From: John De Armond
Subject: Re: Better furnace thermostat
Date: Wed, 13 Oct 1999 04:20:57 EDT
Newsgroups: alt.rv,rec.outdoors.rv-travel

Erik wrote:
> 
> "Dave Thompson" <bdthompson@home.com> wrote:
> > The current model is probably OK.  Adjust the heat anticipation slider
> > until you get what you want.
> 
> Could you tell me in more detail what the "heat anticipation slider" is?
> (I am not sure mine has one.)

This is a little adjustable heater inside analog thermostats that
supplies a small amount of heat input to the thermostat.  This
causes the thermostat to open slightly before the room reaches the
setpoint.  It "anticipates" the extra heat recovered from the
furnace as the heat exchanger cools after the fire is out.  In the
other direction, since the heater is off when the thermostat is off,
the thermostat comes back on a little before it normally would. 
This "anticipates" the room getting cold before it actually does.

Physically, it is typically a small resistance wire with an
adjustable slider.  It will have a calibrated scale.  This scale
should be set to the number of milliamps the thermostat circuit
pulls.  Measure this with a suitable DVM.  Turning the anticipator
to a LOWER number will INCREASE the anticipation effect and tighten
up the deadband (span between turn on and turn off)  Go too far and,
as someone else has said, and the furnace will cycle rapidly.

That said, I can't imagine fooling with an analog thermostat when
digital ones are so cheap and work so well.

There are two major variations - thermostats that are powered from
the loop voltage and those that contain their own battery. 
Loop-powered thermostats will not work on millivolt (self-powered)
furnaces.  Such furnaces only supply about 700 millivolts to the
thermostat circuit.  If you don't know what kind of furnace you have
and don't have a suitable meter, buy a digital thermostat that is
battery powered and is rated for both 24 volt and millivolt
circuits.  Read the label.  Even if an e-stat has an internal
battery, if it is not rated for 700 mv service, it may not work. 
This is because relays not designed for low EMF switching can
sometimes accumulate film on the contacts that the 700 mv cannot
punch through. They work fine on the 24 volt systems.  Relays rated
for low EMF switching have high pressure contacts that will cut
through the film even at low voltages.

Other advantages of digital thermostats include:

* Digital indication of room temperature.
* very accurate and capable of being calibrated if you have a
calibrated reference thermometer.
* adjustable deadband (can hold the room temperature to the setpoint
+- 1 degree)
* Learns the thermal characteristics with intelligent anticipation.
* Programmable - either 5+2 days or 7 days.  Of limited use in an RV
unless you're living in it.
* cheap
* position insensitive.

For the programmable versions, the two variants are 5+2 and 7 day
programmable.  The 5+2 holds 2 programs, one for the weekdays and
one for the weekends.  Pretty useless unless you work a 9-to-5 job
with little variation.  These are the cheapest of the programmables
so if one program for all days is OK, say, warming the RV at 9:00AM
every day, then it will do the job.  Non-programmable ones are a
little cheaper for this application, however.  The 7 day
programmable 'stat will hold a program for each day of the week. 
They usually have shortcut keys to emulate 5+2 mode if your
lifestyle fits.

Many digital 'stats have two control circuits so that two appliances
can be controlled.  This is nice if you want to control your
overhead AC and your gas furnace on the same 'stat.

Many of the high end 'stats have auto-crossover.  That is, it will
automatically cross over from heat to cooling or vice versa as the
conditions dictate.  This is nice for those times when heat is
needed at night but AC is needed in the day.  

I have a Hunter brand e-stat that does all the above except the
auto-crossover.  I bought it a Sam's for about $25 about 3 years
ago.  Battery life has been about 2 years.  Only minor problem is
that it loses its program when the battery is changed.

I use a top-of-the-line White-Rogers Digital Comfort-Set II e-stat
in my restaurant. This does all the above and in addition, can stage
up to 3 cooling and 2 heating stages or units and has a remote
sensor so the thermostat can be locked in the office where no one
can tamper with it.  The auto crossover is very useful because the
heat load varies so much between meal times and prep times that AC
is needed one hour and heat the next in the winter.  At about $200,
probably overkill for an RV.  OTOH, it could be used on a large RV
to stage 2 or 3 AC units according to the heat load.

John