Some basic questions about wires and the thermostat


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Old 04-11-17, 09:35 PM
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Some basic questions about wires and the thermostat

Is it me or is there something wrong with needing AC in April in NJ : ) ?

We have 2 units - 1st and 2nd floor. Both are 5 years old.

My wife turned them on for AC (we have Honeywell Heat / AC thermostats that have 1996 date codes on them : )

1st floor worked fine,

2nd floor ran a bit and stopped. I know I'd have to fiddle last year with the 2nd floor thermostat sometimes.

Looking at it, there's pins on the backplate that mate up to the thermostat. 1 has a broken solder joint to the backplane. I'll solder that later.

1st & 2nd floor are radiator hot water heat system and the AC system - 5 wires total.

we have a 3rd thermostat for a back room that is heat only system.

Took the backplate off that and put it in place on the 2nd floor. hooked up the 5 wires on the 2nd floor - red & white for heat and red, green yellow for AC to that backplate and still no AC. And sliding the fan switch doesn't turn on the fan.

Some basic questions:

can I just join the red, yellow and green wires together to get the AC and fan to start? Yes, it'll keep running till we break that connection. But at least I'll a) get A/C so my wife is off my back and b) know that it's just a thermostat issue.

and joining them - if that doesn't work, are there voltages I should be able to see across them with a voltmeter?

The fact that the backplane had that broken solder joint and the thermostats ARE 20+ years old, that's likely the issue.... but my luck there's something else maybe.

Being that she heard the system on 2nd floor run a little means breaker is on, etc...
 
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Old 04-11-17, 09:38 PM
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The thermostats are honeywell / 34 model CT3400. Manual has copyright date of 1994, and again, date codes on the thermostats are 1996.

what are the odds the new ones we are going to likely buy will last as long as these!?
 
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Old 04-11-17, 09:46 PM
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If you want to check voltage.....
there should be 24vac between red and yellow
and there should be 24vac between red and green.

Connecting red to yellow starts the A/C condensor.
Connecting red to green starts the blower.

You can tie all three together as a test. If the test is positive.... replace the stat.
Don't use the wiring as a switch. The stuttering connection is not good for the compressor plus the thermostat has a time delay built in to protect the compressor from short cycling.
 
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Old 04-16-17, 08:26 AM
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To recap, this is 2 systems - heat and A/C are seperate units going into the same 20 year old honeywell thermostat.

The A/C is for the upstairs in the house. I have a seperate / same arrangement thermostat for 1st floor.

Checked voltages on 1st floor and got 24v across the greeen-red and yellow red of AC.

No voltage on A/C wires at 2nd floor thermostat. Went upstairs and in the (American Standard) unit in attic, there was what I thought was a jury rigged car fuse that was blown. Reading the schematic on the cover, I see they designed it that way : )

Replaced the 3amp fuse (the schematic said should) be 5 amp) and now the thermostat has voltage.

Joining all 3 wires together gets nothing. Joining green-red gets the system to go (just the fan I am thinking). If red-green are connected & fan is running, attaching yellow STOPS the fan?!

So could someone suggest next steps for how to troubleshoot the A/C not working (and even stopping the fan when yellow wire gets added to red-green?)

Any thoughts on what could have blown the fuse? I was fiddling with the wires / backplate - could the heat wires and A/C wires touching & mixing their own 24v pop the fuse?

THANks!
 
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Old 04-16-17, 12:57 PM
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Red and green starts the blower.
Adding yellow blows the fuse.

You either have a short to ground in the yellow wire or the contactor in the outside unit has a burned/shorted coil.

I recommend using a 3A fuse as a 5A fuse can push the transformer and cause it to blow out.
 
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Old 04-17-17, 07:50 AM
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PJ - So Sunday was your birthday? i was looking at your profile.

Belated Happy Birthday!

I googled how to diagnose contactor. Pages talked of measuring resistance across 24v coil and the 1 page I found talked of about 10 ohms.

On this problem contactor, I got something like 1 ohm. On the other, working system I have (1st floor system vs. 2nd floor), I got 17 ohms.

so some questions:

1) In my notes, the systems were installed in April 2008. This problem system had a failed contactor reolkaced uynder warranty 3 months later. Now this contactor failed about 9 years later. Any thoughts of there being a 'bigger' problem to cause 2 failures over this amount of time? Short cycling from a 20 year old thermostat that had a weak contact, couldn't help the situation, right? I'm replacing the thermostat.

2) Here's a picture of a nest in the electricals of the outside box. I'm sure with your experience you've seen more ; 0 but nothing you can do about these things, right? Not sure how they got in there though

3) a big question: So where (here in NJ or online) would you get a replacement contactor? Do you you have a brand recommendation?

4) probably part of question 2 - do you turn off the breakers on the outside (and inside?) parts of the AC system over winter? There's heat that draws animals over the winter if left powered on?

5) is there something you would do on the interlock contacts? some sort of grease? Clean contacts routinely?

6) the papers in the electrical compartment talk of 30amp and 40 amp contactors depending on model. I have 30 amp. Would you go to 40 amp?

THANKS!!
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Old 04-17-17, 09:20 AM
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Contactors are pretty generic. They come in both Single Pole and Double Pole configurations. From the picture of the OEM working system, it appears that you have a Single Pole contactor, however you can always use a Double Pole to replace a Single Pole. Personally, I prefer the Double Pole as that removes all power from the compressor & condenser fan when the contactor is "off". Just be sure to get a contactor with a 24V coil, as they come in different coil voltages. You should be able to find one locally (check with a local Grainger store or electrical supply store). They are also available on Amazon.com.

I don't power down my A/C unit over the winter. When it's "off", there shouldn't be anything generating any heat. A heat pump is a different story, but I assume you're referring to an A/C unit.

There's no guarantee that a 40A unit will last longer than a 30A one, but it certainly won't hurt. The coils are probably very similar, what's different between a 30A & 40A are the contacts.
 
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Old 04-17-17, 11:25 AM
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That's an older Siemens part but it's still available. Like Bob mentioned almost any contactor will work. They are not really a specialized part. One reason to stay with a singe pole contactor is in case a crankcase heater is being used.

40A have slightly heavier contacts but don't necessarily last any longer. Nine years is a good run for a contactor. They are under a lot of strain. The single biggest contact killer is being low on refrigerant and the contactor chatters.

I buy from a local parts distributor. They require a trade account.

SIEMENS-Condenser-Contactor-45EG10AJA/dp/B00DZPHDCU

Siemens-45EG10AJA-Replaced-Contactor-Voltage/dp/B00WN4PT2K

grainger/product/SIEMENS-24VAC-Open-Compact-Contactor-6DRC3
 
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Old 04-17-17, 04:12 PM
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THANKS GUYS!!!! Gotta say it again - the website is saying I was too short!

THANKS!!
 
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Old 04-17-17, 08:20 PM
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Sorry - Is there anything you put on the interlock contacts to keep them clean? not important?

The tabs / connectors on the capacitors, etc have rust on the visible surfaces. But metal to metal is (likely?) clean? Anything you put on those contacts or is rust unavoidable?
 
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Old 04-17-17, 10:33 PM
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Nothing goes on the contacts. They're pretty much self cleaning.
Any kind of oil or grease would act like an insulator and increase the contact arc.

Most contactors are built inexpensively and aren't that expensive.
I consider that a spare part to have on hand.
 
 

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