"HVAC Guy" wrote in message ...
I've got a Honeywell MagicStat thermostat that has 2 jumpers marked as
"A" and "B" that are opened or closed by a screw head that you tighten
(to close the jumper) or turn out (to open the jumper).

There are 2 other jumpers (C and D) - C is to set Celcius or
Farenheight, but I don't know what "D" is for (there is no screw for
that one - so it's open). I can find no reference anywhere what D could
be for.

I'm not quite sure what exact model of MagicStat I have - there is
absolutely no number or name stamped or printed on this thing. Not even
the word "MagicStat". I've seen models like 28, 1000, 2800 and 3200
from web searches. I believe this one was purchased between 1996 and
1999.

This is the most similar picture I could find of the front panel:

http://img.ehowcdn.co.uk/article-pag...10-800x800.jpg

And this is the best picture for the entire unit:

http://www.honeywellcentral.com/medi...A1001-U_lg.jpg

The thermostat is controlling a 20+ year-old GE "Whisper-heat" natural
gas furnace. This is NOT a high-efficiency (or condensing) furnace, but
it does have an electronic control module, combustion damper gate and
electronic ignition.

The furnace is cycling too often to suit me, and so I'd like to know if
the thermostat can be modified to solve that.

The written documentation for the A and B jumpers are as follows:

Warm Air Furnace: Set at the Hot Water setting (A-out, B-in)
Electric furnace: Leave at the Warm Air Furnace setting (A-in, B-in)

(the way the second line reads is confusing)

It also says:

For high efficiency (90% AFUE): A-out, B-in

On the back of the unit these instructions are stamped into the plastic
case:

Warm Air Furnace: A-in, B-in, Fuel Switch F
Hot Water Boiler: A-out, B-in, Fuel Switch F
Electric Furnace: A-in, B-out, Fuel Switch E

On my thermostat, the current settings a

Warm Air Furnace: A-in, B-in, Fuel Switch F

I believe that the only thing the Fuel Switch does is turn on the fan
when heat is called for (in the E setting) otherwise the fan is turned
on by the furnace (in the F setting).

So if anyone knows, I'd like some clarification as to how exactly the A
and B settings work or how they modify the operation of the furnace.

It would be nice if there was a SPAN or hysteresis setting so I could
force a 1 or 2 degree swing so the cycles last longer, but I see no such
setting. I'm wondering if the A and B settings can accomplish this?

And - anyone know what the D jumper does?

Sounds like your trying to use the thermostat to mask the primary
problems.... and that would be having a furnace that is a whole lot bigger
than you need, and not enough insulation.
20 years is a normal *high* life span for a gas furnace. It would be time to
look into replacing the furnace before it becomes a problem and craps out
when its really cold.

The contractor that installs the new furnace should do a complete
room-by-room Manual J heat load/loss analysis to correctly size the new
furnace for your home. You might be surprised at the results.