Need C wire for new Honeywell RTH8580WF

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JOE1934

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I have Base board hot water system.It has 5 thermostats with 5 zones. It operates with Taco SR504switching relay plus SR503 expansion relay. I can not use the power from the taco units and would like to use a external 24v source. I have tried and even spoke to Taco but for some reason I do not get 24v out of transformer. So the work around would an external 24v transformer. Right now I have a Red and White wire going to old therm. I have two extra wires in wall to use going to the boiler. If I use external 24v transformer,where do I put the two wires going into the RTH8580WF.I have an idea but would like some one with more experience to put there two cents .
 

Stuff

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It needs to be an independent transformer and you need to be careful with the connections. With an additional transformer you run the risk of doubling voltage (48v) which will destroy things. If you share the transformer among thermostats you need to make sure that the existing red and white wires are going to identical places on both ends.

One lead from the new transformer should to C on the thermostat and the other to R or Rc - you will need to double up to keep the current wire (or jumper) on R as well.

There is even a video for this -
 

JOE1934

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It needs to be an independent transformer and you need to be careful with the connections. With an additional transformer you run the risk of doubling voltage (48v) which will destroy things. If you share the transformer among thermostats you need to make sure that the existing red and white wires are going to identical places on both ends.

One lead from the new transformer should to C on the thermostat and the other to R or Rc - you will need to double up to keep the current wire (or jumper) on R as well.

There is even a video for this -
 
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Jadnashua

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The safest way to power the thermostat with a external 24vac transformer is to use an isolation relay. The existing R and W leads would go to the relay contacts, and you'd add wires from the thermostat's R and the C to the coil of the relay, and connect the transformer to those same points. THen, when the thermostat calls for heat, instead of completing the circuit to tell the furnace to turn on, it instead operates the relay that does the same thing. Both power supplies are then totally isolated. If your thermostat has both Rh and Rc terminals, if you remove that jumper, it probably will work if you do it like the video. Leaving it in place means 24vac from one source being jumpered to 24vac from another, and that can cause problems.

You need to read the instructions on the thermostat you have...some can handle an external transformer like shown, but not all.
 

JOE1934

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The safest way to power the thermostat with a external 24vac transformer is to use an isolation relay. The existing R and W leads would go to the relay contacts, and you'd add wires from the thermostat's R and the C to the coil of the relay, and connect the transformer to those same points. THen, when the thermostat calls for heat, instead of completing the circuit to tell the furnace to turn on, it instead operates the relay that does the same thing. Both power supplies are then totally isolated. If your thermostat has both Rh and Rc terminals, if you remove that jumper, it probably will work if you do it like the video. Leaving it in place means 24vac from one source being jumpered to 24vac from another, and that can cause problems.

You need to read the instructions on the thermostat you have...some can handle an external transformer like shown, but not all.
 

Mikerf12

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Can a tranformer be added and the R wire be doubled and the C wire added to the Terminal or could this cause damage?
 

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Can a tranformer be added and the R wire be doubled and the C wire added to the Terminal or could this cause damage?

When you have a second power source the two can be either in phase or out of phase. If installed out of phase the voltages add up so 48vac would be seen by the thermostat and if mis-wired your equipment could see 48 and cause damage there. If a thermostat has separate Rc and Rh then it was designed to deal with the 48v difference between terminals. If there is only a single R terminal on the thermostat then it was most likely not designed or tested with those voltages. It can have internal components rated at 24v that break down when exposed to 48v.

So possible but unlikely. If you proceed you can reduce the potential issues. After attaching the Rs together get a multimeter and measure voltage between the second transformer wire (new C) and W, Y, or G. If you read 48vac then swap the transformer wires.
 
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