Why Your 'Furnace' Leaks in the Middle of Summer
It seems backwards: your furnace isn't even running for heat, yet there's water pooling beneath it every time the air conditioning kicks on. The explanation is the shared cabinet design used in most Greater Vancouver homes.
In a typical forced-air system, the air conditioner's evaporator coil sits directly on top of the furnace, inside or just above the furnace cabinet. When the AC runs, that coil gets cold and pulls humidity out of your indoor air — exactly like the water that beads on a cold glass on a hot day. That condensation is supposed to collect in a drain pan and flow out a dedicated condensate line. When it doesn't, the water spills down through the furnace and lands on the floor, making the furnace look like the culprit.
So in summer, a "leaking furnace" is usually an AC condensate problem. Cooling systems in our area are serviced by the same parent company that handles our furnaces, CanroHeat, at 604-359-1081.
The Usual Suspects: Drain Pan, Drain Line, and Float Switch
Clogged AC condensate drain line. This is by far the most common cause. The drain line that carries water away from the evaporator coil clogs with algae and slime. Water backs up into the drain pan, overflows, and runs into the furnace. It's the summer twin of the winter condensate clog.
Cracked or rusted drain pan. The pan beneath the coil can rust through or crack with age, letting water drip straight down regardless of whether the line is clear.
Stuck or missing float switch. Many systems have a safety float switch that's supposed to shut the AC off when the pan fills. If it's missing, stuck, or wired out, the pan overflows instead of triggering a safe shutdown.
Disconnected or poorly pitched line. A condensate line that has come loose at a joint, or that sags, will leak at the low point.
Frozen Evaporator Coil — A Leak That Comes in Waves
There's one more cause worth flagging because it behaves differently: a frozen evaporator coil. If your AC is low on refrigerant, or airflow is restricted by a dirty filter or blocked return, the coil can drop below freezing and ice over. While it's frozen, you may see little water. Then the system cycles off, the ice melts all at once, and a surge of water overwhelms the drain pan and floods the furnace.
The tell-tale sign is intermittent flooding rather than a steady drip, sometimes with weak or warm air from the vents and visible frost on the refrigerant line. Because this involves refrigerant — which only a licensed technician may handle — it's a call-the-pro situation. Don't keep running a frozen system; you risk damaging the compressor, the most expensive part of your AC.
The simplest thing you can do yourself: replace a dirty air filter and make sure return vents aren't blocked by furniture. Restricted airflow is a leading cause of coil freeze-ups in our experience.
What to Do Right Now
Turn the air conditioning off at the thermostat to stop more water from accumulating, and mop up standing water to protect your floor and furnace. Check and replace the air filter if it's dirty. Look for a small float switch near the drain pan and confirm nothing is obviously dislodged.
Then call CanroHeat at 604-359-1081. Clearing a clogged AC condensate line, replacing a drain pan, checking refrigerant charge, and servicing a frozen coil all require proper tools and certification. Running the system while it leaks risks water damage to the furnace's electronics and blower motor sitting right below the pan.
When you call, mention that the leak only happens during cooling — that tells our dispatcher to send a technician equipped for the AC side, so it's resolved in one trip.