bram
May 21st, 2008, 05:40 PM
I’m hoping you guys can help me with a thermostat wiring issue. I currently have a thermostat which controls separate heating and cooling systems with two separate 24VAC transformers. I have wires at the thermostat marked RH, RC, W, G and Y, and I know what those mean. I want to replace this stat with a model that is not equipped for two transformer operation (long story as to why, but I really want to use this particular model), so it has R, C, W, G and Y terminals. The new stat manual has a page on this situation, and recommends a relay running off the cool transformed that, when energized by current between R (R would be RC) and W terminals on the stat closes the circuit between the RH and W wires, starting the heat. Makes sense. Based on a recommendation, I picked up a RIBU1C relay to accomplish this.
The problem is (I realized once I got home) is that I have a 5 wire setup, i.e. no common C wire back to the cool transformer. So, I have no way wiring a relay between the R and W terminals on the new stat, because the W terminal isn’t connected to anything, so there’s no way for current to go “back” to the transformer and complete the circuit. So, the relay doesn’t energize at all. If, for a test, I wire the relay between R and Y, that works fine, because there is a path back to the transformer. The relay is energized all the time, and closes the NO/NC leads appropriately based on whether the stat is calling for cool or not.
I really can’t run a new common back to neutral side of the cool transformer, because it’s majorly inaccessible in a miserable crawlspace.
Thinking about this, I realized that my problem was that the relay was trying to use the R and W terminals for two separate things: It was stealing power from them all the time to run the relay, and it was sensing when the thermostat closed the cool circuit (i.e. when there was continuity between the R and W terminals) and using that to connect or disconnect the load side. The continuity part is OK, because the R and W terminals will have continuity whenever the stat is calling for heat, even if there’s nothing connected to them. So, I guess what I’m looking for is a gizmo that senses continuity across one “sense” pair of wires (R & W terminals on the stat), and uses that as a signal to switch another “load” pair of wires (existing RH and W wires to heating system), without taking power from the sense pair, as most relays do. I would be OK if it was powered by 24VAC off the heating wires on the load side, or if it got its power by some other means (battery, steal power from the cooling wires) but not from 110VAC, as I don’t have that accessible at the stat location.
Does such a device, kind of like a backwards relay, exist? Or is there some other trick I can use to accomplish this?
I really appreciate all replies. Sorry for the long post, I could probably have said all the same in a quick diagram, if only I could draw.
The problem is (I realized once I got home) is that I have a 5 wire setup, i.e. no common C wire back to the cool transformer. So, I have no way wiring a relay between the R and W terminals on the new stat, because the W terminal isn’t connected to anything, so there’s no way for current to go “back” to the transformer and complete the circuit. So, the relay doesn’t energize at all. If, for a test, I wire the relay between R and Y, that works fine, because there is a path back to the transformer. The relay is energized all the time, and closes the NO/NC leads appropriately based on whether the stat is calling for cool or not.
I really can’t run a new common back to neutral side of the cool transformer, because it’s majorly inaccessible in a miserable crawlspace.
Thinking about this, I realized that my problem was that the relay was trying to use the R and W terminals for two separate things: It was stealing power from them all the time to run the relay, and it was sensing when the thermostat closed the cool circuit (i.e. when there was continuity between the R and W terminals) and using that to connect or disconnect the load side. The continuity part is OK, because the R and W terminals will have continuity whenever the stat is calling for heat, even if there’s nothing connected to them. So, I guess what I’m looking for is a gizmo that senses continuity across one “sense” pair of wires (R & W terminals on the stat), and uses that as a signal to switch another “load” pair of wires (existing RH and W wires to heating system), without taking power from the sense pair, as most relays do. I would be OK if it was powered by 24VAC off the heating wires on the load side, or if it got its power by some other means (battery, steal power from the cooling wires) but not from 110VAC, as I don’t have that accessible at the stat location.
Does such a device, kind of like a backwards relay, exist? Or is there some other trick I can use to accomplish this?
I really appreciate all replies. Sorry for the long post, I could probably have said all the same in a quick diagram, if only I could draw.
