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Old 11-08-2011, 07:37 PM   #1
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Single Pole Thermostats


Virtually everything I have read says that you can only set them to Low, that you can never completely turn the heating unit they control off.

What does this mean?

1. That there will always be power in the heating unit because a single pole thermostat only affects one wire, and the other wire is still live? (I assume this is true.)

2. If it gets really cold, the heater will still go on because the thermostat only goes to Low. (I don't understand this.)

It sounds like it's both. But I don't understand how it works.

The single pole thermostat I'm looking at has a metal plate that comes up against metal on the back of the knob to make the connection to turn the heating unit on. But when you turn the knob all the way to Low, a plastic bump breaks the connection. Since electricity can't pass through plastic, doesn't that mean that the Low setting is really Off?

What brought this up is that someone's heating unit turned on when their thermostat was set to Low. They said that all thermostats have two resistors in them, and that if they both fail the thermostat fails.

Thanks,

Peter

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Old 11-08-2011, 07:56 PM   #2
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For thermostats for electric wall heaters, some of these just turn to low, then they would still come on if it gets really cold. Other have an off setting where it is totally off.

And same with plug in space heaters. Some of these have dials which just lower the temperature of when the heater will come on. And they will come on if very cold. Others have an off position or a separate on/off switch which will totally turn it off.

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Old 11-08-2011, 08:04 PM   #3
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Billy Bob,

I understand that, but I'm looking for an explanation of how it can still come on when it's cold if the plastic on the knob breaks the electrical connection.

Thanks,

Peter
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Old 11-08-2011, 08:45 PM   #4
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Line voltage thermostats can be single pole or two pole. If it has an OFF position, both lines will have their contacts held open regardless of the room temperature.
Once dialed away from OFF, one line will be closed all the time and the other line will change state as the room temperature changes.
A single pole thermostat does not have an OFF position and the one contact will open and close based on room temperature.
You would have to post a picture of your t-stat and the "bump." Do you have a manufacture and model number?
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Old 11-08-2011, 09:11 PM   #5
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Originally Posted by peterinwa View Post
Billy Bob,

I understand that, but I'm looking for an explanation of how it can still come on when it's cold if the plastic on the knob breaks the electrical connection.
Perhaps it is the type which is totally off when turned to low?

All sorts of different companies make these. So there can be all sorts of different designs.

Place it in the freezer and see if it turns on by measuring with an ohm meter or continuity tester.

Note: As for myself, I want to be able to turn these things totally off! So I make a point to find a thermostat which does turn totally off.
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Old 11-08-2011, 09:45 PM   #6
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I'm researching this for our condo newsletter and need to be able to explain things a bit. Plus I have a unit myself.

I couldn't find a serial number (without removing it) but took a picture. It's pretty bad, but A and B are "bumps" sticking out from the metal plate you see. When the knob is set to anything but Low, A makes contact with the metal on the back of the knob, making the connection.

But when set on Low, A rests against bump C on the back of the plastic knob. So I'd think Low would equal off.

I was thinking that perhaps the reason I read in so many places that you can't ever turn a Single Pole off is so that people would realize that there is still power in the heating unit from the one wire.

The person with the near fire did have their thermostat on Low, utilizing the plastic bump. I'm thinking perhaps that really does break the connection, so in effect it is off. But electronics within the thermostat might have failed.

Don't have an ohm meter.

Everyone I'd talked to before posting here just said to replace it if in doubt. I just like to understand.

Thanks again,

Peter
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Old 11-08-2011, 10:49 PM   #7
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Got most of it solved!

Just happened to run into my neighbor who works for the phone company and knows something about electrical stuff like this. He'd removed his thermostat a long time ago and saved it, so we took it apart.

I'd assumed that the current running into the part of the thermostat pictured was low current, and that in went into a high current part in the back. And that part had failed.

No such thing!

There's no current in the metal you see in the picture. It simply moves in and out to turn a switch behind it on and off. We even looked inside the switch and it's very simple, no electronic parts. Just like a light switch, contact or no contact.

I assume that the heater unit couldn't have turned on without the switch circuit being closed. So the question that remains is how it closed when it had been just sitting there for years.

Anyway, other than that question will is probably answered with "it broke," I understand the workings of the thermostat now. Electronics 101 for me!

Thanks for your time,

Peter

P.S.

I do think that the plastic bump held things in postion so that the switch couldn't be turn "on" (circuit closed) no matter how cold it got.

And if that's the case, I suspect that the reason the thermostat read Low rather than Off is for one of two reasons:

1. It's just traditional on Single Pole thermostats.

2. It implies that there is still power to the heater, which is true since a Single Pole thermostat only breaks the circuit for one of the two lines.

Last edited by peterinwa; 11-08-2011 at 10:54 PM.
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Old 11-08-2011, 10:58 PM   #8
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What happened with the fire or almost fire?
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Old 11-08-2011, 11:00 PM   #9
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Quote:
Originally Posted by peterinwa View Post
...It implies that there is still power to the heater, which is true since a Single Pole thermostat only breaks the circuit for one of the two lines.
Well that would make sense for not saying off! A "legal" reason to not say off, then they can't be held responsible for 240 volt uses of that switch and someone being shocked, etc.
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Old 11-08-2011, 11:05 PM   #10
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That's exactly what I'm thinking.

Plus, perhaps most single pole thermostats don't have that "bump" that effectively makes it an off switch. So those units might really go on when it gets really cold.

Thanks!
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Old 11-08-2011, 11:12 PM   #11
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I just looked at my 240 volt double pole wall thermostat and it says "off". And it in fact does turn off power to both lines.

I also looked at my "Made in China" plug in space heater and it has a turn dial which says "off" (turned fully counter-clockwise) and it is NOT OFF in that position. I heard it running in my cold garage one day. I installed an on/off switch on it, so now it is totally off when I flip the switch. (Also used high heat range wire for this.)
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Old 11-09-2011, 12:16 AM   #12
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Billy Bob,

I have half a mind to get the thermostat switch that failed and see if I can see why it failed. Might be interesting.

On a not-too-related subject, I use a small space heater in my bedroom during the coldest part of the winter so I don't have to heat my entire condo. The problem with every space heater I've ever had is that as the unit itself warms up, that effects the action of the thermostat.

So if you set it just right when you go to bed, it won't keep the room as warm later in the night when the unit has warmed up causing the thermostat to shut if off sooner.

I've always thought it would be cool if you could buy a thermostat that you could plug into a wall socket, then you'd plug your space heater into that. Since the thermostat would be independent of the heater, it wouldn't be affected by the heater itself warming up.

With what I've learned from this exercise, I suppose I could buy a Double Pole thermostat and splice in into the space heater cord between the plug and the heater. If I didn't get electrocuted, I'd probably burn my condo down.

But I think the idea is sound... if not safe!

Peter
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Old 11-09-2011, 12:20 AM   #13
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Or point a fan at the space heater might be even safer...

Tell us what happened with the almost fire at your neighbors please ???
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Old 11-09-2011, 12:36 AM   #14
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So I'm going to have a space heater and a fan running? And the fan pointing at the space heater?



In our condos we have toe warmers in our bathrooms, actually called baseboard heaters, installed under the sinks. They were all made by Cadet, and recalled because people didn't clean them and the accumulated dust was causing fires.

I still use mine, as I hadn't heard about the recall. But I clean it so it's safe.

Other condo owners who never used theirs also didn't switch them out. But then they sold their condos, and new owners are unaware of the danger.

In the condo in question the owner was aware of the problem but never used her heater. After living there for 11 years, one morning it just came on. She'd installed carpet with a higher pile, and it singed it. Could have actually become a fire if she hadn't been home and smelled it.

My best guess would have been that sometime in the 11 years the knob got turned up a bit, and so the heater just came on when it got really cold. But it wouldn't shut off. They had to turn it off at the circuit breaker. So the thermostat switch really must have broken.

That's what started this discussion. And now I'm writing a newsletter article for our condo board that explains the issue.

From what I've learned, it HAD to be the switch in the thermostat that failed. I say this because as I understand it, the heater couldn't have turned on without the switch completing the circuit.

Peter
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Old 11-09-2011, 10:58 AM   #15
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Quote:
Originally Posted by peterinwa View Post
In our condos we have toe warmers in our bathrooms, actually called baseboard heaters, installed under the sinks. They were all made by Cadet, and recalled because people didn't clean them and the accumulated dust was causing fires.


In the condo in question the owner was aware of the problem but never used her heater. After living there for 11 years, one morning it just came on. She'd installed carpet with a higher pile, and it singed it. Could have actually become a fire if she hadn't been home and smelled it.

My best guess would have been that sometime in the 11 years the knob got turned up a bit, and so the heater just came on when it got really cold. But it wouldn't shut off. They had to turn it off at the circuit breaker. So the thermostat switch really must have broken.
How are these baseboard heaters installed as "toe warmers"?
Cadet Baseboard heaters, when properly installed, shouldn't cause a problem with high pile carpet unless the pile is 6" high. There is no heat at the bottom edge of the heater and there shouldn't be anything above the heater.

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