Off Topic: Connection from Water Heater to Thermostat
December 30th, 2013How about a little home improvement mystery to break up the steady flow of nightmares and doom?
Alrighty then!
The thermostat on our hot water cylinder fried itself yesterday morning. There are a few reasons why I’m baffled by this:
1. This is the second time that this has happened. When it happened the first time, I just assumed that the thermostat (which was the original one that was in hot water cylinder when we bought the house) had gone bad, who knows why, I just replaced it, monitored the situation closely for a few days and thought the problem had been resolved.
About once a month, I checked the water temperature and it was right on when compared to the setting on the thermostat. I would also check the wiring to and from the thermostat for any signs of trouble. No problems. Wires remained at room temperature after the unit had been running. The circuit breaker never tripped throughout any of this.
It worked fine for six months… until yesterday, when it let the smoke out of the wire between the thermostat and the cylinder.
2. On both fried thermostats, the meltdowns occurred between the output terminals on the thermostats and the input terminal on the water heater. The input terminals on both thermostats and the household wiring leading into the thermostats never showed any sign of trouble. You can’t see it on the picture I’ve posted below, but the input side of this thermostat is pristine.
Cooked Cotherm Thermostat
3. So, with all electrical supply places closed and messages left on multiple electricians’ 24-hour response lines being ignored, I decided to conduct an experiment. Rather than trying to wash dishes and my filthy creature sons with cold water, I wired up the hot water cylinder without any thermostat inline and flipped the breaker to ON. (Don’t try this at home, you might poke your eye out.) Then I sat by and watched… as the hot water cylinder uneventfully heated the water without any problems at all. The water had been lukewarm so I just let it run. After about 15 minutes, I felt the wires—Ambient temperature. After a half hour—Ambient temperature. After an hour—Ambient temperature. All the time, the water was getting hotter and hotter. Finally, after about an hour and a half, with the water at 52C, I shut it down. The input wiring still felt right around ambient temperature.
So, people out there in Cryptogon-land:
Why are the thermostats frying between the output terminal on the thermostats and the input terminal on the hot water cylinder?
Why does the system work fine with no thermostat in the circuit?
Is 90minutes normal? Might it be running at half throttle?
I don’t really know if it’s normal. I haven’t timed it from a cold, powered off state to hot before.
Make sure you are getting 220 volts through both heating elements and make sure there’s no water or condensation getting into the thermostat. Beyond that, I’m as confounded as you are. Good luck and Happy New Year.
Yeah I’ve put a meter on it and it’s between 238-240v, right where it should be. (We’re 240v here in NZ.) That’s with both a thermostat in line and without.
Ok, a reader sent the following:
Hi Kevin,
Saw your post on the water heater problem. I am an industrial electrician by trade, with many years experience, so I hope I can help.
IME, there are only 2 things that can cause a terminal to burn up, as pictured. Excessive current, or poor connection. Since the input terminals were fine, and the circuit breaker did not trip, we can safely rule out the former. Which leaves only the latter. Poor connection at the terminal. This can be due to either looseness or contamination of the metal-to-metal surface contact, by corrosion, dirt, or some other substance. On recently installed or replaced equipment a loose connection was the more likely.
If you are very sure the connection was good and clean and tight, I would have to say the thermostat was itself internally faulty. Meaning the connection inside the unit to the terminal was poor due to some factory defect. Did it happen to burn up right after the warranty expired ? That would not surprise me.
Joe
PS: If the wire connecting to the terminal were too small that could cause heating of the terminal too. If the output wire is the same size as the circuit supply wire from the panel, then that is not the problem.
—
I wrote back:
Hi Joe,
I really thought that I made sure that there was plenty of contact on the terminals. Both the thermostat and the breaker that the heater is on are 16 amp parts, but that input side wiring is fine, and the breaker never tripped.
Here’s something, though: I was considering your response and the bit about the new thermostat possibly being internally faulty jogged a memory.
When I got that thermostat home, the screw on the side that didn’t burn loosened normally, but the screw on the side that burned up had been tightened down so hard at the factory that I damn near couldn’t loosen it. I had to apply an absurd amount of pressure to loosen that thing and nearly stripped it. I eventually got it loose, but considering what has happened, maybe that unit was somehow damaged from the start.
The water heater does run like a charm without the thermostat in line, so I really think you’re onto something with the idea that the thermostat was bad.
It should have a one year warranty. Whether or not I can find the receipt is another matter…
Thanks very much for your help, Joe.
All the best,
Kevin
Awesome! Thankfully it didn’t destroy anything but itself.
“Why does the system work fine with no thermostat in the circuit?”
It’s just a switch, just like at the main panel. You wired around it.
And on the subject of wire and more specifically the actual connections, I don’t know how they do things in NZ, but you didn’t mention if your wire was copper or aluminum. There was a time in the US when aluminum wire was common in residential construction, in particular with higher amp circuits such as A/C units, stoves and water heaters with are typically 240v pole-to-pole, not to ground as in many countries outside the US.
The point here is aluminum is a different beast than copper especially with terminations. And aluminum and copper don’t mix, requiring a chemical inhibitor such as Noalox. And aluminum connections require a second torquing at a minimum after about 15 minutes.
Aluminum colored or “looking” termination blocks and copper wire generally don’t need an inhibitor, but it might not hurt. Or alternately on non-aluminum connections, an electrical conductive paste or gel. Thermal expansion and contraction (at different rates for dissimilar metals) might be a culprit here as that can cause carbon build-up between the wire and the termination lug. Carbon = resistance. Resistance = heat.
I tend to agree with Joe above, although the wiring inside your device may not be the same gauge as your as your main wiring, at least in the US that is possible. As a rule of thumb, consumer electrical devices needs to have a certification of being safe. This is done via testing of a specific example of the production version.
If the wiring in your device is not the same gauge as your main, likely less, it does not necessarily indicate a problem. Wire ampacity is based on the available current conducting surface area (gauge of the wire for the most part) AND the ability to dissipate heat. The latter is impacted by ambient temperature and whether or not a conductor ( the wire in this case) is bare or has some sort of protective covering. It is that covering that makes a difference, especially in consumer appliances.
In the US we have the NEC – National Electrical Code that outlines in charts and accompanying calculations what can or cannot be done. I’d guess NZ has something similar.
I’d verify your getting the right part – most electrical equipment is required to have their voltage and current ratings legible on the device itself. You might research and see if that model has a history of failures, factory recalls and so on.
The wire is copper. The terminals on the water heater are copper. The screw down terminal blocks on the thermostat are copper. The only things that aren’t copper in this connection are the screws on the terminal blocks on the thermostat. They are aluminum colored—not copper anyway. But as I already mentioned, the input side looks new.
The wire is all the same gauge.
According to the electrician supply place I dealt with, this exact thermostat is very common here. Manufacturer and specs are the same as the one that was in the unit when we bought the house. It’s very definitely the right part.
The one I have is the TSD in case anyone wants to look:
http://www.cotherm.co.uk/uk/content/download/1173/18736/file/TSDR.pdf
16A – 240V. That’s me.