An air conditioner that runs but doesn’t cool is one of the most common summer HVAC calls we receive in Orangeville, and it almost always means one of five things: a clogged filter, a blocked outdoor unit, frozen evaporator coils, a refrigerant leak, or an undersized system struggling in the heat. The good news is that some causes are easy to rule out yourself. The bad news is that ignoring a few of them, especially frozen coils, can turn a straightforward repair into an expensive compressor replacement. This guide walks you through each cause in order of likelihood, tells you what to check before calling a technician, and explains exactly when to shut your system down to protect it.
TL;DR
- Check the filter first. A clogged air filter is the most common cause and takes two minutes to rule out.
- Shut it down if coils are frozen. Running a frozen system can destroy your compressor. Turn it off at the thermostat and let it thaw before doing anything else.
- Refrigerant leaks need a licensed tech. Low refrigerant always means a leak. Handling refrigerant in Canada requires TSSA certification, and with the R-410A phase-out, repair costs are rising.
- Debris and cottonwood matter more than you think. In Dufferin County, June cottonwood season can plug your outdoor condenser unit in days.
- An undersized unit will struggle all summer. If your AC runs constantly but can’t keep up when it’s 33°C, the system may simply not be sized correctly for your home.
Step 1: Turn It Off If You See Ice
Before you check anything else, walk to your indoor air handler and look at the copper refrigerant lines. If you see frost or ice forming anywhere, either on the lines themselves or around the indoor coil, shut the system off at the thermostat immediately. Do not try to run it through. Do not switch it to fan-only yet.
Here is why this matters: when evaporator coils freeze, the ice acts as a barrier between the refrigerant and your home’s warm air. The system keeps running, the compressor keeps working, but no heat transfer is happening. If you leave it running, you risk burning out the compressor, which can cost $1,500 to $2,500 to replace, compared to the $150 to $400 it typically costs to fix the underlying cause of the freeze.
Once you have shut it off, switch the thermostat to fan-only and let it run for two to four hours to thaw the coils. Then work through the steps below to find out why they froze in the first place.
The Five Most Common Causes (In Order of Likelihood)
1. Clogged Air Filter
A dirty air filter is the single most common reason a central AC runs without cooling effectively. When the filter is plugged with dust, pet hair, and debris, your system cannot pull enough warm air across the cooling coils. The result is reduced airflow, poor cooling, and, if it goes on long enough, frozen coils from the same restricted airflow problem.
What to check: Pull your filter. If you cannot see light through it, replace it. For most Dufferin County homes, filters should be replaced every one to three months during cooling season, and more frequently if you have pets or live near a gravel road.
This is a two-minute fix. Rule it out first before assuming anything more serious is wrong.
2. Blocked Outdoor Condenser Unit
Your outdoor condenser unit releases the heat your AC pulls from inside your home. If the fins are plugged, that heat has nowhere to go, and your system runs continuously without actually cooling anything.
What to check: Walk outside and look at the unit. Clearance matters: you want at least 60 cm of clear space on all sides. In Dufferin County specifically, June brings cottonwood season, and those white seed clusters can pack the condenser fins within days. Grass clippings from mowing also accumulate quickly. A garden hose sprayed gently through the fins from the inside out will clear most buildup. If the unit is surrounded by overgrown shrubs, trim them back.
Also check that nothing is sitting on top of the unit, that the fins themselves are not bent or crushed, and that the unit is level on its pad.
3. Frozen Evaporator Coils
If you caught this early from Step 1, you already know the coils are frozen. But sometimes you won’t see obvious ice and the symptom is just warm air from the vents and a system that runs and runs without cooling.
Evaporator coils freeze when they get too cold without enough warm air passing over them. The two most common causes are a clogged air filter (restricted airflow) and low refrigerant (which lowers the pressure and temperature inside the coil). Once frozen, the ice acts as insulation and the coil can no longer absorb heat from your home.
What to do: Shut the system off and let the coils thaw completely, which takes two to four hours with the fan running on fan-only mode. Once thawed, replace the filter and restart. If it freezes again within a day or two, the cause is likely refrigerant, and you need a technician.
4. Low Refrigerant (Refrigerant Leak)
Refrigerant does not get “used up” like fuel in a car. Your AC runs on a closed loop, meaning if the refrigerant level is low, there is a leak somewhere in the system. Common signs include ice on the copper lines, a faint hissing or bubbling sound near the indoor unit, and an AC that runs constantly but cannot reach the set temperature even on moderate days.
There is an important practical reality here for 2025 and 2026: R-410A, the refrigerant most Ontario central AC systems use, is in the middle of a regulated phase-out. New systems must now use lower-GWP alternatives like R-454B, which means R-410A stockpiles are finite and its cost is rising. If your older system has a refrigerant leak, you may be looking at a more expensive repair than it would have been a few years ago, and it may be worth having a conversation about whether repair or replacement makes more sense.
Important: Handling refrigerant in Ontario is regulated by the Technical Standards and Safety Authority (TSSA). Only licensed, certified technicians can legally add, recover, or work with refrigerant. Do not attempt this yourself.
If you suspect a refrigerant leak, call Hyde-Whipp at 519-941-8429. We can diagnose the leak, advise on repair versus replacement, and handle the refrigerant work legally and safely.
5. Undersized or Mismatched System
If your AC runs constantly from 9 a.m. to midnight but the house never drops below 26°C on a hot July day, the problem may not be a malfunction at all. It may be that the system is simply too small for the space it’s trying to cool.
This happens more often than you’d expect in Dufferin County, where older homes have been renovated to add square footage without upgrading the HVAC system, or where a previous system was replaced with something close in size rather than properly sized using a heat load calculation.
Signs your system may be undersized: It runs almost constantly during any day above 30°C. The temperature inside tracks the outdoor temperature more than it should. Upstairs rooms are significantly warmer than downstairs. The system was installed without a Manual J or heat load calculation.
If this sounds familiar, a proper assessment by a qualified technician is the right next step. For more on cooling options suited to different home sizes, see our page on Central A/C and Ductless A/C.
When to Call a Technician
Some of the causes above you can rule out yourself in under ten minutes. Others require a licensed technician. Call Hyde-Whipp if:
- Your coils froze, you thawed them, and they froze again within 48 hours.
- You hear hissing or bubbling near the indoor or outdoor unit.
- There is ice on your refrigerant lines after the filter has been replaced.
- Your outdoor unit’s fan is not spinning or is making grinding or rattling noises.
- The system trips your circuit breaker when it tries to start.
- Your AC is more than 12 years old and has needed refrigerant twice or more.
For true emergencies where you have no cooling on a hot night, Hyde-Whipp offers 24-hour emergency HVAC service year-round. You can reach us any time at 519-941-8429. For more on how we can keep your system running reliably season to season, see our Protection Plans.
Frequently Asked Questions
Why is my AC blowing warm air but the unit is running?
The most likely causes are a clogged air filter restricting airflow, a blocked outdoor condenser unit that cannot release heat, frozen evaporator coils, or low refrigerant from a leak. Start by checking and replacing the filter, then inspect the outdoor unit for debris. If you see ice on any part of the system, turn it off immediately and let it thaw before diagnosing further.
How do I know if my AC has a refrigerant leak?
Signs of a refrigerant leak include ice forming on the copper lines or indoor coil, a faint hissing or bubbling sound near the unit, the system running continuously without reaching the set temperature, and noticeably higher hydro bills. Low refrigerant always indicates a leak in the closed-loop system. Only a TSSA-certified technician can legally diagnose and repair a refrigerant leak in Ontario.
Is it safe to run my AC if the coils are frozen?
No. Running your air conditioner with frozen evaporator coils puts serious strain on the compressor, the most expensive component in the system. If you see ice on the coils or copper lines, shut the system off at the thermostat, switch to fan-only for two to four hours to thaw the coils, and then investigate the underlying cause before turning cooling back on.
How often should I change my AC filter in Ontario?
For most Ontario homes, every one to three months during the cooling season is the standard recommendation. Homes with pets, multiple occupants, or gravel driveways should change filters monthly. In Dufferin County, the June cottonwood season can dramatically accelerate filter clogging, so check your filter during that window even if it was recently replaced.
My AC runs constantly but never cools the house below 26C. What is wrong?
If your system runs without stopping but cannot reach the set temperature on hot days, the most common explanations are a condenser unit blocked by debris or cottonwood, a refrigerant leak, or an undersized system that was never properly matched to your home’s heat load. A technician can diagnose which applies and recommend whether repair, cleaning, or a system assessment makes sense.
What is the R-410A phase-out and does it affect my AC?
R-410A is the refrigerant used in most Ontario central air conditioning systems installed before 2025. Due to environmental regulations, manufacturers can no longer produce new R-410A for residential systems, and stockpiles are shrinking, which means the cost to recharge or repair a leaking R-410A system is rising. If your system is older and develops a refrigerant leak, it may be worth discussing whether repair or replacement with a newer R-454B system makes more financial sense.
Action Steps
- Check and replace your air filter before calling anyone. This is the most common cause and the easiest fix.
- Inspect your outdoor condenser unit for cottonwood buildup, grass clippings, or debris blocking the fins, especially in June and after mowing.
- Shut off the system immediately if you see ice or frost anywhere on the unit or copper lines. Switch to fan-only and allow two to four hours to thaw.
- Do not add refrigerant yourself. It is regulated in Ontario and illegal to handle without TSSA certification.
- Book a tune-up before peak summer heat rather than waiting for a breakdown on the hottest day of the year. Ask about Hyde-Whipp’s Protection Plans for priority scheduling.
An air conditioner that runs but doesn’t cool is almost always a solvable problem, but the urgency depends on the cause. A clogged filter is a two-minute fix. Frozen coils that aren’t caught early can become a compressor replacement. If you’ve worked through the checklist above and your system still isn’t cooling, call Hyde-Whipp at 519-941-8429. We have served Orangeville and Dufferin County since 1983 and offer 24-hour emergency service when you need it most.