Electrical Assistance with a furnace please

[Boddhisattvha]

Hi all,
I just installed a new Suburban NT20 furnace and I need some help with the wiring. There's only four wires, red (hot) yellow(grnd) and two blue which are for the t-stat. My issue is that the damn installation manual makes no real differentiation between the two blue wires. There's a red and a white wire coming off the tstat and I assume red is hot and white is something else and they would need to attach to the corresponding blue wire(s). I don't imagine they are interchangeable? The wiring diagram only shows where the wires go in the actual furnace, but unless I take the furnace apart and track the wires I have no way of knowing which is which.

Anyway I got it installed and tried to turn it on and nothing happened. I switched the blue wires and still nothing happened.

I'm going to call Suburban on Monday but I figured I'd see if I can get it resolved with your help first.

[time2roll]

Ah-ha! Glad to hear it is working.

[Boddhisattvha]

It ended up being an issue with my power converter. Thanks for all the help and advice. I couldn't have done it without your help!

[wa8yxm]

Many T-Stats are nothing more than a switch (or a relay) and do not care which way they are hooked up.. So it matters not which wire is "HOT" and which is "NOT" when the T-Stat is off.

Some electronic ones do care.

So, how to tell 'em apart?

IF the T-stat manual clearly indicates the POSITIVE wire is to be hooked to THIS screw.. Use your volt meter... Ground (Known good ground) and measure for voltage. the "HOT" lead will be hot.

Do not have a voltmeter.. A test lamp will light when hooked to the HOT lead, but not the other one.

Method two (If the install manual for the T-Stat does not specify)

Hook it up, try it, if it works good, if not try reversing the leads.

Odds are it will work both ways, but if it does not.. mark the leads with stick on labels or heat shrink tubing.

[time2roll]

Boddhisattvha wrote:
If I set the meter for 20 DCV range and touch the red lead to the end of the live wire do I just ground the black lead?

Yes. Although if no voltage you should also consider if ground is good.

[Boddhisattvha]

Again don't hate me please. I get the setting for the multimeter thanks for that. I'm hooked up to shore power. I've got a converter and circuit breakers up towards the front of the rig. The furnace sits under a bench/seat about mid rig. I have one live wire that runs that span to power the furnace. If I set the meter for 20 DCV range and touch the red lead to the end of the live wire do I just ground the black lead?

[enblethen]

The red lead of the voltmeter would go to one wire and the black to the other. Meter set on DC volt scale around 20 volts or so. The meter will read voltage. If there is a negative bar on the left side of the reading, it is reversed.
On a analog meter, one with a needle, the meter will deflect to the left if reversed.

[time2roll]

To check continuity set the scale to ohms. Touch the leads together and you should get zero. Now set the thermostat to switch heat on and touch the leads to the terminals. You should get very close to zero ohms.

To check for voltage set to 20 vdc scale. Put the leads on the battery terminals and look for about 12.5 to 13.5 volts. Now move the lead to what you want to check. Make sure the ground is good or extend the lead all the way back to the battery if needed.

[Boddhisattvha]

Forgive me but I'm a novice at electrical work. I have a multimeter and to check continuity I'm supposed to connect a lead to each end of the wire or device. I'm not sure what that means. I need to trace the hot wire back to wherever it came from? Is there a way to just test the lead itself to see if it's got 12v? I used to have a voltmeter with a bulb that was much simpler. The multimeters throwing me off.

[enblethen]

Have you verified that there is 12 volts to the furnace?
Verify polarity of 12 volts.
Check the circuit breaker normally mounted to fan housing. It should have small red or white reset button

[time2roll]

That thermostat should be fine and is just connecting the two blue wires together. Check continuity with an ohm meter if you have doubts.

[Boddhisattvha]

Okay, I tried it and no go. I have a suspicion that I didn't get it grounded well enough. If that's not the case than either A. the furnace I just bought is bad or B. the hot lead coming in has issues. Are there any other possibilities I should look at?

I actually just installed a new thermostat a few days before the old furnace died. (The old furnace just stopped lighting the blower motor was fine) The new thermostat is Heat only White Rodgers

[Chris_Bryant]

What thermostat do you have? On a Coleman thermostat that operates the rooftop air conditioner, it does matter, as the thermostat gets its power from the furnace.
Agree to try hooking the blue wires together first as a test though.

[donn0128]

Try it first then well discuss other things.

[jtel]

no 12V power