Air Handler Fuse Blowing


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Old 01-05-18, 03:02 PM
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Air Handler Fuse Blowing

I had trimmed some of the thermostat wires shorter to gain wire to try adding a C wire for a wifi thermostat (lazy and whoever wired it left feet of extra wire in there). Use one of the extra wire lengths to run a C through the wall. Unhooked all of the thermostat wires from handler wires and wired it all back up with the new C and the 5 amp air handler fuse is blowing now (never did before).

Here is the diagram.

My wiring in the handler (old unit no board) is exactly wired like the attached 2nd photo before I touched it and looks the same again now.

I have:

large green unit to small green tstat
large red unit to small red tstat (with an overflow sensor spliced in; isn't this reversible as it's just a switch and cannot be backwards?)
large white unit to small white tstat
large blue unit to small white (out to the condenser unit)
small blue tstat (yellow wire) to red small (other out to the condenser unit)

I ran the new C off of the blue wire on post of the 24v side of the transformer; on the bottom photo the middle block bottom left terminal with the blue wire. My understand of the diagram is that is the 24v common. Wire from that spot through the wall to c wire on the t-stat.

Am I missing something here why the 5amp fuse inline with the 24v red line (below the far right block on the diagram) is blowing now?

double checked all of the connections and wire nuts and all are tight; see no shorts and nothing else was touched/moved. I've used this wifi thermostat for years so I know it's not defective.

My best guess is my C hookup is in the wrong spot and shorting it out. With the system off the tstat has power, clicked on the heat and after a few min popped the fuse.
 
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Old 01-05-18, 03:12 PM
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Welcome to the forums.

Yes... definitely wired wrong.

Focusing on this picture.
I see a four wire thermostat cable....
red to red, green to green, and white to white. Is that correct ?

What is the small two wire cable for..... the outside condensor ?
I don't see any small yellow wire in the picture.

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Old 01-05-18, 03:15 PM
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Correct sorry. 4 wire tstat cable, adding a C. That's a borrowed photo but mine is essentially the same. The blue small wire is the yellow small for the tstat (I didnt wire the place just a rental)
  • R large unit to R small tstat (with a shutoff switch inline which shouldnt be causing the issue and is reversible by my understanding)
  • G large unit to G small tstat
  • White large unit to white small tstat
  • Blue large unit to the white small out to condenser
    • red small from condenser back into blue small tstat (which serves as the tstat yellow wire for some reason how they wired it).
    I then added a C from transformer 24v side to C on the tstat and it's blowing the fuse. I suspect my C hookup is the issue bridging something wrong
 
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Old 01-05-18, 03:25 PM
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You can't add a C with only 4 wires. You need......
Red = 24v (R)
White = heat (W)
Green = blower (G)
Blue = A/C (Y)
You need a 5th wire for C

Are you sure there aren't more wires in the stat cable ?

What stat are you trying to install ?
There is a way of getting C with 4 wires but you give up the FAN ON mode at the stat.
 
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Old 01-05-18, 03:28 PM
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Only 4 wires on each side of the tstat cable. Stripped more wiring back and nothing hiding. The unit is clearly ancient from the internals.

Honeywell RTH 6500 wifi thermostat which has no batteries and requires C.

Correct, I was running a 5th wire from the transformer 24v to the thermostat to act as C. The handler and thermostat are only about 2 feet apart and I already have a 5th wire fished through the wall. The issue is where on the handler would this new C hook to
 
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Old 01-05-18, 03:38 PM
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You can buy and install an add-a-c module in the future. That gives you a C connection on 4 wires.

For now..... and if you don't mind losing the FAN ON mode forever........
Keep the small red wire where it is and on R.

Keep the small white wire where it is and on W

Keep the small blue wire on Y and connect to one wire going to the condensor and to the heavy green wire.

Connect the small green wire to C at the stat, the other wire going to the condensor and the heavy blue wire.
 
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Old 01-05-18, 03:55 PM
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Did that

red to red
white to white
green large/blue small (yellow tstat)/1 condenser wire
blue large/small green (now C)/1 condenser

Tstat has power and set to auto but unit doesnt come on heat or AC making sure the temp is above/below the current. The tstat clicks like its connecting to turn on the unit but unit doesnt fire up.

No blown fuse and can hear the slight transformer buzz so I know there is power to the handler unit.
 
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Old 01-05-18, 05:15 PM
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Do you have Rc and Rh jumpered at the stat.

You'll need to check the lines for 24vac. You may have damaged the stat with the shorts on it.

Check for 24vac from large blue to large red (C to R)
In heat mode.... check from large blue to large white (C to W) for 24v
In cool mode.... check from large blue to large green (C to Y) for 24v.
 
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Old 01-05-18, 05:20 PM
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Yes the jumper is in. I dont have a multimeter with me but Im sure it's not the tstat; it still works quickly touching the 24v transformer blue to tstat C and turns the unit on/off.

Could it be the system being so old requires G energized for the fan to run the AC/heat?

But back to my original question, why is a wire from the blue wire 24v prong of the transformer/ground to C on the stat shorting the system the way it was wired originally? Is that not where C comes from (as far as everything I understand on the subject about it)? I have a 5th wire run to the tstat for C, but it shorting the system somewhere on the handler side.

ie. doing the attached but is for some reason blowing the fuse
 
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Old 01-05-18, 05:27 PM
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You need to keep something in mind. All terminals and colors in the thermostat are switched to 24vac. The C is like the negative of the system. No terminal should every be shorted to C.

Are you trying to say that the heavy blue wire is not C ?
In many units one side of the transformer secondary is connected directly to the metal chassis. This is ground and common. Shorting the C to any colored wire at the air handler can create a short.
 
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Old 01-05-18, 05:42 PM
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My understanding is on the handler, the 4 wires are red (24v), blue (which is yellow at the tstat and thus A/C and where the condenser was hooked in series, not C/ground), white (heat), and green (fan). The tstat connects red 24v to yellow for AC, red 24v to white for heat, and the green gets in there as well to run the fan for both

I meant blue as in the blue wire coming off of the transformer in the diagram; the 24v/ground connection marked in my diagram. That is where I had run a new C wire where my blue line is drawn. So I had this hooked up to C on the tstat with R unit as R, G unit as G, B unit as Y, and W unit and W. And got a blown fuse.

Clearly this tstat is not going to work with this handler as I read some older ones require the fan to be energized for heat/AC to run which wont be an option using G as C.

So Im looking for some other solution. I have a 5th wire run but no clue where on the handler to hook it up. There must be some way here.
 
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Old 01-05-18, 05:52 PM
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We are not on the same page here. You again mention a yellow wire,
What are your four thermostat wires ??
 
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Old 01-05-18, 06:01 PM
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Red, white, green and yellow terminals are used on the manual thermostat; the standard colors. They ran a thick blue from the handler and wire nutted to 18 guage blue tstat wire that hooks to the Y terminal in the tstat. There is no yellow colored wire present anywhere. It appears this blue is the yellow wire and just not standard color when wired.

Red handler wire hooks via wire nut to red tstat wire
Green handler wire hooks via wire nut to green tstat wire
White handler wire hooks via wire nut to white tstat wire
Blue handler wire hooks via wire nut to condenser 1 and condenser 2 to blue tstat wire into the Y tstat terminal

I was trying to add a 5th, or C wire, from the 24v/ground point in the handler through the wall to the C terminal on the tstat.

But doing that caused the blown fuse and Im not sure why.
 
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Old 01-05-18, 06:09 PM
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Red handler wire hooks via wire nut to red tstat wire - correct
Green handler wire hooks via wire nut to green tstat wire - correct
White handler wire hooks via wire nut to white tstat wire correct
Blue handler wire hooks via wire nut to yellow tstat wire (and has the condenser wired in series) - wrong

The yellow stat wire goes to one of the condensor wires. (this is the switched 24vac TO the condensor)
The other condensor wire goes to heavy blue. (this is the ground or common to the condensor)

 
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Old 01-05-18, 06:11 PM
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Got it. Then how do we get C wire connection to power this tstat

Have no other option but use to old battery powered tstat or get an add a wire box?
 
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Old 01-05-18, 06:22 PM
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You mentioned you ran a 5th wire or did I misread that ?
(Trying to juggle many threads on multiple sites.)

I had explained a way to do it by losing the FAN ON mode thinking your unit had a sequencer. It doesn't. It requires a blower command for heating and cooling.
 
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Old 01-05-18, 06:24 PM
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Yeh I ran a 5th wire.

The unit doesnt turn on with using the G as C, it appears it needs G to use AC or heat (whatever the technical term is.)

If I'm understanding what you were saying, putting everything back to stock, cant I just connect that 5th wire to the blue thick wire which is C to C in the tstat then?
 
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Old 01-05-18, 06:27 PM
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If I'm understanding what you were saying, putting everything back to stock, cant I just connect that 5th wire to the blue thick wire which is C to C in the tstat then?

Yes. That was easy..........
 
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Old 01-05-18, 06:33 PM
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Lol. Thanks. I can at least say I learned a lot about HVAC now
 
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Old 01-05-18, 06:45 PM
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I had that original picture in my mind and it confused the issue. Sorry.
Glad you learned something. That's most important.
 
 

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