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Discussion Starter · #1 ·
I have furnace that is currently controlled by a simple electronic thermostat on the fritz, and an evaporative cooler. I'd like to combine the control into a modern wifi thermostat, but I am lost with the wiring:

The furnace is a simple 2-wire control. connect the wires and the furnace runs, disconnect and it stops. The thermostat I am using now is battery powered, so it simply opens and closes a relay.

The evaporative cooler is controlled by a "White Rodgers 8A18Z-2" Unit, which uses 4 wires (can't post a link here, but a description is available online. Google "White Rodgers 8A18Z-2" ) that connect to W, R, Y, and G terminals on the mercury switch. This switch also controls fan speed, and to switch between "cool" and "vent".

I have bought a Honeywell RT9580 WiFI Thermostat, but I can't figure out how to connect the wires.

2 complications: I can't find a c-wire anywhere, which I apparently need to run the thermostat, and

To avoid the cooler and the furnace battling to death, they are on a manual switch that only powers either the cooler OR the furnace. I suppose I would have to bypass that switch, as the thermostat would prevent both from running at the same time.

Hopefully someone here can shed some light on this for me. Thanks.
 

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The White Rodgers control provides some features that are not available on your new thermostat. Speed selection, evap pad pre-cooling and vent only option. If you'd like to keep these features the "easiest" way would be to have your new thermostat operate an isolation relay from the Y terminal. This relay would break the R connection between the W/R thermostat and control panel. You would have to have the W/R thermostat set where you like it as far as fan speed, lowest temperature setting and cool/vent mode. Basically your new stat would control power to your W/R stat and would not allow for dueling systems but would control the cooling. I'm not thinking all of this is worth the effort if pulling wires and installing a relay is a major pain.
 

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Discussion Starter · #5 ·
Thanks. Perhaps you are right. Do you know of any WiFi Thermostats that do support the features?

The other option might be to have the thermostat just control the temperature settings for the evap, but not the features? So, I would have the thermostat turn on and off the cooler, but control the speed and other features from the thermostat that is installed now. Is that possible? (And have the Wifi thermostat also control the furnace, of course).

Something like running the R and W wires to the WiFi thermostat and keep the others on the current thermostat?

And while I am at it (and you seem to be very knowledgeable in these things): IN AC systems you typically have a hot and a neutral wire. In 110V systems, the black wire is the typically the hot wire, the white is the neutral one. On the 24V transformer, the common wire is the black wire. Does that mean that the "common" wire is the hot wire? I am just wondering as I have to run a c-wire to the thermostat and want to make sure I understand it correctly. I saw a diagram that seemed to indicate that the common wire is the neutral wire.
 

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I don't know of any wifi capable thermostats that would control an evap cooler. The diagram below shows how to use your new stat to control your evap cooler using your existing W/R control. If you think you could do this without much headache go for it.
The primary side (110V) of the transformer the black should be hot and the white common/neutral. Do not connect the C wire from the thermostat to the primary side of the transformer.

 

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Discussion Starter · #7 ·
Hello 57Tinkerman. I really appreciate your help. Will mull this over and see what I do.

Thanks for the warning with the primary side of the transformer. But my question was slightly different: the SECONDARY side of the transformer has 2 wires, a black and a white wire. The black wire is labeled "common". I am just wondering if the common wire is the hot wire of the 24V line, or if the black/white convention of the 110V lines is not the same for 24V.

So, if I read your drawing correctly, in heating mode, the stat closes loop between W and R if the temperature drops too low, and the furnace turns on. In cooling mode, the stat the stat closes between R and Y, which in turnenegizes the Isolation Relay and fires up the evap.

The stat actually has an R and an Rc terminal (as it can handle a system that has 2 transformers. Wouldn't I be able to run the red lines that you drew directly to the Rc and Y Terminals of the stat? There is no Y terminal on the furnace (just a jumble of cables, it's old). And there is no C terminal on the Furnace either. I could look to see if I find a 24V signal there, but that goes back to my question about the C-wire. Is the C-wire the hot wire of the 24V AC or the neutral?

And if it is the hot wire, could I not simply bridge the Rc terminal to the C-terminal to (in the setup I though of) to power the thermostat?

I am trying to make a drawing and upload that. Let's see if that worked ...
 

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Yes you can use the RC and Y terminal. I used the isolation relay because your furnace is old and you may want to upgrade some day and use the G terminal (fan) on your new furnace. Just make sure you've isolated the RC and RH on the new stat. The C terminal will have to come from the furnace as the stat is powered by RH and C terminals. If your black wire on the secondary side of the transformer is common then use that for your C terminal connection. I'm aware that your furnace does not have a Y terminal just showing the Y terminal to isolation relay connection. See new drawing below, do not connect RC and C together.
 

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Discussion Starter · #9 ·
Perfect!!! Thank you.

I just have to figure out the common on the furnace. The transformer I was referring to was actually not the transformer on the furnace but a separate one that is on the furnace to drive the drum humidifier (we are not using that humidifier anymore, so the transformer is actually disconnected). I am sure there is a transformer on the furnace, but when I open the panel it is all a jumble of wires with wire nuts (as I said, the furnace is old ...). But it's a Lennox, so I will try to find out from them what the common wire is, unless you have an idea how to identify it.
 

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Discussion Starter · #11 ·
Never mind. I found some information on the web where to find the C-wire. Seems to ne the yellow wire on the secondary side of the transformer. Took a bit, as the furnace is older than I thought (1977) and Lennox did not have any information about it anymore ...
 

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After a bit of thought, using the G terminal (with isolation relay) will run the furnace fan while the evap cooler is operating. Don't think that's necessary or even wanted. So, for total separation of the systems don't use the G wire. And yes, by your diagram yellow is common.
 
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