Older systems were wired without a common conductor, which most smart thermostats need. Pull the faceplate and count. There are adapter kits, but knowing before you buy the thermostat saves the return.

This is most common in homes built roughly 1940 to 1980. Post-war and mid-century — copper supply, aging cast iron stacks, 60 to 100 amp panels, first-generation central air.

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

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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.