Ask Me Help Desk - Wiring water heater switch

I want to replace my water heater switch with a timer. I need to identify which live wire comes from the mains and which leads to the heater.
I unscrewed the switch from the wall to test both wires with a test-pen. With the switch at ON position - the pen lit up each time as expected. Then with the switch at OFF position, I tested the two wires again, thinking the one to the heater would not light up the test-pen, but it did. There was no difference with the switch at either position. How come? How then do I identify them? Please help!
Thanks.

First I would recommend using a voltage meter and not a test pen. What switch are you turning off? You should be able to turn off the circuit breaker or fused disconnect box that feeds the heater and not have any voltage at the switch or heater. Some hot water heaters use a night rate conductor that comes in from the electric meter and if you don't understand 230 volt electrical, you may want to consult a electrician. Where is your switch located? Is it by the heater or your main electric service? What type and what color wires do you have in your switch box, is it cable, conduit, two blacks, and two whites?

How big is the water heater? The way you describe the switch it sounds like a 120 volt model. Maybe in the 10-20 gal range.

Jimmy

Which side of the pond are you on? English and American wiring is different. Most larger American water heaters have 2 hots 120 volts above ground 180 degrees out of phase and 240 volts to each other. I hope you aren't planning to replace a defective thermostat with a switch.

Quote:

Originally Posted by diynewbie

I want to replace my water heater switch with a timer. I need to identify which live wire comes from the mains and which leads to the heater.
I unscrewed the switch from the wall to test both wires with a test-pen. With the switch at ON position - the pen lit up each time as expected. Then with the switch at OFF position, I tested the two wires again, thinking the one to the heater would not light up the test-pen, but it did. There was no difference with the switch at either position. How come? How then do I identify them? Please help!!
Thanks.

Since you said wire, I will Assume 120 volts. For 240 volts,(You should switch both Hots)
Verify at Name Plate, or meter. You have voltage with switch off, Because of other conductor(Neutral for 120 already goes to water heater, through Element and Back to Switch.

Turn thermostat on water heater to Cold(All the way counterclockwise, usually) to open the return circuit. Now only one should Be Hot(Line side)You may need a neutral for timer to work. If battery powered, Connection should not matter. Good Luck

From a PM:

''Hi guys,
Thanks for your prompt responses. Things have become more technical than I expected. Let me try explaining again.
Incidentally, I hail from Singapore - way yonder. I'm trying to replace the wall switch for my heater with a timer-switch which is supposed to save energy because it cuts off automatically after a preset time (up to 20 min.) When I unscrew the wall switch, I see 2 sets of 3 wires: black (Neutral), green (Live) and yellow/green (Earth). I need to identify which set comes from the mains in particular the green one. Is there a way for me to tell? I'm not sure if the electrical system here differs greatly from the US system other than that we operate on 220-230V. The timer-switch I'm installing is quite a separate thing from the thermostat.
Thanks.''

Oh, the other side of the other pond. The wire colors you give could kill somebody used to the American. It sounds to me like what you have is the black and green both hot and of opposite poles like an American water heater where the black and white are both hot. In which case, with the switch off, power would flow through the other wire to the heater, through the element, and back to the switch. A voltage detector (test pen) would find both wires still hot. What you could do is turn the thermostat down as Stratmando said. Recheck for voltage at the switch. The wire to the heater should now be dead.

Where you have 2 hot wires it is best to use a 2 pole switch. A single pole switch will work if that is what you have available as the timer.

It is the policy here to answer questions in the public forum. Such answers are available to help others and to be critiqued. It is your best protection from those sounding like they know what they are doing, but are posting poor answers. Perhaps with more information available, tkrussel will give an answer. I don't trust some of the others here.

I need to correct. On 120 volts you would NOT be getting voltage backfeeding. 240, You would, If heating.

If not done yet, connect wires at switch. Install Timer at water heater. Should be easy to determine line and load.
Note: What I do find is sometimes, on this site is, not enough info in questions, The replies sometimes, sound stupid. As a troubleshooter, like the others are here to help, no money in it, Have seen enough Bad Service people Take advantage of the uninformed, Good People. This is my way of trying to balance it out, Plus you always can learn something.

Quote:

Originally Posted by diynewbie

I want to replace my water heater switch with a timer. I need to identify which live wire comes from the mains and which leads to the heater.
I unscrewed the switch from the wall to test both wires with a test-pen. With the switch at ON position - the pen lit up each time as expected. Then with the switch at OFF position, I tested the two wires again, thinking the one to the heater would not light up the test-pen, but it did. There was no difference with the switch at either position. How come? How then do I identify them? Please help!!
Thanks.

So sorry for not keeping you posted. I managed to sort the situation out with some help from the local manufacturer
Thanks for your kind responses.