How to replace an old mercury thermostat with a programmable one
Old mercury tube thermostats often use just two or three wires for furnace-only heating systems, which can make wiring a new programmable model confusing. Accidentally touching the wires together without turning off the power can blow the low voltage fuse, requiring a fuse replacement before the new unit will work.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Checking furnace wiring connections or posting pictures of the wiring for guidance.
- 2Checking the control board fuse visually or with a multimeter and replacing it with a new one after shutting off power.
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Looking for advice on replacing my thermostat to one i can program its an oldschool mercury tube one with i presume two wires, two browns and one white.”source ↗
“Possibly- what am I looking for here?”source ↗
“Edit: ok I likely blew the low voltage fuse from being a dumbass and not killing power first. Going to replace it today.”source ↗
Where this came up
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