Older returns often have no proper filter rack, so the filter sits loose and air bypasses it entirely. Check that yours actually seals against the frame. A filter that air goes around is doing nothing for the coil.

This is most common in homes built before about 1940. Pre-war construction — galvanized supply, cast iron drains, cloth-wrapped wiring, ungrounded circuits.

You can handle this one yourself. No call needed.

Related

• Ductwork threaded through a house built for floor grates
• One central return for the whole house
• Water heater vents into the chimney and the furnace no longer does
• Silver-wrapped tape on old duct joints
• Additions still on window units
• Uninsulated walls and an oversized condenser
• Furnace runs but the house never warms evenly
• Electrical panel cannot support a modern condenser

Back to Hvac.

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Written by Jaylan Jones. I Like To Fix earns a referral fee when a call connects to a contractor. See our business-model disclosure.