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Plumberpro, Plumber
Category: Plumbing
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Experience:  Heating and plumbing answers. 20 years experience
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Thermostat wiring question: I have replaced an old

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Thermostat wiring question: Hi I have replaced an old Honeywell T40 mechanical thermostat with a programmable thermostat (TH-2005) and I do not get this new thermostat to work. The old thermostat has 4 wires: L/N/E and switched live. The new thermostat
has 2 sets of connections: set 1: L/N and set 2: NO/COM/NC. I have tried connecting the wires in two ways. First attempt: L to COM and switched live to normally open. In this case the display of the thermostat does not even work. Second attempt: L to live
and N to neutral. Now the display works but the boiler does not fire up. What is the correct way of wiring? shouldn't I use the switched live cable somewhere?
Hi , the new prog stat doesn't need neutral. Where the old stat ( t40) was live in on 1 and out or switched on 3
the new one is live in 2 and out / switched from 3
Customer: replied 2 years ago.
Apologies, could you be a bit clearer as there are no 2 and 3 on the diagram I have attached?
Sure . the new stat .live and neutral to live and neutral . the new stats are designed to have the live and switched live come from the boiler . so you will have to make a small link wire from live to COM then the switched wire from NO
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