Honeywell MagicStat thermostat -> questions about jumper settings on the back

[HVAC Guy]

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:

And this is the best picture for the entire unit:

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 sec 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 are:

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?

[Steve]

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.

[Steve]

>>

He still has a 20+ year old furnace.... doesn't matter where he lives... and short cycling still indicates a grossly oversized furnace no matter where you live.

[Dave M.]

I've got the Honeywell CT3200, which matches your pictures but not your description. My unit has 3 dip switches and a larger fuel switch. The manual offers no suggestions about altering the duty cycle. The furnace is expected to cycle about 5 times per hour and is meant to hold within 1 deg F of the setpoint.

Dave M.

[HVAC Guy]

Why are you such a trollish Klown?

Or are you naturally stupid when it comes to furnaces and thermostats?

Any boob knows that it's the thermostat that controls the cycling of the furnace.

You can make ANY furnace short-cycle given a tight temperature span.

[HVAC Guy]

Your explanation indicates that the amount of turns of the screw is somehow sensed by the thermostat's circuitry.

This assumption is incorrect.

On this thermostat, the screw is not functioning as the core of an inductor or coil (as per your explanation). The only function the screw has is to bridge the wire contacts on the upper surface of a machined plastic block. The wires connect to the PC board.

If variable operating was desired, then Honeywell would have used a potentiometer instead of a significantly more complicated reactance circuit.

All the printed manuals for these thermostats indicate that the screw has basically only 2 settings: Fully turned in (tightened down) or unscrewed (turned out) by one turn. The one turn is sufficient to raise the underside of the screw head enough so that it is not contacting (bridging) the wires.

[HVAC Guy]

I obtained the documentation for a MagicStat CT3300 and it shows a diagram of the back of the unit that is identical to what I have. I copied that diagram and it can be seen here:

The PDF file itself can be downloaded from here:

That appears to be the case from what I read as well.

That's a pretty tight temperature range and to be able to hold it within

1 degree F and keep to only 5 cycles per hour really depends on a lot of factors (how the premesis is constructed, how drafty it is, how cold / windy is it outside, how many BTU's the furnace puts out, etc).

This thermostat basically has 4 binary-selectable settings:

A-in / B-in Conventional warm-air (nat-gas?) furnace A-in / B-out Electric furnace (or electric base-board?) A-out / B-in Hot water boiler (radiant heat?) A-out / B-out not defined (hmm...)

Just guessing here, but why would a thermostat need to know between which of the above 3 systems is it controlling?

The speed of response of the system? Nat-gas forced-air can respond faster than a boiler, for example. To anticpate the call for heat (reduce the control span?)

The thermostat doesn't want the ambient temp to drop too far below the setpoint, and it doesn't want to over-shoot the setpoint. It's not clear which of the above options would give me a wider span (and hence fewer cycles per unit time). I guess I'll just have to experiment - or buy a new thermostat with a user-settable span or hysteresis setting.

[Dave M.]

Guy,

I agree with your conjecture that the screws (in your case) or DIP switches (in mine) change the anticipator circuit and thus adjust the cycling. I suspect that experimenting with them will be harmless but unrewarding. The settings recommended in the manual are those that will work best. Let me know if you find better settings.

Dave M.

[Steve]

Sure... go ahead on... you know what your doing, so you don't need to worry with posting questions about ancient POS big box stats.... when you can get a good one for $50 or so.

Just cause your "HVAC Guy" doesn't mean squat.... you should have a decent stat on your truck that you can use instead of screwing with a POS.

[ilbebauck]

Whenever i adjusted the 'A' screw in or out partially...it DID make a difference on the cycle time (differential) . I wouldnt give Honneywell too many accolaides as far as engineering goes because this thermostat of theirs is a poor example of quality..certainly one Engineers wet dream to flood the market with a most inexpensive thermostat without much consideration to accuracy. A proper electronic thermostat that is designed well, is one where the Person can make a definitive selection between 1,2,or 3 degrees F. differential for heating AND for cooling....and not a psuedo- thermostat like a Honneywell Magic Stat. Sadly, these are things the typical homeowner isnt up to par on.

[ilbebauck]

Totally agree . The Hunter wouldnt properly control anything beyond a .4 amp device in the heating mode.

[HVAC Guy]

My gut feeling, based on turning out the A screw, is that the furnace cycle time has been lengthened to a more satisfactory condition, with no discernable change in overall ambient air temperature.

A-out / B-in Hot water boiler (radiant heat?)

So this is, in effect, telling the thermostat that instead of controlling a conventional mid-efficiency forced-air natural gas furnace, that it's controlling a hot water boiler.

This seems to have the effect of not turning on the furnace as often.

[Grumpy]

[Steve]

I still haven't figured out why nobody has posted the link for the instructions for this antique big box store POS stat.

Either the OP is an idiot (this is a given), and/or he's just too freakin lazy to actually look for it, and/or hes nothing but another troll. A simple yahoo search gets over 19,000 hits. How hard is this??

You can also go to the Honeywell site and get it from there....

[Daniel who wants to know]

Because not every Honeywell manual is perfect, especially for a cheap HwHwST

The manual for mine, a Honeywell RTH221B1000 that I got 3 of for $5 each from the Orscheln Farm and Home bargain bin makes no mention of cycles per hour. If you Google the model RTH221B, you get this manual:

However if you Google RTH221B cycles per hour you get this manual:

Or Google RTH221B1000 cycles per hour and get this manual:

The latter 2 manuals both explain how to press both temp arrows together for

3 seconds to enter the hidden settings menu.

If you want a real Honeywell manual you have to step up out of the HwHwST class and into the professional class, Google Honeywell t8600d t8601d and get:

Now That's what I call a manual.

HwHwST=Honeywell hardware store thermostat.

[HVAC Guy]

Of course not.

Because the number of cycles per hour will depend on

- the temperature span or hysteresis around the desired set point

- the heat loss of the space being controlled

- the BTU capacity of the furnace

If the stat has a setting for cycles per hour, then you're going to have a variable hysteresis, and the stat is going to take some time to learn what that's going to be for a given set of conditions.

To make it more complicated, the heat loss of the space is going to be affected by ambient outside weather (wind and temperature) and how well the space is insulated. As that changes, the thermostat will have to re-learn how to control the furnace to keep the desired set-point while maintaining the desired cycles per hour.

It's far simpler to set the hysteresis (2 or 3 degrees instead of 1 degree) and live with what-ever cycles per hour you end up getting, rather then aim for a set or fixed cycles-per-hour.

[Daniel who wants to know]

Did you read any of the PDFs?

Anyway the problem here isn't Honeywell's choice of CPH over hysteresis degrees, it is how this stat (RTH221) seems to function, when set to 5 CPH it calls for heat every 12 minutes as expected, but when set to 3 or 1 CPH in my house it only drops to calling for heat every 13 minutes, and this is after several hours to adapt. I have a 90+ condensing gas furnace and this style is only supposed to be cycled about 3 times per hour.

Also interesting with this model is that when set to 62° F as it cycled its reading would swing from 58° to 65° with each cycle while 3 other thermometers, a non-connected Lux digital thermostat, a digital meat thermometer, and an indoor/outdoor thermometer and hygrometer weather station all placed right beside the RTH221 would all show the swing as being from 61-63°. When the temp setting on the RTH221 was first set down to 62° from 68° its reading quickly dropped to 61° and stayed as the 3 others showed the normal gradual drop. When the others started to show 61° as well the RTH221 dropped to showing 60° and shortly thereafter called for heat.

It is an odd model for sure.

[Steve]

Yup... big box store junk..... There is a sucker born every minute that will buy one.

[Grumpy]

degrees, it is how this stat (RTH221) seems

expected, but when set to 3 or 1 CPH in my house

hours to adapt. I have a 90+

times per hour.

reading would swing from 58° to 65°

******************************************************************** Ok enough of this crap apparently your cheepy Tst is much more sensitive

Than the others, to make it more stable you can cover Tst with something

To see how much of effect will have, or/and if you have forced air heating

Make sure that your duct discharge is not blowing on to Tst.

********************************************************************** with each cycle while 3 other

thermometer, and an indoor/outdoor thermometer

show the swing as being from 61-63°.

reading quickly dropped to 61° and stayed

show 61° as well the RTH221 dropped to

[Daniel who wants to know]

That's what I thought at first so I wedged some plumber's putty in the corner where the thermistor is located (it sits epoxied into a small recess in the bottom left corner of the plastic housing) to add thermal mass, this showed no change. Besides have you seen how fast a meat thermometer reacts to temp changes? In air it only takes a few seconds per degree for a small change, in meat they don't call them "instant read" for nothing.

I thought about also putting a clear storage bowl or one of those anti-tamper plastic boxes over it.

Nope, the supplies are on the other side of the room.