noma electronic thermostat model thm303m

rickevin29

I have two new a/c units and furnace by Trane, the problem I have is that my new noma thermosat wont turn on the a/c unit. I have only two wires to thermostat from furnace, but one has a pigtail and I am guessing there is a resistor in the pigtail for the a/c unit. Is it possible to connect to my electronic thermostat keeping this two wire with pigtail set up? things ive tried:: switching from gas to electric on
thermostat
removeing jumper from rc and rh

mdshunk

Sounds like you had one of the new digital communicating control systems before. What was the model of the Trane stat that you removed?

rickevin29

the stat is a common white rodgers type 1e56w-356

mdshunk

There must be something special at the other end of that two wire setup. I've never seen such a thing. Normally, you need at least 3 wries for heat and ac. Normally more.

rickevin29

if their is a resistor in the pigtail would it not send lower voltage signaling the a/c unit only to start? thier is something in the furnace end called " ADD A WIRE" by total line.........

#CARRIERMAN

Hi rickevin29

I would reccomend goin to the furnace and finding out how it is wired. mdshunk could be right the thing you are calling a thermostat that you removed could be a dc interface. The thing you saw called an add a wire, goes to the g circuit and allows the a/c switch to operate the blower circuit. If you have a 24 volt circuit for your thermostat you will need to leave the rc-rh jumper in. I will give you the termanology so you understand what those letters mean.
RC= 24 VOLT HOT, COOLING CIRCUIT
RH= 24 VOLT HOT, HEATING CIRCUIT
W= 24 VOLT OUTPUT, HEAT
Y= 24 VOLT OUTPUT, COOL
G= 24 VOLT OUTPUT, BLOWER
C= 24 VOLT COMMON OR NEUTRAL, NOT TO BE COFUSED WITH GROUND.

Let me know if you need any more help
Rusty

Spuds105

Hi Carrierman,
Judging from your posts it sounds as if you know a thing or two about thermostats so I'm hoping you can help me out. I am trying to change my existing thermostat (Weathertron) for our heat pump with a Noma THM303M. The wires that are attached to the existing thermostat are labelled as follows: R, G, Y, X2, B, W, F, O
There is a wire running from X2 to W in addition to the wire that is going to X2. This wire does not come from the wall but appears to connect X2 to W (hopefully that makes sense). The connections for the Noma unit are as you have specified them in your 11:40 am post.
Any help you can provide would be appreciated!
Murray

rickevin29

hey carrierman, All i did was connect the one pigtail wire to the a/c swith on the thermostat and it worked just fine, the reason it wasnt working before is that there is a 5 minute time delay when switching to a/c to protect the a/c compressor..............thx for all the help guys........