Why can I not find a 120V programmable 2-pole thermostat

Question

I'm wanting to connect a 120V wall heater to a programmable, 2-pole thermostat. The difference between 1 vs. 2 poles involves whether or not the thermostat can truly, completely cut power or not. With 2 poles, all power to the heater can be cut. With 1, the "off setting" is really a very-lower power setting.

I can find 2-pole programmable thermostats that use 240V, but so far, have been unable to find the 2-pole arrangement for a 120V load. It's a pure guess that this may because 120V heaters are easier to appear "off" due to lower heat output, but 240V heaters would draw twice the current and generate twice the heat so it might be more noticeable. Still, its hard to believe it is only that reason that would prevent manufacturers like honeywell or cadet from making such a thermostat.

Does anyone know why a 2-pole, 120V programmable thermostat is next to impossible to find? Is there some technical or design reason why they wouldn't work?

Of course if anyone knows where one might be found...that would be excellent! I only need 24-hour programmability, not 7-day. But I really would like the full-power cutoff of the 2-pole thermostats.

I hope this isn't too technical for home improvement section, as I've run into this as part of a home-improvement project. Thanks.

UPDATE: many ask or question my conception of single v. double pole thermostats. I'm using a definition from a Cadet heating @ http://cadetheat.com/blog/faq-single-v-double-pole/.

Double pole stats have a true off setting. Single pole thermostats don't.

I've seen restatements of that definition on other sites and it seemed to be true for Honeywell thermostats as well. The manufacturer statements would seem to be definitive. If they are not, I'd want to know why.

Answer 1

Single pole stats don't because they don't need to. Just buy one of these and call it good: https://www.amazon.com/gp/product/B0028JJLSS/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

Good luck!

Answer 2

A 120V heater runs on two conductors: hot and neutral. (I'm ignoring the grounding wire which is there for safety, not operation of the heater.) The heater is switched on and off by disconnecting the hot conductor. We call this a single-pole switch. A 120V single-pole thermostat is just a single-pole switch that is controlled based on the ambient temperature. Once the hot conductor is disconnected no power will flow through the heater since the neutral conductor is connected to ground at the service panel.

A 240V heater also runs on two conductors but both are hot. Naively we could disconnect either conductor and the heater would no longer draw power. However, each conductor is 120V from neutral ground. Even if there is no electrical fault the metal frame of the heater can pick up a charge (capacitively) and create an electrical shock hazard. So we disconnect both hot conductors with a 2-pole switch (or thermostat).

The number of poles reflects the number of hot conductors and therefore you won't find a 120V two-pole switch unless by two-pole we mean a switch that disconnected two hot conductors at the same time (DPST).

Answer 3

The problem with starting from a fallacy is that it leads no-where useful.

The difference between 1 vs. 2 poles involves whether or not the thermostat can truly, completely cut power or not. With 2 poles, all power to the heater can be cut. With 1, the "off setting" is really a very-lower power setting.

Patently false. Incorrect, wrong, and ignorant. A single pole switch. relay, or thermostat will cut all power to a 120V or 240V heater - the reason we don't use single poles for 240V heaters in the USA/Canada power system is only for an extra degree of safety, not because any power magically makes it through an open switch. If you use a single pole switch on a 240V heater in our system, the "switched off" heater would be at 120V relative to ground - it would still be firmly off, not dissipating any power, as no current is flowing, but it would present a slightly higher shock potential, or a much higher shock potential if someone were to open up its connections (believing it to be "off" at the thermostat) without shutting off the breaker for the circuit.

The reason you cannot find the item you seek is that your starting assumptions are incorrect, and there is no need for, and thus no market for, a 120V two-pole thermostat. Properly connected, the hot line is switched, and the neutral is not, and when the switch is open the entire heater is at approximately 0 volts with respect to ground, and thus there is no safety advantage to using a two-pole switch on a 120V heater.

This makes me think of James Thurber's Grandmother who was sure that electricity was leaking out of switched-on sockets with no bulbs in them...

Answer 4

I'm going to overlook a few misconceptions and get right to your question.

120V thermostats are rare ducks because 120V heating is a weak idea. It takes a lot - a LOT - of electricity to make a useful amount of heat. It isn't worth it to strain a 120V/20A circuit at 2000W (which will barely heat a single room if even) when a 240V/15A can handle that same power with ease. when you're up over 2kw, 240V is just the smart way to go.

You can use mechanical 240V thermostats with 120V just by wiring both poles in series. That won't work with electronic 'stats, as they need a way to power themselves and they are "geared" to poach power off the 240V.

Your best option is probably a $12 transformer and $9 relay which would let you use any of thousands of 24V C-wire-powered 'stats including the Nest.

Answer 5

More about Cadet's page, which applies to Cadet thermostats, and also IME Honeywell.

The first pole is temperature-based. There is no way to turn this pole off. If ambient temp falls below the setpoint, it will turn on.

The optional second pole is only an on/off switch. It is only "off" if you turn it all the way to the left past the detent. It does not turn off the other pole.

All heaters have 2 wires. Current flows in loops, so block either wire and current can't flow. These thermostats operate on the principle that if you sever either pole, you stop current. However a pole can still have voltage (electrical pressure) on it, and that is what zaps you.

Neutral is designed to be harmless and it is a waste of time to switch it. Only 120V heaters have neutral.

With 120V on a 2-pole old mechanical 'stat, do not switch the neutral and do loop the hot through both poles. This won't work on a digital 'stat.

Answer 6

I disagree with Cadet's explanation, and would like to ask them how electricity flows through the device when you only have one wire attached to it... single pole and double pole mean one "switch" or two "switches" inside the device. One pole means one wire is switched, and two pole means two wires are switched. Most light switches in a house are single pole, and you only disconnect the hot wire, yet no power flows to the light. How can they maintain that single pole switches in the off state are just "low power" modes.

What nobody has explained so far, is that with a single pole 240V thermostat, one leg is disconnected, while the other is always connected. 240V systems are 120V above and below neutral. Negative voltage and positive voltage are still a voltage difference, and both will zap you. No electricity will flow with one leg disconnected, but if you were to work on the heater relying only on it being "switched off" at the single pole thermostat and touched a neutral or ground wire in your house, you would get hit by the other leg which is still hot.

The double pole means that when it is off, both legs of the 240V are disconnected instead of just one. In both cases the power flow is zero if the thermostat is off.