C- wire in the oldest A/C ever 1978


Old 08-20-15, 07:01 PM
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C- wire in the oldest A/C ever 1978

Hello All,

I just bought a condo with a rooftop ac / heater combo installed on the roof deck. I recently purchased a smart thermometer and it's just my luck that no c-wire was run down to the original thermometer.

I've been doing a lot of research to figure out what I have, but am not sure the brand of the ac / furnace. I have learned a lot about contactor boards and transformers though!

I've checked the unit upstairs and it looks like I have tracked down all the wires I need other than the c-wire. My hunch is that I should use the neutral wire coming out of the transformer and that should be sufficient. It seems like the transformer is already powering a bunch of other devices up there, based on the number of hot and neutrals wires I see going to a number of different boards.

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This is the transformer. There is a set of terminals in the back with a lot of different red wires coming out which I assume mean these are all the hot wires. One is actually being used as the "R" wire for the thermostat. I'm thinking to piggy back another spade terminal similar to what was done to the top terminal to use as the c-wire.

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Here is the contactor relay board. One of the neutral wire is going there and the other end is the Y wire. Probably will replace this soon as well but surprisingly with the rust, it's still working fine (fingers crossed!)

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Here's a bunch of the other fun. Luckily a lot of the other wires are already labeled.

Please see all the pictures and let me know your thoughts.

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Old 08-20-15, 08:55 PM
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Welcome to the forums.

You may not find a brand or model. That could be a home brew custom built unit. Sort of like a science project.

Be careful handling the terminals on that transformer..... it looks like they may snap off easily. It looks like the bottom row of slide-ons is for the R or 24v side and the top row looks like the C row of slide-ons.

At your contactor looks to be a yellow wire which is Y on the stat. The other wire is the common C wire and should end up on the top row of the transformer.

That contactor is from an age when they were well built and designed to last. However, it does look like it's getting tired and could probably use a replacement.

One last recommendation.... in the last picture you have a big capacitor just laying there. All capacitors should be fastened so that they don't move and create shorts.
Old 08-20-15, 11:34 PM
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That PJMax.

I wouldn't be surprised if it isn't a bit of a frankenstein with the way the intake fan, a/c and heater are cobbled together. I was quite shocked and perplexed when I first opened the panel, expecting all the nice labeled boards that I saw online and ended up with this!

I'll be careful with the transformer terminal. I saw that the piggy back end terminal is fairly common and looking to get a 14-16 gauge one like this: Push-On Piggyback Terminals - Vinyl Insulated

You're right that bottom row does look to be all R and the front row seems to be the C. The contactor does terminate at the C as well. Interestingly, the transformer looks to be pushing out 30v of power. I asked the smart thermometer vendor if it would be an issue and they said it would be okay.

I'll let you know how the install goes, as there is a power extender kit that is provided with the Ecobee3 which allows me to connect the 5 main wires at the furnance and leave the 4 existing wires going back to the the thermostat and still operate appropriately.

Thanks for the advice on the capacitor. I'll look for a metal brace to secure it.
Old 08-21-15, 03:50 AM
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What make and model thermostat do you have?
Does it have a battery back up?

Most digital residential thermostats have the C connection but can run just fine from the battery.
I know many Honeywell models will.
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