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bonzaco
bonzaco,
Category: UK Appliance
Satisfied Customers: 1416
Experience:  Owner at Dales Electronic
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I too have an oldtheremostat with red, blue and yellow

Customer Question

Hi I too have an oldtheremostat with red, blue and yellow wires. Have bought a Honeywell DT90E - This has A B and C connection points. A for L B for N and nothing for C. Which Live to I attach to A Red (Live) or Yellow Switched Live). I assume Blue to be insulated and not attached
Cheers
Submitted: 10 months ago.
Category: UK Appliance
Expert:  bonzaco replied 10 months ago.

Assuming you are connecting it directly to a boiler, then A = live in, B = switched live to boiler and C is not connected. On the original you needed a neutral for the thermostat to operate.