Installed humidifier - now furnace won't go on

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Old 05-07-09, 02:23 PM
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Installed humidifier - now furnace won't go on

I installed an Aprilaire 700 humidifier, and now it seems my entire HVAC system is kaput! It's a 1998 Tempstar NTC6125KJA1 80% furnace.

The first thing I noticed is that the door panel on the top of the furnace sheared off a connector from the pressure switch (a Tridelta .65 model they no longer make). Alas, even touching the two ends of the broken connector doesn't turn on the unit. (Reading online, it seems these old 2-wire pressure switch units can be basically bypassed by connecting the wires. that didn't work either).

The thermostat "power" light on the main floor is not lit either. I checked the breaker, it hasn't tripped. The thermostat is functioning (by that I mean hte clock works, it tries to send the furnace it's desired function [heat, cool, fan on, etc]) but nothing happens. In the basement it's as if there's no power getting to the furnace anymore.

I'm not sure what to try. I don't know if replacing the pressure switch will solve this problem? We need the system up and running ASAP , the wife is getting whiney about the heat already.

Thanks for any advice.
 
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Old 05-07-09, 05:39 PM
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Is the blower compartment door on all the way? There is a safety switch behind the top of the door. If it is ajar, the furnace will be dead.

Even if the door is not ajar, this still is a good place to look, as I have seen them where the sheetmetal door just does not activate the switch completely, anymore. You can remove the blower door and press in the switch button all the way, then see if furnace runs.

If this does not help - do you own a voltmeter so you can test and see if you have voltage in the furnace? If you test across the outgoing wires from the transformer and you have 24 volts, you know you have both 24 volts and 120 volts in that furnace, as you should. Then at least we are on our way to getting this problem of yours solved.

Then from there you move on. In the blower compartment you should see where the thermostat wires hook up. On the strip that they hook up to, one will be labeled R. See if you have 24 volts from R to any metal ground. Do you? If you do, you can bypass the thermostat by jumpering a wire between R and W, and see what happens. That will activate the heat mode of the furnace.

If you do that, you still have to make sure to activate that blower door switch by putting the blower door back on. You can leave the big burner compartment door panel off though, as they do not have a safety switch.

If you manage to get 24 volts from R over to W, and the furnace still does not go, then with the jumper still on, then see if you have 24 volts from any of the 2 pressure switch wires, to metal ground, with the voltmeter.

If you answer these questions and perform any of the tasks, if needed, then post back what works and what does not, we will be on the way to solving your problem.
 
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Old 05-08-09, 12:28 PM
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I really appreciate your help! I'm a bit of a newbie but I can run a multimeter and turn a screw

Originally Posted by ecman51` View Post
Is the blower compartment door on all the way? There is a safety switch behind the top of the door. If it is ajar, the furnace will be dead.

Even if the door is not ajar, this still is a good place to look, as I have seen them where the sheetmetal door just does not activate the switch completely, anymore. You can remove the blower door and press in the switch button all the way, then see if furnace runs.
The switch seems to be closing with the door - even pushing it manually didn't start the blower. Using the multimeter, I confirmed the door switch is working fine.

If this does not help - do you own a voltmeter so you can test and see if you have voltage in the furnace? If you test across the outgoing wires from the transformer and you have 24 volts, you know you have both 24 volts and 120 volts in that furnace, as you should. Then at least we are on our way to getting this problem of yours solved.
I do - and I confirmed that the small transformer box that sends to the humidifer - it's at 18.6v. The wires inside the furnace are getting 122v. I followed those wires down to the door switch, and when pressed, it's sending on 122v to the control board. On to the next...

Then from there you move on. In the blower compartment you should see where the thermostat wires hook up. On the strip that they hook up to, one will be labeled R. See if you have 24 volts from R to any metal ground. Do you?
No. The R doesn't show any voltage whether I touch ground or W with the other lead.
 
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Old 05-08-09, 08:42 PM
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Is there a small fuse on the board that you could have blown?
 
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Old 05-12-09, 07:54 AM
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Not that I can see, and I don't see anything disconnected or anything burned out on the board either.

(But I did solve the "how is my thermostat still working at all" puzzle - it has backup batteries in it)
 
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Old 05-12-09, 01:42 PM
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What else can I test? The board looks pristine (even looked at the back of it) - there is definitely power getting to it, and all wires seem connected.. I'm hoping someone can provide other things to check.
 
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Old 05-12-09, 03:35 PM
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Originally Posted by wantonsoup View Post
I really appreciate your help! I'm a bit of a newbie but I can run a multimeter and turn a screw

The R doesn't show any voltage whether I touch ground or W with the other lead.
As stated by another psoter, carefully look to see if you have a fuse. These are on those open boards only. Not on enclosed ignition control modules. Really look good. It should actually have the word "fuse" and or 3 volt printed on the circuit board. This is the most important thing you need to check before we move onward, since you apparently are getting line voltage to the board, yet no 24 voilts to the low-volt terminal strip.

What happens if you touch R and the neutral transformer wire itself? If say you discover you have about 24 volts here, disregard the paragraphs below and we will simply move on with more questions at this time.

Do you have a wiring diagram say on the inside of the blower door?, and a digital camera where you could print it here?

If R is truly dead, yet you have power to the furnace, this is a problem I have never experienced (without curing the problem for this test first without realizing it). So therefore I cannot "see"(envision, sight unseen, as I normally can). Off the top of my head, I am uncertain of the pathway most furnaces use to get from the transformer to the low voltage terminal board, since I have really never have had to check that out.

If I had a high efficiency furnace handy here, I'd take a look to see if what I see is something obvious in where the wires go. Tomorrow I may be by a furnace or two I can take a peek at, unless someone else comes up with the answers in the meantime.
 
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Old 05-12-09, 04:54 PM
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I've taken pictures of the schematic and the circuit board. I cannot find a fuse anywhere in the system.

(Obviously with door switch closed):
Black (hot) and any thermostat connection (R, W, Y, G, C) = 120V
White (neutral) and any thermostat connection = zilch.



 
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Old 05-12-09, 05:23 PM
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Can't see fuse either.

Schematic is fine. I enlarged it. This will take me a while to injest and I have to get off this computer now. Hopefully someone else can help you and I will try to get back on here before work tomorrow and check on the progress.

Did you try R to transformer wire test, rather than W to transformer test?
 
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Old 05-12-09, 05:50 PM
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Previously in this thread, when I was testing the 24v transformer, it was the one mounted on the main junction box, which runs wires to the humidistat and humidifier. Not the internal transformer in the furnace itself. But, that may indeed be the problem! 120V going in, NOTHING coming out. I took the cover off the transformer and tested the wires there - 120v in the line primary side, but nothing on the 24v secondary side at all (load). I'll wager a guess that this is my problem - somehow when I snapped off the pressure control valve connector, it shorted out the transformer. A better design would have a fuse somewhere here to protect the transformer, but not this furnace. What do you think?

Clear image of the schematics:


Transformer I think may be bad:
 

Last edited by wantonsoup; 05-12-09 at 06:07 PM.
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Old 05-12-09, 10:12 PM
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Talking

......................................
SOLVED!!!!
......................................

$10 transformer from Radio Shack - and the system works! Thanks for all of the help.
 
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Old 05-13-09, 06:42 AM
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Congratulations! I too had figured you were testing the main transformer when I was asking you to test this and that. At least now I don't have to ponder over a schematic, when I have a million and one things I should be doing this morning -

(off topic) -like tearing apart an out of date 1-ton a/c motor and trying to inject grease in it for an experiment. Two motor shops said for them to do bearing work will cost $2-300 and not worth it on this old unit (I did that successfully with a better type bath fan, that cannot easily be replaced, a while back, that had just the right kind of bearing set up.) Motor shop guy told me I have nothing to lose by trying, since I have the motor out anyway - a 2 shafted motor you can not get now. But I may still try to see if company B, who bought out company A, has a handle on that motor, first. And seeing if the 12 sticks! of epoxy putty, I used to fill a rotted support column on an historic house yesterday, worked out good (my sore thumbs!)

Now that you are an HVAC pro , wantonsoup, stick around and try to injest some of these threads on a regular basis, as the increase in knowledge may help you out in the future.

....................................................................

Update: That quick since my last post - I have been online with parts houses, even proclaimed largest one (no luck) and have since set up shop behind the computer here and took motor apart and injected grease, and reassembled and put stuff away. What a simple-type and excellent looking motor! No shaft damage. Bearings cigarette-filter type packing in bearings was all dried out. Maybe what I did will actually hold up. The whole thing took me less than an hour. This will really make my day. Even if I have to redo the job once a year -still not bad cost factor. But now I have to be a contortionist and assemble motor brackets to unit again, basically by feel, while standing on my head.
 

Last edited by ecman51; 05-13-09 at 08:52 AM.
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