Installing Honeywell RTH6500WF Wifi Thermostat "c" wire question


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Old 05-12-13, 09:54 AM
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Installing Honeywell RTH6500WF Wifi Thermostat "c" wire question

Hi, I'm trying to replace an old mercury switch 4 wire thermostat with a new wifi Honeywell rth6500wf. I do not have a "c" wire. I do not have any extra wires running through the wall to use as a "c" wire. I also do not have access to the AC unit itself to use the "g" wire as a "c" wire as suggested in the Honeywell alternate wiring videos.

I've seen online that I can take a 24v power supply, for example a replacement 24v power supply for a phone, and cut the connector off and wire those two wires to "c" and "rc" and that will work. The problem I have is I cannot find how many amps or watts the new thermostat can handle. I've found a power supply at radio shack, its 24v 1 watt. Will this work? Or is it too much power? Or not enough?
 
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Old 05-12-13, 10:17 AM
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Welcome to the forums.

As long as the transformer is 24 VAC the wattage isn't too critical. I would recommend at least a 5 watt transformer.

You found one at RS but 24v 1w doesn't sound correct. Maybe 24v 10 w ? That would be fine.
 
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Old 05-12-13, 10:46 AM
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Ok, here is the power supply in question, Enercell Switchable 18/24V/1A AC Power Adapter : AC Adapters | RadioShack.com

It's 24v up to 1 amp, which means it does around 24 watts right? 24v * 1amp = 24 watts

Would that be too much?
 
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Old 05-12-13, 06:51 PM
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no, its not too much..... You really can't oversize the power supply, you can undersize it though.
 
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Old 09-17-13, 09:23 AM
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im replacing a honeywell t7300f2010 with a rth6500wf there are two extra wires hooked up to the old one in the a1 and a2 connectors any ideas where they will hook into the new thermostat
i bel;ieve they are for the clean air exchange i have
 
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Old 09-17-13, 11:48 PM
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Welcome to the forums.

This isn't my department....

That's a specific function of that thermostat. I'm not sure if your new one can control it.
One of the pros should be able to help you.
 
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Old 10-19-13, 04:31 PM
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Same problem

Were u able to resolve your problem with the Radio Shack transformer.If not which transformer did u use.Does the thermostat work as advertised? Honeywell will not advise using this solution . Thanks sal
 
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Old 10-19-13, 04:47 PM
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Welcome to the forums.

What is the problem you are having there ?
 
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Old 10-21-13, 08:37 AM
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No C wire

I have the same problem as poboy975 I would like to know if the 1amp 24v from Radio Shack would work with the Honeywell 6500,even though the reps. At Honeywell say it is not recommended. Has anyone used this solution when there is no C wire?
 
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Old 10-22-13, 12:33 AM
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I spent an hour going thru Honeywell videos and tech specs and could find not any mention of a separate power supply causing an issue.

I see no problem in using that power supply. Just make sure it's 24 vac.
 
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Old 10-22-13, 01:13 PM
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Thank u! Will try it & let u know how it works
 
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Old 11-12-13, 10:50 AM
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It's a clever idea, but I don't think it will work. The "C" post on the thermostat is supposed to connect to the C or common lug on the furnace control board. Here's the quick run down on the letters and what they mean:
R = 24v supply (Some have Rh (heating) or Rc (cooling) which is why they can be jumpered.)
W = return wire for heating - usually white
Y = return wire for AC - yellow
G = return wire for fan control - green. This is why you can "steal" this wire for "C". When you do this, you can't run just the furnace fan.

The Honeywell Thermostat needs the "C" return because it doesn't use batteries. If you connect the R from the furnace and the "hot" wire from your external 24v power supply, you now have 48 volts entering the thermostat. If you just run the hot from the external power supply to the R and connect the neutral wire to the C, I don't see how you'll be connecting the circuit with the furnace to turn it on.

If you really have no other choice, you might try just connecting the C thermostat terminal to the neutral or common wire on the external power supply. It might just be able to power the thermostat that way.
 
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Old 11-12-13, 09:16 PM
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The C terminal is ONLY used for supplying power to the clock circuit. Some furnaces either don't have a readily available C terminal or there are not enough wires between the thermostat and the furnace or they don't want to give up the fan function.

If the external power supply is connected to C and R then there will only be 24vac on the thermostat. You couldn't use an external supply and a C connection.
 
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Old 11-25-13, 08:33 PM
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Would this work for a Honeywell Wireless tstat?

Hi everybody:

I read through this thread, and was wondering if I could get your opinion on the following scenario we have. I've got two programmable thermostats that I'd like to convert to wireless stats. There are long wire runs back to the furnace, and to run another wire for the C terminal would be tough.

I found this video online:

C Wire - How to power the thermostat via an external transformer - YouTube

The video talks about using a 24VAC adapter, and connecting one to the C terminal and the other to either Rh or Rc (whichever one is open). If this would work, this would be a great solution for us as there are power outlets right next to the stats. Any opinions as to whether or not this would be doable?

Thanks for the help!
 
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Old 10-22-14, 09:14 AM
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No power from C terminal to thermostat on RTH8580WF

When I installed my Honeywell Wifi thermostat I am having trouble getting power to my thermostat. I have two additional wires and fastened one of them to the c terminal on my furnace and the c terminal to my thermostat. I used a meter and found that there is no power. Any help?
 
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Old 10-22-14, 09:33 AM
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Did you measure for 24V across the R and C terminals? If there is not 24V there you may have blown the low voltage fuse on the control board. It is an automotive type blade fuse, either 3 or 5 amp and purple or amber in color.
 
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