How to calibrate mercury switch thermostat

[Jeffrey Slezak]

I have a central a/c and heating unit (I live in Florida) and replaced our
electronic thermostat (which died after Hurricane Andrew) with a
White-Rogers mercury switch thermostat. I have noticed that whether we use
A/C or heat, the unit runs for an inordinate amount of time each time
heating/cooling is called for. There is almost a 5 degree differential
between the time when the unit switches on and it finally turns off.
Consequently, we either freeze for a while in the summer, or roast in the
winter.

I noticed that there is a small dial in the thermostat. I understand that
this is used for calibrating it, but have no instructions. I have tried
playing with the settings (i.e., longer, shorter), but have noticed no real
difference. I have also checked the level of the thermostat and found that
it is perfectly level on the wall.

Does anyone know exactly how to calibrate it? Does this "wheel" affect
heating and cooling, or only heating? I assume that it sets up some type
of resistance in the mercury. Your help is appreciated.

Jeff Slezak

[John H. Alderman III]

Jeffrey,
That gadget is your anticipator and not the point of temperature
calibraton. Calibration of Temperature is a separate adjustment IF you
have the capability. It inputs a bit of heat that has an effect based on
humidity on the bimetallic heat sensor ..which has an effect on your
comfort. You adjust it with an ammeter with a rating between 0 and 2.0
amps. The amp rating is usually stamped on the system control ..for you
the gas valve. If you dont have that data you take the white "heat " leg
current with the thermostat off the base. You set the anticipator to
match. If the burner run time is too short you set the anticipator to a
higher value in .05 increments and recheck cycle rate. Two stagers run 3
cycles per hour...one stagers run six cycles per hour. Heat Pumps you
set anticipator at 140% of draw to prevent short cycle to about 2.5 to 3
cycles per hour.
If you add a system humidifier you will dampen your own body swings and
widen your comfort zone at a lower temperature due to reduced
transpiration of water vapor thru your skin. This will save you money
and be a joy to your sinuses and lungs not to mention skin,hair, wood
work, static, general cleanliness. A setting of 45-50% is the best of
all possible worlds but sometimes you have to reduce if sweating and
condensation is a problem.

AND (GRIN) I sell solutions for such problems in my AlderStore on the
Internet at:
http://www.randomc.com/~mountain/

I would like to thank Honeywell for their Service Handbook for HVAC
Controls which I used as a guide for your answer. I believe in always
stopping for a moment if you can to review especially when you are
teaching someone else. I wish I had this book on CD I would put it on
site for all to reference, WITH Honeywell's Permission of course<L

John Alderman
CN3634 GA

[Vic Dura]

"Jeffrey Slezak" <jsl...@worldnet.att.net> wrote:

>I noticed that there is a small dial in the thermostat. I understand that
>this is used for calibrating it, but have no instructions. I have tried
>playing with the settings (i.e., longer, shorter), but have noticed no real
>difference. I have also checked the level of the thermostat and found that
>it is perfectly level on the wall.

I might be wrong about this, but I'll give it a try.

The "longer, shorter" is probably the "anticipator". It should adjust
what you are trying to adjust, i.e. the amount of overshoot before the
thermostat stops calling for heat or cool. Moving to "shorter" should
decrease the amount from the 5-degrees that you are seeing. Since
you've already tried that, then it may not be working properly. Try
calling a local heating/ac shop and talking to one of the techs.

--
Vic Dura (vpd...@hiwaay.net) DuraHaven, Rogersville AL 35652

[Dan Hicks]

In <01bc07f5$dc462de0$1869...@worldnet.worldnet.att.net>, "Jeffrey Slezak" <jsl...@worldnet.att.net> writes:
>I have a central a/c and heating unit (I live in Florida) and replaced our
>electronic thermostat (which died after Hurricane Andrew) with a
>White-Rogers mercury switch thermostat. I have noticed that whether we use
>A/C or heat, the unit runs for an inordinate amount of time each time
>heating/cooling is called for. There is almost a 5 degree differential
>between the time when the unit switches on and it finally turns off.
>Consequently, we either freeze for a while in the summer, or roast in the
>winter.
>

>I noticed that there is a small dial in the thermostat. I understand that
>this is used for calibrating it, but have no instructions. I have tried
>playing with the settings (i.e., longer, shorter), but have noticed no real
>difference. I have also checked the level of the thermostat and found that
>it is perfectly level on the wall.
>

>Does anyone know exactly how to calibrate it? Does this "wheel" affect
>heating and cooling, or only heating? I assume that it sets up some type
>of resistance in the mercury. Your help is appreciated.

The little wheel/lever is the "compensation" setting. It controls the
amount of current that is routed through a small electric heater in the
thermostat when the unit is calling for heat. There are numbers which
correspond to the current drain of the attached gas valve, and the
wheel/lever is supposed to be set to match this current drain.

It could be that you have a heat pump but you installed a thermostat
designed for a conventional gas furnace. If this is the case, the
compensation heater won't work right (because the heat pump doesn't draw
enough current to operate the heater).

Dan Hicks
Hey!! My advice is free. Take it for what it's worth.
http://www.millcomm.com/~danhicks/