24v secondary 3amp fuse blows on heat and cold.


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Old 12-10-17, 09:26 PM
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Question 24v secondary 3amp fuse blows on heat and cold.

Hello to all,

I have a 2013 HVAC system with an indoor furnace and an outdoor AC/Heat pump unit (all electric). Inside the house, I have a Honeywell Thermostat (nothing fancy). Every time I turn on the AC or Heat the 3amp fuse blows. The fuse is located on the indoors furnace unit on the red wire coming out from the 240v-->24v transformer (fuse on the 24v red wire). There is no Circuit board on the indoor furnace (only on the outdoor unit). When I turn the fan on through the thermostat the fan works and the fuse is fine, as soon as I turn heat or cold on the thermostat the fuse blows. I looked up the wires outside an I can't seem to find any cut wire or similar. I have a multimeter and the transformer gives 24v (protected by the 3amp fuse). How can I tell if the problem is the thermostat the contuctor or the circuit board inside the AC unit/heat pump outside? All and every help will be greatly appreciated. Thank you all in advance.

David
 
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Old 12-10-17, 11:00 PM
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I would turn off power to the indoor and outdoor units.
Remove the 2 low voltage wire to the contactor.
Ohm across the contactor coil ( it should be around 13 to 16 Ohms)




Snap some pictures then disconnect the control wires to the outdoor unit outside.
Ohm each condenser control wire to the other and to ground.
( Probably the Y wire)



The short could also be a pressure switch wire rubbed against the door or a copper pipe...
 
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Old 12-10-17, 11:17 PM
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Welcome to the forums.

Doubtful its a thermostat problem. Since the compressor runs in heat and cool mode..... it more than likely an outdoor problem. Probably in the yellow wire which is the compressor line. The yellow wire is hitting ground.

I go directly to the air handler as all the wiring meets there. Look for the two thermostat cables. Then look for and open just the yellow splice. See if the fuse blows now or use an ohmmeter to check from the yellow wire to ground without opening the connection. You should measure around 15 ohms or so. If you see a number very close to zero.... it's shorted. There will be a yellow wire from the thermostat and a yellow from the outside condensor. You need to determine which one has the short on it.

You can go directly to the outside unit. You can pull the main power on the condensor when testing. Find the thermostat cable where it enter near the AC power. There will be around 5 colors used there..... red, white, yellow, orange, maybe a blue or black. You'll want to test or open the yellow splice.


In order to see save fuses.... we'll use the meter method. If you want to trial and error with fuses.... that's ok too but get extra 3A fuses DON'T get tempted to use a larger fuse. The transformer will blow out.

If you use an ohmmeter.... set it to it's lowest ohms scale. You are looking for a measurement close to a dead short which is around 1-2 ohms.

Wow.... a double response.
 
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Old 12-12-17, 05:18 PM
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Hi PJmax and Houston204

Thank you for the replies, and for your time.

Looking at your replies I went ahead and disassembled the cover for the outside unit, I have noticed the yellow and red wires coming from the furnace (junction) were nicked a bit. I am not sure however if that was the problem (I did recut at the nic just in case). I checked continuity on all the wires from the thermostat to the furnace and from the outdoor unit to the furnace (all beep ok). I do not have a circuit board on the furnace so all the wires there are connected with wire twist connectors.

I have five wires coming into the ac unit outside - White(W2), Blue(C), Orange(O), Yellow(Y) and Red(R). all clear for continuity. The wires go to the circuit board.

"or use an ohmmeter to check from the yellow wire to ground without opening the connection. You should measure around 15 ohms or so. If you see a number very close to zero.... it's shorted."-->> So here... I leave everything connected where it is and set my meter on resistance. Where do I touch with the prone? one ground and the other at the yellow wire connected to the board/twist connector? Do I disconnect at the furnace? or all stay connected.

The yellow wire comes from the thermostat and connects directly to the outdoors unit yellow.

I have bought a pack of 3amp fuses just in case (after burning through one transformer while using a 5amp fuse).

Not being able to buy any parts locally (no one will sell to a non HVAC/ELEC professional) makes this just that much more fun.

So many issues... In the process of shutting off and on the shutoff switch I found that it was bad (one main pole was broken inside the metal casing). Have to get a new one from HomeDepot.



As of Houston204 reply -->> I have taken the contactor off the unit and checked for resistance (none) so I believe that would be bad (the coil looks burned out). I have connected it directly to the 24v coming from the transformer and the fuse blew right up. Is this my problem or did I cause this somehow... (I have that tendency of breaking stuff). While manually pressing the contactor the fan runs on the outside unit. (it is a single pole 30a with a straight "shunt" #23 on it. FLA(32), VAC(240/277), LRA(150) RES(40)) The model number is HN51KC024. I have found online similar specs yet the model number is HN51B024 --> Is that ok to use the "B" model?

Looked at all the Pressure Switch wires and they all seem intact and properly channeled.



So... With your great help, I believe the problem is the Contactor. I ordered a new one and below are the following steps I would like to take.
  1. Install new contactor as the old one was.
  2. Rewire the outside unit as it was before (after recutting the wires below the nic).
  3. Rewire the wires from outside to the furnace and from the thermostat to the furnace.
  4. Installing new 3amp fuse.
  5. Replacing the old 240v Safety disconnect with a similar one (QO 60 Amp 240-Volt 7.4 kW Non-Fuse AC Disconnect).
  6. Turning the power back on from the main breaker panel >> then outside disconnect and inside disconnect.
  7. The last stage would be to pray for the best and turn heat on... (getting 0 degrees tonight)
I think I covered it all. Any advice furthermore would be highly appreciated, and I thank you greatly for your reply, and time spent on my problem.

David

PS. I am going to try and take a modified picture of my wiring (the upper lines of wires is my setup). Hope that would help.
 
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Old 12-12-17, 05:55 PM
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Looks like your computer has a virus. I can barely read your post
///<t>????(?)
 
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Old 12-12-17, 08:38 PM
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Hi Houston204,
Yes it seems like there was a formating issue (not sure its on my side). Tried to edit the post and Iwas denied. Please see below corrected post. Thank you again for your time.
Thank you for the replies, and for your time.
--------------------------------------------------------------------------------------------------------------------------------------------------------------
Looking at your replies I went ahead and disassembled the cover for the outside unit, I have noticed the yellow and red wires coming from the furnace (junction) were nicked a bit. I am not sure however if that was the problem (I did recut at the nic just in case).
I checked continuity on all the wires from the thermostat to the furnace and from the outdoor unit to the furnace (all beep ok). I do not have a circuit board on the furnace so all the wires there are connected with wire twist connectors.
I have five wires coming into the ac unit outside - White(W2), Blue(C), Orange(O), Yellow(Y) and Red(R). all clear for continuity. The wires go to the circuit board.
As for the Recommendation from PJmax "use an ohmmeter to check from the yellow wire to ground without opening the connection. You should measure around 15 ohms or so. If you see a number very close to zero...." So here... Do I leave everything connected where it is and set my meter on resistance. Where do I touch with the prone? one ground and the other at the yellow wire connected to the board/twist connector? Do I disconnect at the furnace? or all stay connected.
The yellow wire comes from the thermostat and connects directly to the outdoors unit yellow through a twist connector at the furnace.
I have bought a pack of 3amp fuses just in case (after burning through one transformer while using a 5amp fuse).
Not being able to buy any parts locally (no one will sell to a non HVAC/ELEC professional) makes this just that much more fun.
So many issues... In the process of shutting off and on the shutoff switch I found that it was bad (one main pole was broken inside the metal casing). Have to get a new one from HomeDepot. (just did... Installing now.)
As of Houston204 reply -- I have taken the contactor off the unit and checked for resistance (none there) so I believe that means its bad (the coil looks burned out as well). I have connected it directly to the 24v coming from the transformer and the fuse blew right up. Is this my problem or did I cause this somehow... (I have that tendency of breaking stuff). While manually pressing the contactor the fan runs on the outside unit. (it is a single pole 30a with a straight #23 on it. FLA(32), VAC(240/277), LRA(150) RES(40)) The model number is HN51KC024. I have found online similar specs yet the model number is HN51KB024 Is that ok to use the one with the "B" instead of "C" in it (they have the same specs).
Looked at all the Pressure Switch wires and they all seem intact and properly channeled.

So... With your great help, I believe the problem is the Contactor. I ordered a new one and below are the following steps I would like to take.
Install new contactor as the old one was.
Rewire the outside unit as it was before (after recutting the wires below the nic).
Rewire the wires from outside to the furnace and from the thermostat to the furnace.
Install new 3amp fuse.
Replacing the old 240v Safety disconnect with a similar one (QO 60 Amp 240-Volt 7.4 kW Non-Fuse AC Disconnect). DONE!
Turn the power back on from the main breaker panel then outside disconnect and inside disconnect.
The last stage would be to pray for the best and turn heat on... (getting 0 degrees tonight)
I think I covered it all. Any advice furthermore would be highly appreciated, and I thank you greatly for your reply, and time spent on my problem.
David
PS. I am going to try and take a modified picture of my wiring (the upper lines of wires is my setup). Hope that would help.
EDIT: Pss. I believe since I copy paste some text the code formating went into my post instead of what was suppose to. I dont believe it is a virus, just a glitch in the interface. Should be all fine now.
David
 
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Old 12-15-17, 01:00 PM
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Dave.... that's a formatting issue. It's usually caused when you post from a mobile device. The techs are still working on some of the mobile interface issues.

If you found your contactor with a burned coil...... that is your problem.
 
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Old 12-15-17, 07:59 PM
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Hi PJmax and all.
First I want to thank you all for the reply and time to help me with my HVAC issue, after replacing the Contactor with a replacement one all is now working great. (My wife is super happy.)
I have in the process learned so much on HVAC systems and electrical concepts.
During the repair I have found some faults I had above the problem posted (bad disconnect unit at the furnace, no ground connecrtion was done from the disconnect to the outside unit, and two nicked wires coming from the furnace to the outside unit).
I have also burned through one transformer by using a 5amp fuse instead of a 3amp one.
This had been a week long adventure of which four five days are just wainting for parts (No local store would sell me the parts as a DIY, so had to order online).
Total Cost is around $25 (Contactor and Transformer)
Estimate for repair by HVAC technician was $200-$600 with a visit cost of $60+/-

PS. PJmax >> One last question if you wont mind. If original contactor was FLA32 and replacement is FLA30 (thats seems to be the only difference between the two), would that be ok? Everything seems to work fine.
Again, thank you all for your time and help. The information given heped a lot on undertanding and correcting the problem.
David
 
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Old 12-15-17, 08:51 PM
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There are many contactors from different manufacturers that will all work in your unit.
The major factor is single pole or two pole switching. Other than that the only differences are in the way the wires connect. Different lugs or screw terminals or push-on terminals.
 
 

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