What is the box on top of my furnace? The evaporator coil
That box sitting on top of your furnace is the evaporator coil, the indoor half of your central AC. The refrigerant line from the unit outside runs into it, the furnace fan blows air across it, and the coil pulls the heat and the humidity out of that air.
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From one of our jobs
An evaporator coil sitting on a gas furnace
Right now: Check the floor around the furnace and the white drain pipe. Water there or ice on the black pipe, turn the AC off at the thermostat and call.
Look for the black insulated pipe going into the box: that’s the refrigerant line from your outdoor unit. The white plastic pipe coming out the side is the condensate drain. All summer the coil drips water like a cold glass, and that pipe carries it to a floor drain or a pump.
When the drain clogs, water backs up and spills around the furnace, or a float switch shuts the AC off to stop it. When the coil gets dirty or the system runs low, the coil and that black pipe can frost over. Both show up in a tune up before they show up on your floor.
Quick answers
What people ask about evaporator coils
Why is water leaking around my furnace?
In summer it’s almost always the AC: the coil’s condensate drain line is clogged and the pan overflowed. In winter, a high efficiency furnace makes its own water, and its drain can clog too.
What is the float switch on my AC drain?
A small switch in the drain line or pan that shuts the AC off when water backs up. If your AC quit and the pan is full, that’s what tripped.
Why is there ice or white frost on my AC pipe?
The coil is too cold, from a dirty coil, a clogged filter, a weak blower or low refrigerant. Turn the AC off and run the fan to thaw it, then call.
Does an evaporator coil need cleaning?
Yes, every few years in most homes. A dirty coil can’t pull heat out of the air, so the AC runs longer and the house stays sticky.
Who services AC coils in Northern Virginia?
Call SwiftPro at (703) 997-9757. We clean, repair and replace evaporator coils and clear AC drain lines across Northern Virginia.
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SwiftPro Heating, Cooling & Plumbing
4230 Lafayette Center Dr N, Chantilly, VA 20151
44679 Endicott Dr, Suite 211, Ashburn, VA 20147
SwiftPro Plumber & Water Heaters
4200 Lafayette Center Dr, Suite N, Chantilly, VA 20151
Virginia Class A contractor license #2705192276