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XTension Discussion

Cat-5 for thermostat wire

From: Bob Vennerbeck <bvennerb-AT-risd.edu>
Date: 22 Oct 2001 12:37:53 UTC   (08:37:53 AM in author's locale)
To: xtensionlist-AT-shed.com
Cat-5 MAY work fine, or may not - depends a bit on distance to
thermostat, and particular type of control...

Most of the heating controllers I've dealt with expect to see a certain
amount of current through the closed thermostat contact - this causes
the adjustable resistance wire 'heat anticipator' in the thermostat to
dissipate the desired amount of watts of heat and cause the thermostat
to release 'in anticipation of extracting residual heat from the ducts,
pipes, furnace, boiler, etc' and thus not overshoot the desired
temperature. The same principle applies to various solid-state add-on
set-back hacks....

Also, many of the relays in heating controllers are current sensitive -
this is so you can hang an electronic thermostat on a 2 wire thermostat
circuit, and use the 24 volts coming from the heating controller to run
the thermostat brain ( or good old fashioned clock motor!), without the
current draw of the thermostat closing the controller relay until you
want to.

Anyway - thermostat wire is generally 18 gauge (if I remember
correctly), and the smaller 24 or 26 or even 28 gauge Cat-5 wire may be
just a little undersized for the purpose, and cause too much resistance
at the wrong place in the thermostat circuit. This is not likely to be
any kind of safety issue, just an engineering nicety - just like central
heat is an engineering nicety.....

Rob Lewis wrote:
> CAT 5 ought to work fine. I expect the signals are DC levels, so bandwidth
> isn't an issue.
> >
> > I have easy access to the control unit, with an outlet nearby as well. I
> > should be able to fish some CAT-5 cable up to where the thermostat is, will
> > that work, or should I use different wire?
> >
> > Thanks!
> >
> > Sean
> >
> >
> > On 10/21/01 3:50 PM, "Steve Fyfe" <SFyfe-AT-WindwardSG.com> wrote:
> >
> >> Sean hammer-AT-empire-group.com on 10/20/2001 1:40 PM said:
> >>
> >>> the TX-15b
> >>> from RCS and it seems like what I'm interested in. The .pdf manual seems to
> >>> indicate that you need 4, 5 or 6 wires at your thermostat. I took mine off
> >>> the wall and it only has two wires connected to it, plus a third that comes
> >>> out of the wall and doesn't connect to anything
> >>
> >> The TX-15b is not just a simple replacement for an existing thermostat.
> >> You cannot just remove your old one and put the TX unit in its place.
> >>
> >> The TX-15b comes with a control unit that is mounted near the hvac
> >> system, and a display unit that goes where the old thermostat is. The
> >> cable between the display unit and the control unit needs 4 wires, if I
> >> remember correctly. This is true even if you don't have an AC unit.
> >>
> >> There is also a cable that goes from the control unit to the hvac system.
> >> I don't remember how many wires this one needs, but this one many not
> >> need them all if you don't have AC. In any case, if the control unit is
> >> near the hvac unit, it is usually not a problem to run a new cable if
> >> needed.
> >>
> >> The control unit also needs access to power. It has a wall wart unit to
> >> supply it power and it uses a TW523 to get x10 signals, so you need an
> >> electrical outlet or two nearby.
> >>
> >> HTH
> >> Steve
> >>
> >> _____________________________________________________________
> >> Steve Fyfe
> >> Windward Services Group
> >> Ashby, Massachusetts
> >>
> >>
> >
> >

Bob VENNERBECK = <bvennerb-AT-RISD.edu>; RI School of Design Film/Video Tech
2 College Street, Providence, Rhode Island 02903-2784 USA (401) 454-6236

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