Fan coil 4 pipe system. I connected the wires and double checked them. I did everything right but the digital model display did not light up.

Then I measured the votage and it showed zero. I reinstalled T4039 (old one) and it is working fine but no voltage reading. Both models are 120 V. I must have lack of knowledge at some point. Does it matter if I shut the system off from the thermostat first then the heater switch before removing the old one? Can please someone tell me what I'm missing.

Thanks in advance.

You failed to provide the letter behind the number. They made 14 or so of the same thermostats but with different features. See your old stat diagram below.

http://customer.honeywell.com/techlit/pdf/PackedLit/60-2241.pdf

New stat

http://customer.honeywell.com/techlit/pdf/PackedLit/62-0278.pdf

NOTE: if you blew the fuse in the new thermostat it cannot be replaced so a new thermostat will be in order.

T4039S1040

Could the problem be the neutral line? The old one did not require one so there is no neutral wire coming to the stat. However new one requires neutral. I can pull an extra wire and connect one end to the stat but I don't know exactly where to connect the other end. I believe it will be somewhere in the fan. Or I can use the wire for HI fan as neutral and don't connect anything to HI fan terminal. Would that confuse the new stat?

Thanks!

If all the required wiring is not present then that could be a problem.

So what would you suggest?

A fan coil thermostat operates a bit different for a regular system thermostat. Unless you can provide a complete detailed drawing with all control items listed I would be just guessing since a fan coil system can be setup many different ways.