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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Coleman AC quit on me
Message-ID: <>
Date: Tue, 22 Apr 2003 03:38:15 -0400

On Sun, 20 Apr 2003 14:27:02 -0500, Bob Giddings <> wrote:

>Do you find the digital Tstat works better than the analog Coleman?  I
>find the Coleman leaves the AC on too long before cutting off.  At
>least with the high speed fan going.

Bob, I've tried a digital thermostat and went back to the analog one.  The
digital thermostat controls the temperature better but there was one fatal
problem.  When I half woke up in the morning, slightly warm or cold, I could
no longer groggily reach up and bat the thermostat over just enough to kick on
the conditioning.  With the digital 'stat - every one I've ever seen, anyway -
one must look at the display to set the new temperature.

You might try a better analog unit. The classic Honeywell Round (home depot,
etc) is a good one and has some deadband adjustment.  Even that cheapo Coleman
has a little deadband adjustment.  there's a little adjustment screw inside
the 'stat that varies the bias tension on the contacts.  On mine it's near the
deadband magnet just under the contacts.

For the tightest control you also need to pay attention to the location.  If
the t'stat is in stagnant air, particularly when the fan is off, it won't see
a representative sample of room air.  Ideally the t'stat should be located in
an area where there is some air movement, such as from normal movement.

You should also look at what the t'stat is mounted on.  If the t'stat is
mounted on an uninsulated wall, it will respond as much to outside air as
inside.  Had that problem in my rig with the original mounting.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: And so it begins...
Message-ID: <>
Date: Fri, 30 May 2003 00:36:16 -0400

On Thu, 29 May 2003 10:54:14 GMT, "Mara" <> wrote:

>Given that my husband not only installed all the plumbing in his
>parents' house, but also did all the wiring, I don't think a
>thermostat is going to be that much of an issue.  I just wanted to
>make sure we don't need a *particular* thermostat.

As usual and unfortunately Mara, you've been given incorrect advice here.  The
type of thermostat does matter.  There are several things to consider,
depending on the type of thermostat, that are the result of differences
between stationary and mobile control circuits.

There are basically 4 major types of HVAC control systems.  They are:

24 volt AC heat/air
24 volt AC heat pump
12 volt DC (mobile/RV systems, for the most part)

There is some interchangeability but not always.

The millivolt system is used in self-contained gas appliances, sometimes in
RVs but mostly in stationary applications.  In this system, there is a
thermopile that when heated, generates about 750 millivolts.  This pile, which
is about the diameter of a 1st grade pencil, is in the pilot light flame.
This 750 millivolts is routed through a thermostat and then to a gas control
valve.  It allows electric control without having to have an outside source of

The 24 volt AC systems constitute the majority of systems installed in
stationary applications such as your furnace and AC at home.  The heat/air
thermostat has two switching circuits, one for heat and one for cooling.  This
type is used for electric AC and either simple resistance electric heat or gas
heat.  The heat pump thermostat has the extra switching circuits to control
multi-stage heating and cooling, operation of the aux electric heating coils
and operation of emergency heating if the heat pump part quits.

The 12 volt DC systems constitute the majority of systems installed in RVs for
the obvious reason that 12 volt power is conveniently available.

Heat pump thermostats generally don't apply to RVs so we'll not pay them any
more attention other than to say that you don't want to buy a heat pump type
of thermostat.

The rest of the thermostats are divided up into two major classes - mechanical
and electronic.  The mechanical thermostats are fairly simple - just some sort
of temperature sensing element, usually a coiled bi-metal strip, and some
contacts to operate the heat and AC.  Some of the mechanical thermostats
contain gravity-dependent elements.  One element is the mercury switch.
Others involve "dry" (not mercury wetted) contacts that are counterbalanced
with weights.  These won't work in an RV because the RV isn't always level.
If the thermostat label says that it is mercury-free, is suitable for
millivolt and gas heat and AC, then it will work in the RV.

The complication arises with electronic thermostats (digital and electronic
analog.)  Again, there are two major types - loop powered and battery powered.
Most electronic thermostats designed for stationary applications are loop
powered.  That is, they derive their operating power from the control loop
running from the furnace and AC to the thermostat.  This is very convenient
because the power comes from the utility and thus is always there.  Some loop
powered thermostats contain backup batteries to preserve the settings when the
power goes off.

The problem arises in the vastly different nature of stationary (24 volts AC)
control power vs RV (12 volts DC) control power.  A thermostat designed to run
on 24 volts AC power will not run on 12 volts DC power.  And 24 volts AC
applied to a 12 volt DC thermostat will probably damage it.  Thus a stationary
residential electronic thermostat will not work in an RV.  An RV-type
electronic thermostat obviously WILL work.

The other type of electronic thermostat is the battery powered type.  This
type of thermostat obtains its operating power from built-in batteries,
usually a few penlight batteries or a 9 volt one.  This type of thermostat
will work in both stationary and RV applications.

The big trick is telling which kind of thermostat you're looking at.  The
Hunter 'stats that Home Depot and so on sell are not well marked.  However,
there is a dead giveaway.  If the label says that the thermostat is suitable
for use on millivolt (also known as thermopile and thermocouple) systems, then
it is battery powered and WILL work in the RV.  Most thermostats rated for gas
heat service are battery powered and will work.  But the dead giveaway is the
rating for millivolt systems.

I've tried the digital thermostats in my rig and ended up going back to
analog.  The reason is simple.  One has to be looking at the display on a
digital thermostat to set it.  That means light and glasses and stuff like
that.  I have my thermostat mounted over the head of my bed so that I can just
reach up and adjust the temperature without having to fully wake up.
Impossible with a digital unit.  This is particularly important around here
where during much of the year it is cool enough at night to need a bit of heat
but hot enough after the sun comes out to need AC.  It's a real pain to have
to wake up, find the glasses, sit up in bed and fumble with the thermostat.
Soooo much nicer just to bat the lever a little until something turns on.

Installing these things is trivially easy.  The terminals are marked with
standard letter designators and the wire colors are standardized (usually - no
telling what some RV makers will do.)  Simply write down what wire is
connected to which terminal and then connect them the same way on the new
thermostat.  The voltage isn't enough to shock so unless you really feel
fumble-fingered enough to cause a short and blow a fuse, there isn't even any
need to kill the power.  Even going very slowly and taking careful notes, the
process shouldn't take over a half hour.


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