According to Filipo :
I'm trying to replace the built-in mechanical thermostat on an old
Fasco Industries 8- foot, 240 volt electric baseboard heater with a
programmable line voltage thermostat, either a Honeywell CT1950 or an
Aube Technologies TH106. Problem is I'm supposed to choose between 2
programmable thermostats - the Honeywell CT1950A which is for 2 wires
or the Honeywell CT1950B which is for 4 wires, and I have no idea
which one I need.

A bit of background: a line voltage thermostat is one that slips
in series with the power feed to the heater and controls the heater
by switching the power feed off and on. As contrasted with other
thermostats which activate a seperate relay to switch the power feed
to the heater.

The CT1950x thermostats are line thermostats, which means that they
take power in (via the Lx terminals) and switch the output power on the
Tx terminals.

The only difference between the CT1950A and B is that the CT1950A
switches only one wire (single pole switch), and the B model switches
two wires (double pole switch).

[Ie: electrically, the CT1950A simply connects/disconnects wire T1 from
L1. Electric simply connects/disconnects wire T1 from L1 and T2 from
L2.]

http://content.honeywell.com/yourhom.../CT1950faq.htm

You use single pole thermostats on 120V heaters, because there's only
one hot wire present (total of two: neutral plus one hot), and that's
the only one you need to switch.

240V has two hot wires (no neutral). Preferably and ideally you should
use two pole thermostats for 240V heaters, so that both hot wires are
simultaneously detached - otherwise, the element would be live even when
the thermostat has the heater turned off. However, most codes do not seem to
insist on that - which has always seemed very odd to me.

I always use 2 pole for 220V.

Questions:

1. Is the CT1950A or B just one thermostat that can be wired in 2
different ways, or are we talking 2 different thermostats altogether?

2. In any case, how can I tell if I need a 2-wire thermostat or a
4-wire thermostat? I looked at the existing wiring and noticed that
there are only 2 wires coming out of the wall directly from the
breaker panel. Then there appears to be 4 wires, all of them colored
red, connected to the built-in thermostat mounted on the heater itself
- 2 red wires going in and 2 red wires coming out. I have not traced
where these wires going in and out of the existing thermostat are
coming from.

It sounds as if the preexisting thermostat is 2 pole. You will need
to replicate how it interconnects.

As you can tell, I am really electrically challenged, so please pardon
me if the question sounds stupid. But I really have a problem
understanding electrician's English - when they say 2-wire or 4-wire,
are they talking about the wires coming out of the wall or the wires
coming out of the thermostat?

When they say "4 wire X", they really mean "4 wires connected to X".
What that actually means electrically depends on what X _is_.

Poor terminology.

The electrician should _really_ be saying "two pole" (CT1950B) versus
"single pole" (CT1950A).

Any help will again be appreciated. Thanks.


--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.