On Dec 29, 8:52*am, HVAC Guy wrote:
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...a08/7r/3r/inst...

And this is the best picture for the entire unit:

http://www.honeywellcentral.com/medi...3200A1001-U_lg...

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?

I remember that Thermostat . The A and B screws act as a heat
anticipator does ; the more you back out the A screw with the B
screw turned all the way in....the greater the differential will be
and the greater the room temperature swing will be . It is all done
by trial and error using the screws , so, you have to set the screws
then cycle the system thru a couple times to see what kind of room
temperature drop youre getting before the next heating cycle starts.
Since the furnace has electronic ignition, youll want to set it up as
if it were a high efficiency gas furnace which ordinarily would
correspond to a .8 amp anticipator setting ; thats what you have to
shoot for by trial and error by adjusting the Magic Stat screws.
(They dont call it an 'inexpensive' thermostat for no reason !) .
If , after youve done that and you still get a rapid cycling
response...then you could install a simply spst start relay at the
furnace which would effectively reduce the heat anticipator (amp)
demand of the Magic Stat and possibly bring you within the operating
range of the A / B screws for more consise adjusting . ... the Magic
Stat would operate the added relays coil directly , with the relay
contacts making and breaking R to W at the furnace control board
strip . Good luck. ALso, theres a Honneywell Thermostat Tech 800
number for Contractors ; if you need it i can try and find it for
you.