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replaced thermostat, now ac will not work, did lots of research, have it narrowed dow

replaced thermostat, now ac will not work, did lots of research, have it narrowed dow

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  #1  
Old 10-23-06, 07:55 PM
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replaced thermostat, now ac will not work, did lots of research, have it narrowed dow

I replaced my old theromostat with a newer digi one. Like any DIY newbie, I managed to mangle up the job and shorted the two power wires (5 wire system central air/heat) together a few times.

After frying the first digital thermo because of my newbie mistakes, I began work installing the second one, this time knowing what I was doing.

Heat comes on, works ok, fan comes on when put in on position, but ac does not come on.

I researched, and found that a 24 volt signal is sent down the wires to the thermostat. I opened the furnace and found where the wire bundle comes in from the thermostat. It goes into a small circuit board of some type where the wires from the thermo are screwed down into this board, and the board, in return is bolted down to some sort of transformer, or the sort.

Well, with my voltmeter, I got a 28 volt signal from what obviously had to be the heat wire, however, on the other terminal, I'm getting nothing, dead coming right out of that screw on the board in the furnace.

what causes the signal to be lost, and how do I get my 24 volt signal back so I can get my ac going in this very hot so cal october??

thanks ahead of time for the help!!
 
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  #2  
Old 10-23-06, 09:44 PM
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ok, first thing is set the t-stat to cool and check at the circuit board terminal between R and Y if you have 24 vts. your t-stat is not closed and not callling for AC, but understand most digital t-stats have a 5 min time delay built in to act as a surge protector. If you don't have 24vts across R and Y check from Y- C on the board for 24vts. If you do, go outside to the condensor and check the wiring outside.
 
  #3  
Old 10-23-06, 10:19 PM
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OK, attached is a pic of what is going on.

the lead in the upper right corner was the one that has constant 28 volts to it, and I assume that's the heater lead?

Now, I did what you said, and put one probe on the yellow, and the other on the red, which is the bottom right screw, and I got nothing.

I'm not sure what "c" is, so I put one probe on the yellow, and touched the probe to all the other screws and got nothing, except for the 28v I've been getting from the upper right one, is this right?

In case it's hard to tell from this pic, yellow is the lower left one, green the middle one, and white the upper left.

thanks again for your help, I especially love being able to learn stuff and fix things on my own.

edit: now that I think about it, c = common or ground right? I tried one probe to ground and the other to yellow, and got nothing.

ok, pic didn't work, please click here

http://www.50stangs.com/picturehosting/jeremy/heater%20ac%20wiring.JPG

 
  #4  
Old 10-24-06, 07:07 AM
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OK, Lets talk a little about the basics. Your transformer has a hot and a common. The common is 0 volts, it's basically the return for the 24 volts to complete the circuit. Next, lets talk about the yellow wire. It goes to the relay coil of the condensing unit (outside unit). You do not get power from this relay. It is the job of the thermostat to supply power to the yellow wire.

Next, is the white wire, on a gas furnace it connects to the gas valve. Again, the stat is supposed to supply power to this wire on a call for heat. It's the same deal for an electric furnace, except a relay is substituted for a gas valve.

Next we move on to the green wire. This controls the blower. Again, this is done through a relay and it's the job of the thermostat to send 24 volts through this wire on a call for a/c or a call for fan "on" at the stat switch. There is a bit more information on this, but we'll leave that alone for now.

Next is that red wire. This one is easy. It connects to the hot side of the transformer and supplies the 24 volts to the stat. Your voltage will vary and will appear higher under no load conditions. That's ok, I won't go into the detail as to why right now.

Now that the wires have been defined, we are set to go. Right? Well not exactly. If you have an old mercury stat you would be. But, you switched to a digital stat. To power the electronics in this stat you need that 24 volts (the red wire) and a common to return that power back to the transformer to complete the circuit for the board. The "c" terminal of that stat needs to go to the common of that transformer. Looking at the photo, I can't tell if any of the stat cable wires are connected to it. If you look at the transformer, the hot is on the upper right, the common is on the lower right just below the hot. If no wire from the stat cable is connected here, then use on of the unused wires. It looks as if the blue wire is available.

In summary, connect red to "r" terminal on stat. Connect white wire to "w" on stat. Connect yellow to "y" on stat. Connect green to "g" on stat. Connect whatever color wire that goes to the common of the transformer to the "c" terminal of the stat.

This is the basics, you may need to follow the stat's instructions for additional information. This may include a possible jumper for "rc" and "rh". In most cases this is already done. Or forced blower on for some electric furnaces.

I also suggest you shut down the power to the furnace when connecting stats. You could easily burn out your transformer or damage your stat.

I think this will get you going. Good luck.
 

Last edited by jim-connor; 10-24-06 at 07:21 AM.
  #5  
Old 10-24-06, 07:59 AM
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Wink

Jim has it all layed out for you

I did what you said, and put one probe on the yellow, and the other on the red, which is the bottom right screw, and I got nothing.
sgthvac
Wanted you to try a jumper from R to Y at the tstat. This should have turned the AC on for you but not the inside fan
 
  #6  
Old 10-24-06, 09:16 AM
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I really appreciate all the help. This makes complete sense, and I understand how it works.

I've already had everything wired like you stated. The common wire was already wired up, it's a brownish or lighter red color (i'm colorblind slightly) and it is indeed wired to that common terminal at the furnace.


I then did as ED suggested, and jumpered the yellow and 24V red wire together, and the compressor indeed came on.

What does this tell me? I'd guess either a bad t stat or bad common wire? Thanks all, I'm finally getting somewhere on this!
 
  #7  
Old 10-24-06, 09:20 AM
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Id say more like a bad tstat. when you did the R to Y you jumped around the tstat. The C goes from the transformer to the compressor. You just closed the power to to compressor by the jump R to Y.

ED
 
  #8  
Old 10-24-06, 09:27 AM
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what a joke, it would have been handy if the instructions told me to power off before connecting, but oh well, this will be the third honeywell unit i've tried, and it's a $79 piece too, one of the better units on the shelf. Perhaps I should try a better brand?

Bummer but I'll head BACK to home depot and return it again, and keep you all posted.
 
  #9  
Old 10-24-06, 09:42 AM
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also, just a general question, so as to understand this system a little better, you stated that the common was to return the 24 volt signal back to the transformer, if that's the case, shouldn't it be reading 24 volts at the transformer on the common terminal? I know this isnt' the problem as the heater works fine, not the ac, so that tells me the 24 volt loop is ok, just trying to get a better functional picture of this.
 
  #10  
Old 10-24-06, 12:59 PM
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It would be easiest to think of Common as sort of like a ground connection. It can never be above zero. If your 24 volt hot was connected directly to common, you would have a short. Still, common will be zero. If the 24 volt hot was connected to one wire of a light bulb and the other wire (of the light bulb) was connected to common, you would have a complete circuit. The light bulb will light. If you connected a volt meter across the light bulb, you will measure 24 volts.

I hope this answers your question.
 
  #11  
Old 03-11-07, 11:16 PM
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this thread is back from the dead....I gave up on this because of winter, and the heater was working, however it's in the mid 90's in cali, and it's HOT.

I've now gone through several tstat's (all of the same brand) and NONE will turn on the a/c...however when I twist the yellow, green, and red wires together the a/c works fantastic.....I'm using it in this ghetto fashion to cool the house, but I'm just finding it hard to believe all those tstat's are garbage....

compressor comes on, house cools, maybe the tstat's are all garbage? Any more ideas here???thanks so much..Jeremy
 
  #12  
Old 03-11-07, 11:56 PM
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Ok,I'm learning more and more, I whipped out my voltmeter and put it on the ohm mode so I could hear when it has a closed circuit.

I then took this honeywell tstat, and turned it to the heat mode......on the back, the red and the white pins ohm out, telling me that it's supplying power to the white, or heater wire...

when I put the a/c switch on, it ohms out across the green and yellow pins, but not the red pin.....

and to make this better (the fan doesn't work when put into the "on" position) when I turn the switch to fan "on" the green and common pin ohm out.

is it me, or does this sound all wrong? I'm guessing the heat one is the only one that works right, and if I'm getting this right, when the cool is switched on, the yellow and red pins should ohm out, when the fan is on, the red and green pins should ohm out, and when the heat is on, the white and red one...correct???

so given that my theory is correct, perhaps I need to go back to home depot armed with my volt meter and rip every package apart till I find one that works??

Additionally right now I"m using the a/c by twisting the green, yellow, and red wire together....should I also be twisting the common in with it too??

thanks all for the help again, Jeremy
 
  #13  
Old 03-12-07, 06:31 AM
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NO do not add comon to your red, yellow, green hookup! Red and whatever color is common are the secondary winding of the control voltage transformer. If you tie red and your common color together, you will burn out the transformer because you will not have any current consuming devices wired across the transformer. Exactly where are yuo tieing R Y G together? Is it where the wires come out of the wall where the transformer is located? If so, then the thermostat is the problem. Honeywell is a good brand of thermostats so I suspect that your not hooking it up properly or you may have the wrong thermostat. You mentioned that you have a furnace so I'm thinking you have gas, oil or electric heat? If so, make sure your not trying to install a HEAT PUMP thermostat into your system. The thermostat is simply a switching device. It makes a circuit from R to Y to bring on the cooling, R to W to bring on the heat and R to G to bring on the indoor fan. So your theory is right. The red wire goes to the R terminal, the white wire goes to the W terminal, the green wire goes to the G terminal and the yellow wire goes to the Y terminal. I suspect the reason your not getting continuity between R and Y on the thermostat is because the stat probably has a time delay and has to be powered in order to work. What is the model number of the thermostat you using? Also, when you have just the wires sticking out of the wall and everything is turned on, do you have 24 volts from red to the fifth wire? blue wire? Jim laid it all out and I'm just repeating. Check for the RC to RH jumper or see if there is a dip switch on the back of the thermostat that needs to be switched to match the type of heating unit you have.
 
  #14  
Old 03-12-07, 08:04 AM
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yes, I'm twisting the wires together right where the thermo plugs in at the wall.

I definetly do not have it wired up wrong, it's plain as english, and has the color codes where to hook each wire. I've checked, rechecked, checked again, and rechecked again and again and again.

I ohm'd out the actual pins in the back of the thermostat itself, and that's where it's switching all wrong.

I'm not convinced honeywell is a good brand from my experience, I've been through several, and each time the thermo has been the problem......

I guess I have a mental block against that common, I don't understand how it comes into play if I just twist the red, yellow and green wire together, and the thermo never shorts out the common wire, It seems as if it never gets used? I understand the concept of the 24v return, but how does it return anything if it never gets tied into anything?

anyhow, it is a gas furnace, the thermo i'm using is a honeywell rth6300b1005, i'm quite disappointed that a near $100 thermostat is nothing but a pile of junk...I'm seriously about ready to go out and buy another $9 manual dial type thermostat, this is silly..Jeremy

oh, as well, I'm familiar with the time delay, it's a five minute time delay or so....at the end of that 5 minute time delay, I hear the click, then the only pins on the thermo that are shorted out are the green and yellow pins, NOT the yellow and red pins, which indicates to me it's working improperly.

when I hook my volt meter from the red to common wire directly, I get about 28 volts ac.
 
  #15  
Old 03-12-07, 08:25 AM
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You never have said for sure what you have there . AC? heat pump? furnace gas ? electric AUX.
Now R is power to R or jump RC &RH
W is heat
Y is cool or compressor
G is for fan
O is for the 4 way valve on a heat pump
C is common this is just for the tstat to work if you dont use Batt. in it
You see that everything already has the common to them . All the tstat is doing is send the R or power to them so they work.
 
  #16  
Old 03-12-07, 08:38 AM
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Originally Posted by Ed Imeduc View Post
You never have said for sure what you have there . AC? heat pump? furnace gas ? electric AUX.
Now R is power to R or jump RC &RH
W is heat
Y is cool or compressor
G is for fan
O is for the 4 way valve on a heat pump
C is common this is just for the tstat to work if you dont use Batt. in it
You see that everything already has the common to them . All the tstat is doing is send the R or power to them so they work.

gas furnace with central air, standard central air (electrical I believe)
 
  #17  
Old 03-12-07, 10:33 AM
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Ok, finally got this figured out...lol first time a phone call that wound up routed to an indian tech center actually worked for me....

found out this particular thermostat doesn't use the common wire, and the rc terminal is for something else...put the jumper back in between r and rc, and everything works.....well now if the instructions were written like they should have been, I would have had air conditioning 4 months ago, lol....

anyhow, I MORE than appreciate all of your help, i've gained quite a good knoweledge of how these systems work..Jeremy
 
  #18  
Old 03-12-07, 10:58 AM
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Its all there in the paper work. When you have only one transformer for the whole set up you go to R or a jumper to RC RH . Now If you cam along later and say put in a new AC and it had its own transformer. THEN the furnace transformer R would go to the RH only and the AC - R would go to the RC only.
 
  #19  
Old 03-12-07, 11:06 AM
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it all makes sense now
 
  #20  
Old 03-13-07, 09:55 AM
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Your thermostat still uses common but not in the conventioal sense that your thinking. It's a "power stealing model" which means it is borrowing common from a backfeed circuit thru one of the inactive circuits. I kind of figured that the jumper was the problem. I think the confusion came in because it was labeled R and RC. RC means power for the cooling circuit. 2 more things to check for- if the is a slide or dip switch on the back for the heat, make sure it's in the right position. Also, if there is a cph [cycles per hour switch] make sure it is set to the recomendation in the install instructions. Glad to hear you got it working. Sometimes a small project turns into a learning experience!! Life would be dull otherwise!!
 
  #21  
Old 03-30-14, 06:54 PM
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OK...I'm having the same problem as jthompson listed above, with a couple of slight differences, (my 2 red terminals are jumpered).

Background:
Regular A/C [ single unit ], Gas Heat, Central unit.

Same thing, switched out my old mechanical merc-stat for a digi-stat [Honeywell], last fall when we replaced the compressor. New
compressor was installed fine, A/C & Heat all worked fine with old merc-stat. A friend told me that I should goto a digi-stat with
the delay to protect this new compressor.

Noticed that when I hooked it up the new digi-stat, heat worked fine, but when it called for A/C [after delay], the compressor comes on,
but blower fan does not. Talked to a friend in the heating business, he said the stat was bad, so I took it back, got another, same
thing. Later on in the middle of winter the main circuit board on furnace control unit got fried [ when it was -20 ], and so we replaced
that. It was not hard. I noted the wires from the control board for the stat. Also noticed there was a jumper for the type of stat
used "mechicanical or digital", and so I switched that jumper from mechanical to digital.

Still in same boat, heat works fine, A/C compressor comes on but no fan. Now, if I force the stat to "fan-on" instead of "fan auto",
then I get the cool A/C air, but the fan must be forced on with the A/C.


-A/C compressor Harness with 3 wires [ Red, Yellow, & Green ],
-Circuit Board (for the stat ), [ White, Red, Yellow, Common, Green ]
-Stat [ White, Red(2x Jumpered), Green, Blue, Yellow]


Again there are 2 harness
Stat (5-wire)
A/C Compressor (3-wire)

The blue wire in the stat harness is not connected on either end.

How its currently wired [ please excuse the use of under-bars used to preserve
spacing]:

[_Heat ] White[stat]<---------------->White[circuit board]
[Power]____RC[stat]<---------------->Red [circuit board]
[Power]____RH[stat] Jumper above
[__ Fan] Green[stat]<---------------->Green[circuit board]
_________________________________Yellow[circuit board] EMPTY
[ __ A/C] Yellow[stat]<-------------------------------------------------->Red[A/C harness]
[???????] Blue [stat] empty
__________________________________Common[circuit board]<----------->White[A/C harness]
_________________________________________________________________Green[A/C harness] empty


When I look at this, knowing just enough to be dangerous...
I question:
#1 Why is the Yellow from the stat attached to the Red from the compressor? [ I did not do that ]
#2 Why ISN'T the Yellow from stat attached to the Yellow on the Circuit Board? [ what happens if I do that? ]
#3 Why isn't the Green Wire from the compressor harness attached somewhere? [ Fan problem with A/C, Green not used, HMMM!! ]
#4 Where should the 3 wires from the compressor go (Red, White, Green).


Any Help would be Greatly Appreciated!!

Eric
 

Last edited by ericm24; 03-30-14 at 07:17 PM. Reason: Correct tabs & spacing
  #22  
Old 03-30-14, 09:23 PM
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Color codes vary but this is a common example.

stat - furnace- condenser is easier to understand than your diagram.
 
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