Calibrating old Honeywell "round" thermostats (for furnaces)

[harlan.b.braude]

I need some calibration information for one of these thermostats.
The newer "round" thermostats may be functionally different than
what I have, so I'll first describe the "algorithm" of mine (as I
understand it) and then present my question.

algorithm:

There is a small vial of mercury attached to one end of a
temperature-sensitive coil. At the other end of the vial
are wires for a low-voltage selenoid (it's for an oil-fueled
steam system.) When the vial tips far enough so that the
mercury in the vial completes the circuit, the furnace fires
up.

My question relates to the use of two small levers in the thermostat
that are in addition to the temperature selection dial and can be
seen when the front cover is removed. One lever points to a position
on a scale from .2 to .8 at .1 increments and the second lever has
a scale from 6 - 10 that is marked by an arrow with the word "longer"
(I think.)

Clearly, these levers are supposed to be set in some complementary
fashion to define the optimal temperature range around the set
temperature; the idea being to limit the on/off cycling of the
furnace.

What is the optimal balance and how is it achieved?

Currently, the thermostat allows the temperature to drop too low
between cycles. Is trial and error the only way or is there a more
direct path to comfort short of replacing the thermostat with
something more modern?
--
Harlan B. Braude
h...@karin.att.com

[David Lesher]

{what's them other things in a thermostat?}

Plastics, son. Plastics.

Oops - wrong advice....

Hysteresis. Anticipatation.

The room gets cold. The t-stat signals the furnace for heat. It starts.
It works hard, getting its plenum (airbox) hot. When it is, a second
t-stat, called the plenum t-stat {what a conincidence ;-!} starts the
blower. The blower blows warm air into the room. Eventually the room
t-stat senses it's warm enough, and shuts off the gas.

But the blower keeps going, because the plenum is still warm. By the
time it cools and the plenum t-stat shuts it off, the room is TOO
warm.

So the room t-stat has a little heater inside of it. When the gas valve
is open, the heater makes the t-stat a little warmer, so it shuts off
the gas a little sooner to make up for the overshoot. That's what the
"Longer/Shorter" refers to.

Two more points: The same thermal intertia also applies to other
systems such as boilers. Only the names are changed... Ooops.
And REAL electronic setback t-stats are often smarter. Some 'learn' how
much overshoot happened last cycle, and act to prevent it. I like
that.....

--
A host is a host from coast to coast..wb8foz@skybridge.scl.cwru.edu
& no one will talk to a host that's close..........................
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Chris Best]

> My question relates to the use of two small levers in the thermostat
> that are in addition to the temperature selection dial and can be
> seen when the front cover is removed. One lever points to a position
> on a scale from .2 to .8 at .1 increments and the second lever has
> a scale from 6 - 10 that is marked by an arrow with the word "longer"
> (I think.)

I'll think of the name of these things before I'm done, but it's something
like "compensators". I always understood them to be used to set some lag/
hysteresis into the system, to allow the thermostat to ignore a quick burst
of cold air (like from an open door), and only start the furnace when the
AVERAGE temp is too low. I think they also allow a full furnace cycle,
even though the thermostat may be warm enough partway thru the cycle.
"Anticipators".

----------

> Currently, the thermostat allows the temperature to drop too low
> between cycles. Is trial and error the only way or is there a more
> direct path to comfort short of replacing the thermostat with
> something more modern?

I think you're on to something. If you can find an instruction
sheet for it, it would explain it better. Or ask at a hardware
store. Otherwise, trial and error may be the way to go.

[MaddenTC]

Yeah, "anticipators" is what they call those rascals, and trial
& error seems to be the only way to get 'em set up right. The
one calibrated in tenths (of a degree) lets you set how far in
advance of the control point the furnace will kick in. (If your
thermostat is set at 68 and the first anticipator lever is at
0.6, the furnace will come on when the temp drops to 68.6
degrees.) The purpose of the second lever seems to be stuck in
write-only memory right now, but I think it keeps the furnace
from short-cycling. If used properly, the anticipators will keep
the temperature from dropping below the set point. (Permitting
us to brag about keeping our thermostats at 67 degrees while
assuring that the house never gets that cold!!) :-)

I haven't read the other postings on this thread, so the
following might be old news: the thermometer mounted out on the
cover of those round Honeywells has nothing to do with the
operation of the thermostat. *That* is controlled by the
gas-filled or bimetallic spiral which tips the mercury switch. A
more likely reason for a thermostat not controlling at the
set-point is if it (the thermostat) isn't mounted dead level.
Rotating the thermostat body a few degrees one way or the other
causes the mercury switch to actuate too soon, or too late, so
you're effectively "controlling" at a different temperature than
the one you want. Simple and effective devices, as long as you
understand their limitations.

Tom "think warm thoughts" Madden

____________________________________________________________________
Thomas C. Madden t...@drutx.att.com AT&T Bell Laboratories Denver
These are my opinions only!!