1000 Ω resistor

Alan (California Radiant) Forbes

If you only have 2 wires going to a power stealing thermostat, you can place a resistor between the "C" and "W" terminals at the controller and and it will trickle current back to the thermostat to keep the rechargeable battery charged. That is hard for me to understand given that the "R" and "W" wires connect when there's a call for heat and you would expect a connection between "R" and "C" to short, no? Is this "trickle" of current not enough to cause a short?

Help me understand this phenomenon, please. It seems like magic.

ChrisJ

Alan (California Radiant) Forbes said:

If you only have 2 wires going to a power stealing thermostat, you can place a resistor between the "C" and "W" terminals at the controller and and it will trickle current back to the thermostat to keep the rechargeable battery charged. That is hard for me to understand given that the "R" and "W" wires connect when there's a call for heat and you would expect a connection between "R" and "C" to short, no? Is this "trickle" of current not enough to cause a short?

Help me understand this phenomenon, please. It seems like magic.

1K ohm and 24 volt is only 0.024 amps or 1/2 a watt.

mattmia2

brown black red

HomerJSmith

Alan, There wouldn't be short. One lead of the transformer goes to the R connection on the thermostat. The electrical energy goes thru a charging circuit from the R connection to the C connection. The energy goes from the C connection to the other lead of the transformer. That would be a 3 wire setup, a circuit.

In a two wire setup, one lead of the transformer goes to the R connection on the thermostat. The electrical energy goes thru a switch to the W connection. The energy goes from the W connection to a relay winding which activates the relay switch and send power to another device (pump). The other lead from the relay coil goes back to the other lead on the transformer. That would be a two wire setup, a circuit.

For the charging circuit to work, the C connection must get back to the other lead of the transformer. So, the only way to get back to that other transformer lead is to connect to some wire that is connected to that other transformer lead. That would be the W connection in a two wire system. But remember, the W wire goes thru the relay coil. You don't want enough energy to activate the relay, so you trickle a little current thru the coil below the coil threshold.

In the thermostat base you would put a 1/4 watt resistor between the C and W connection. Taco put a 910 ohm charging resistor in their early model of their zone valves to keep the charging capacitor charged.

I have never done this.

You could measure the current flow. Remember, the idea is to charge the thermostat without activating the relay.

hot_rod

Alan (California Radiant) Forbes said:

If you only have 2 wires going to a power stealing thermostat, you can place a resistor between the "C" and "W" terminals at the controller and and it will trickle current back to the thermostat to keep the rechargeable battery charged. That is hard for me to understand given that the "R" and "W" wires connect when there's a call for heat and you would expect a connection between "R" and "C" to short, no? Is this "trickle" of current not enough to cause a short?

Help me understand this phenomenon, please. It seems like magic.

Curious where this suggestion is coming from? The thermostat folks?

ratio

Honeywell's installation manual is here.

Basically (and generically), you use the resistor to jump out electronic controls that might not have enough leakage through them to power the thermostat normally, while a heat call is not present. Once the heat contacts close, of course, the stat must run on stored energy.

NB: this is why Nest stats occasionally (and not so occasionally!) have trouble running in power-stealing mode. WiFi radios use an awful amount of power when they're transmitting. They simply run out of energy before the heat call goes away and they can get back to charging their onboard power storage.

Alan (California Radiant) Forbes

Hot Rod: It's an instruction sheet from Taco.

Solid_Fuel_Man

Without the resistor the Tstat is just a switch. With the resistor enough power to passed through to the T-stat so it can run/charge it's on board capacitor (short term battery). During a heat call, the stat makes contintinity and must run off its stored energy in the capacitor.

Power stealing stats are kind of a red-headed-stepchild..... 3 wires are really needed to have a good electronic thermostat.

Tim_D

Every circuit must have a power supply and a load. Most also have a switch. The resistor is the load and therefore there is no short.