Honeywell RA117A protector relay and wifi thermostat install help?


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Old 10-19-15, 11:28 AM
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Honeywell RA117A protector relay and wifi thermostat install help?

I have an older furnace that has a Honeywell RA117A stack relay coupled with an old mercury bulb style thermostat. Last winter, I had my furnace cleaned and I asked the tech if I could swap out the old thermostat for a wifi one and he thought it was possible.

I have a 3 wire cable that runs from my thermostat to the RA117A but only 2 wires are in use.

Within the RA117A there are 3 available terminals. R, W(T) and B(T). The W and B are currently the only terminals in use.

With the thermostat, the W terminal connects to the B terminal on the RA117A and the R terminal connects to the W terminal on the RA117A.

I purchased a Sensi wifi thermostat and need help wiring it up. I contacted sensi but they weren't any help.

Can I utilize either the R, W(T) or B(T) terminals within the RA117A as a "C" terminal to power the Sensi wifi thermostat?


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Old 10-19-15, 12:15 PM
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I'm still looking into it but it doesn't look like there is a true C (common) transformer connection which means your stat won't work correctly.

honeywell/resources/techlit/TechLitDocuments/68-0000s/68-0119.pdf
 
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Old 10-19-15, 12:27 PM
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I know in some cases others without a C terminal have been able to grab the needed power from another terminal but I am not sure if this is one of those cases.
 
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Old 10-22-15, 07:29 AM
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I figured if anyone could tell me if this was possible, Honeywell could, so I contacted them. Unfortunately, that turned out to be a waste of time. Honeywell told me to call an HVAC tech.

I looked through the schematics in the pdf you sent me but I don't understand them enough to answser my own question. I've done some research and leaned some manufactures refer to their C terminal as a B terminal instead.

What would/could happen if I were to attempt to install my thermostat and connect the unused wire on on each of the different terminals to see if it works?

Obviously I don't want to do anything that'll damage my furnace or thermostat.
 
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Old 10-22-15, 10:36 AM
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There is no clear C connection on that control. Connecting the stat would be risky. You would have to connect inside the control and I don't recommend that.

I would recommend you purchase a separate 24vac transformer to run the stat.
You would connect the two wires to the burner control and two to the power supply.

Many choices of transformers. One choice listed here.
amazon/gp/product/24vac wall adapter
 
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Old 10-22-15, 01:06 PM
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Thanks for the reply. I thought Venstar made the only adapter out there and I really didn't want to go that route. I didn't like the idea of a power cord hanging from the thermostat to a wall outlet. The adapter you mentioned is a much better solution. I'm going to purchase it right now.

Thanks again
 
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Old 10-22-15, 01:57 PM
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You lost me. I did recommend a plug in the wall power supply. It doesn't have to go under the stat but it does need two wires between it and the stat.

If you have a four wire thermostat cable you could out by the controller.
 
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Old 10-23-15, 06:33 AM
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Yes, I'm aware of that, however, the plug you reccomend will allow me to run an outlet in my basement below and wire it up through the wall to the thermostat with some 18/2. The Venstar option's already wired with a 6ft cord and would of been more work to hide the plug and wires.
 
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Old 10-23-15, 09:45 PM
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The best thing you can do is to take that stack switch primary out and kick it into the next county. Replace it with a decent cadmium sulfide cell primary controller. The stack switch is ANCIENT technology and because it takes so long to react it is prone to create a dandy environment for a furnace explosion.
 
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Old 10-26-15, 05:21 AM
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I still have the original furnace that was installed when the house was built in 1942. It's a massive gravity fed octopus style furnace. Eventually, I plan on replacing the whole furnace.

The stack switch was actually replaced last year so it's safer than the older ones. This one shuts down in 15 seconds if the burner doesn't light vs the older one that waited 90 seconds.
 
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Old 10-26-15, 06:55 AM
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Correct me if I'm wrong but the c is just a ground.... the r wire is the power. I just installed a thermostat yesterday. It doesn't have a c wire running to it but that is bc the ground is at the furnace control center side. If the furnace worked before as it stands there's no reason it won't work now. It just needs to common to complete the circuit.

If your worried about the c just ground it out to basically anything metal.
 

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Old 10-26-15, 12:27 PM
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Ok.... you're wrong.

On Goodman equipment usually the metal chassis ground is C. Same goes for a lot of other companies but it cannot be assumed. It needs to confirmed.

On most of the mixed voltage controls like this one.... the ground would never be used as a C connection. This is the schematic for the RA117A control. Look at the transformer. The wires on the bottom are the secondaries. You'll notice there is no direct connection between those points and the stat connections. That means there is no confirmed C connection here.

In this instance..... W and B are the stat wires. I believe the R connection is for a remote heat anticipator inside the old thermostat. Not used in most applications.


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Old 10-27-15, 05:04 AM
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From that diagram there is no common. I was merely suggesting if for whatever reason the thermostat required direct access to a ground due to its power requirements ect. To run one to a ground source. From the last conversations I thought we were talking about the thermostat side not the wiring as it applies to the furnace. I'd assume he not even mess with that.
 
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Old 10-27-15, 10:23 AM
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The thermostat requires a C connection. That means common with the transformer inside the furnace control.... not ground.
 
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Old 11-03-15, 05:51 AM
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Update:

I purchased the Elk TRG2440 24VAC adapter that PJmax recommended along with some 18/2 wire. I ran an electrical outlet in my basement just below where my thermostat is. Fortunately for me, we're in the middle of a remodel and the wall that my thermostats on was already opened up. I was able to easily fish the 18/2 wire from the thermostat down to the new outlet in the basement.

The only confusing part was the Elk adapter has 3 terminals. 2 AC terminals and one G terminal. After some digging around online I found I only needed to connect to the 2 AC terminals. Essentally, 1 AC terminal connects to the C-wire terminal on my thermostat and the other AC terminal connects the RC-wire terminal. Figuring that out was the toughest part of the install.

I've been using my Sensi wifi thermostat for a few days now and it works great.
 
 

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