit are problems that get worse with time. If the roof is due for replacement in the next few years, line the rooftop unit replacement up with it, so the roofer isn't working around a unit that's coming off anyway.
Repair or replace: the simple math
The question isn't only "what does the repair cost?" It's "what does the repair buy?" Divide the cost by the years of service you can reasonably expect from it.
A hypothetical example: a 17-year-old unit needs a $6,000 repair, and you expect maybe two more years from it. That's about $3,000 per year of service. A replacement priced at $30,000 that runs 15 years is about $2,000 per year, and it uses less energy and comes with a new manufacturer's warranty. In that case, the repair is the more expensive choice, even though the check is smaller. Run the same math with your own quotes. A $1,500 repair on an 8-year-old unit is usually an easy yes.
Also check whether your utility offers incentives. Ameren Missouri lists business energy efficiency incentives, including instant incentives on some heating and cooling equipment and custom incentives. Look before you order equipment.
What a rooftop unit replacement involves
- Survey and sizing. Don't assume the new unit should match the old one. In the Advanced RTU Campaign white paper, Walgreens ran a whole-building load analysis before sizing replacements and found many stores could use smaller units. Lighting upgrades, new windows or a change in how the space is used may have changed the load.
- The curb. If the new unit's footprint matches the existing curb and the curb is sound, it can be reused. If not, an adapter curb or a new curb is needed, with roofing work at the curb.
- Structure. If the new unit is heavier or sits differently, the roof structure under it should be checked.
- The crane lift. Most rooftop units are set by crane. OSHA's crane rules cover operator competence, qualified riggers and signal persons, ground conditions under the crane, and a check of whether the crane or load could come within 20 feet of a power line. Lifts are often scheduled early in the day or on a weekend, with the area below closed off.
- Connections. Gas piping, electrical, ductwork, condensate and controls are reconnected to the new unit.
- Startup. The unit is started, checked and tested, and its controls are set up, including economizer and ventilation settings.
- Permits and inspection. Mechanical work needs a permit. In the City of St. Louis, for example, mechanical permits are issued to licensed mechanical contractors certified by the city. Your contractor should pull the permit.
Most of the time goes into planning, ordering and preparing the curb and connections. The swap on the roof is the short part, so the space is without that unit only briefly. Schools usually replace units over the summer, and businesses often schedule the lift around their slowest hours.
Plan it before it fails
Waiting for failure costs more. The Advanced RTU Campaign white paper describes how running units until they failed led to expensive emergency replacements, with high labor rates, small one-off equipment purchases, missed rebates and missed chances to downsize. A planned replacement lets you bid the work, schedule the crane, line up incentives and pick the season.
If several units are near the end of their life at once, it's also the right moment to price an alternative. A geothermal system puts heat pumps indoors and a well field underground, and the federal credit covers a large share of it. See why geothermal makes sense for businesses and how geothermal HVAC works.
Next step
We set and replace rooftop units on existing buildings, including curbs, connections and startup. If your units are showing these signs, request a free estimate or call 636-327-4238. If you're weighing geothermal instead, the geothermal calculator takes your conventional replacement price and shows what geothermal would cost after the credit.