I broke my heater - what'd I do wrong?
February 22, 2016 3:00 PM   Subscribe

I am in the process of replacing all of the 240V single-pole wall thermostats in my house with programmable thermostats. This has all gone well until I got to the very last thermostat, which I can't get to work. What'd I do wrong?

My house is exclusively heated by 240V wall heaters. I have been progressively replacing their thermostats with no issue. I got to the last heater of the house - the one I use the least - and neither the thermostat nor the heater turns on. I tried replacing the new thermostat with the old mechanical one and the heater still doesn't work. Unfortunately, I didn't try the heater out before replacing the thermostat.

There is a 12/2 line going into the thermostat box and a 12/2 line going to the heater. When I measure the voltage between the two conductors (expecting 240V), I measure ~0V. When I measure the voltage between each of the two conductors and the ground conductor, I measure ~120V. It seems to me that both of these conductors are connected to the same phase, which would make sense why the heater doesn't work.

I am relatively certain I have used this heater in the past successfully, so I don't really think that the wiring has been incorrect forever. However, I'm not entirely sure what I could have done in connecting the other thermostats to cause this problem. None of the other thermostats had their circuits "daisy-chained" - in particular, all thermostats had one 12/2 in and one 12/2 out (to heater) except one. The thermostat that didn't had two heaters on it, with two 12/2 in and two 12/2 out.

I can - and probably should - call an electrician here, but it seems like there isn't much I could have done to cause this problem. Any idea what I might have screwed up?
posted by saeculorum to Home & Garden (9 answers total)
 
you did check ground is ground?
posted by andrewcooke at 3:15 PM on February 22, 2016


Response by poster: I haven't, but neither the thermostat (all plastic) nor the heater (in-wall) have a ground connection. The ground conductor is just capped unused (except for measurement).
posted by saeculorum at 3:22 PM on February 22, 2016 [1 favorite]


Are these all on the same breaker with this one the last in line?

If so, I'd guess you somehow mildly screwed up at the one before and connected both of the two wires that lead to this one to a single side of the one before.
posted by jamjam at 3:34 PM on February 22, 2016


They wouldn't all have to be on one breaker I suppose, just one other at least on the same breaker as this one.
posted by jamjam at 3:36 PM on February 22, 2016


Best answer: Here's a scenario;

The heaters may be hooked up to a single circuit in parallel (to keep the voltage constant). If so, there should be two wires going into each heater and two wires coming out. Hot In, Hot Out - Return (Neutral) In, Return (Neutral) Out. One wire will go straight to the heater from the panel, and one will go to the thermostats before going to the heater.

The thermostat should be on a branch of the hot side. The key is that there has to be an additional wire to bridge the hot wire around the thermostat to the next heater otherwise the first thermostat will control any heaters further down the wire(s). This is true for all the heaters except the last one.

Since the two wires in the thermostat are on the same side of the circuit, same as a light switch, there shouldn't be any potential (voltage) between the wires. Voltage will appear across the Hot and the Return (Neutral). There will also be voltage between Hot and Ground, since Ground is part of the return to earth of generated electrical power. But only one of the sides if the circuit passes thought the thermostat. The exception is a double pole thermostat with both sides passing through - But that would have four wires- two in two out and you say only two wires are shown.

Have you tried shorting the thermostat leads together to see if the heater comes on? Or turn on all the thermostats to see if the heater in question comes on? Including the green/bare ground wire there should be three wires going to each heater. You say that the grounds are capped so that means that there should be two other wires hooked up to each heater.

Check the wires in the last heater to see that (aside from ground) there are two different colours - black/white or red/black for example. Also all the thermostats should all have the same colour wire going to each. Example black wire in, black wire out. The heaters should have two different colours going to each.

Whoa, this answer got long quickly. Apologies for the wordiness. Hope this helps.
posted by Zedcaster at 4:15 PM on February 22, 2016 [1 favorite]


Best answer: saeculorum: "The thermostat that didn't had two heaters on it, with two 12/2 in and two 12/2 out.
"

You shouldn't have power derived from two different places unless you had two different thermostats. Can you explain further what you mean by two in and how you wired that thermostat?

Having said that I can't think of how a mistake there could get the symptoms you are reporting at your last heater. Usually this symptom is a misconfigured breaker panel. Have you done anything in the panel ever?
posted by Mitheral at 4:19 PM on February 22, 2016 [1 favorite]


There is a 12/2 line going into the thermostat box and a 12/2 line going to the heater. When I measure the voltage between the two conductors (expecting 240V), I measure ~0V. When I measure the voltage between each of the two conductors and the ground conductor, I measure ~120V. It seems to me that both of these conductors are connected to the same phase, which would make sense why the heater doesn't work.

When the thermostat switch is closed, you should always measure 0V between the two thermostat wires; when it's open and properly connected to a working heater you should measure 240V. (I misread what two wires you were measuring, before).

I'd guess you have an open circuit between thermo and heater or in the heater itself, or in the 120V leg which is supposed to be connected to the side of the heater not controlled by the thermo.
posted by jamjam at 4:20 PM on February 22, 2016


Your readings seem consistent with a burned open heater. With the power off, if you can measure the resistance across the heater (preferably at the heater) it will tell you if the heater is good. I'm guessing the resistance should be between 10-30 ohms. While you are doing this, check the resistance to ground, should read open.
If you short across the thermostat you should have 120v to ground at each end of the heater (which equals 240v across the heater) and the heater should heat. Unless something has gotten rewired you probably aren't on the same phase.

BE CAREFUL- 240v is much more dangerous than 120v !!!
posted by H21 at 4:57 PM on February 22, 2016


Response by poster: Zedcaster and Mitheral have this here - the branch with two heaters was miswired. I had naively figured that the two 12/2 lines coming in were one for each heater. One was for the thermostat that doesn't work and I was effectively switching the power into the thermostat that wasn't working. I figured it out when I noticed that I could turn on the broken thermostat by turning on two-heater branch.

Thanks for the quick and useful help!
posted by saeculorum at 6:14 PM on February 22, 2016


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