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Old 02-26-2010, 03:58 PM   #1
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Adjusting '69 Honeywell Thermostat

I'm cleaning up my '69 Safari's electrical a bit in this thread:

http://www.airforums.com/forums/f449...eps-61514.html

..and in the process I seem to have messed up the function of my appears-to-be-original Honeywell thermostat. The arrow points to the screw I tried to "adjust" (yeah, right)...:

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the larger metal part above it is the magnet..

I'd sure appreciate some basic insight into this adjustment...I really like the patina of the old Honeywell and would like to keep it alive as long as possible....thanx
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Old 02-26-2010, 04:27 PM   #2
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Quote:
Originally Posted by fotochop View Post
I'm cleaning up my '69 Safari's electrical a bit in this thread:

http://www.airforums.com/forums/f449...eps-61514.html

..and in the process I seem to have messed up the function of my appears-to-be-original Honeywell thermostat. The arrow points to the screw I tried to "adjust" (yeah, right)...:

Attachment 97021

the larger metal part above it is the magnet..

I'd sure appreciate some basic insight into this adjustment...I really like the patina of the old Honeywell and would like to keep it alive as long as possible....thanx
Tsk, Tsk.

You had the right idea, but you adjusted the wrong thing.

As you face the thermostat, look at that screw, which is really a contact. To the left of that, is a bi-metallic strip, which is the other contact.

Now the good part, which is the solution. Immedistae to the left of that contact, is a small aluminum bar, that the backside of the left hand contact rests against.

Simply bend that small bar, wich moves the left hand terminal closer to the screw terminal.

The closer you place that very left hand terminal, to the screw terminal, the better the thermostat works, "AND" the more sensitive it becomes.

Make that adjustment with the thermostat set as low as possible.

You can, if necessary clean up the two terminals with a nail file, "GENTLY".

That should put the thermostat back in operation for you, better than new.

Questions???

Andy
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Old 02-26-2010, 04:36 PM   #3
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Here's a link for insight

Quote:
Originally Posted by fotochop View Post
.. I'd sure appreciate some basic insight into this adjustment...
I'm not sure, but I think you adjusted the heat anticipator.

While I think Inland Andy may have imparted good advice, you might want to review this link:

How & Why to Adjust the Thermostat Heat Anticipator

Tom
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Old 02-26-2010, 10:09 PM   #4
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Mr. Tom is right on about the anticipator function, although it doesn't work exactly as described in the link above.

This thermostat is a magnetically latched switch. The little screw is both the electrical contact, and an adjustable stop for the end of the bimetallic coil which is the other side of the switch. The screw controls the stand-off distance between the magnet and the coil.

If the screw is backed out far enough the coil can actually contact the magnet. At this setting the coil is most strongly attracted to the magnet and it will take considerable force in the opposite direction to break the magnetic connection.

Conversely, if the screw is screwed in, it will not allow the end of the coil to contact the magnet. The larger the separation between the magnet and the coil the less force will be required from the coil to break the connection.

The force opposing the magnetic attraction between the magnet and the coil is provided by the heat applied to the bimetallic coil. For the sake of argument, let's say the torsion on the coil is 2 gram of force per 10 degrees of temperature.

Further, lets say the magnet has a maximum attraction of 3 grams, and the magnetic attraction drops off at a rate of -1 gram per millimeter separation. In other words, if the coil is directly in contact with the coil the attraction force is 3 grams, and if the magnet and coil are separated by 1 mm the force drops to 2 grams, and at 2 mm it drops to 1 gram, etc.

You can easily see that if the screw is backed out, the coil force needed to overcome the magnet is 3 grams. This would require a temperature change of 15 degrees using the coil torsion equation of 2 grams per 10 degrees F.

If the screw is screwed in so that the coil and magnet separation is 1 mm, the required force would be 2 grams, or 10 degrees F.

So by this explanation you can see that the position of the screw controls the "dead band", or temperature change required to change the thermostat from the closed to open position. This is exactly the same as an "anticipator" function in a more conventional mercury or electronic thermostat.

I would suggest that you need to return the screw to near its original position in order to restore operation of the thermosat. Bending the aluminum tab changes the sensitivity of the thermostat in the "make" (open to closed) direction but does nothing to correct a misadjustment of the "break" (closed to open) function.

You may need to discover the correct setting by trial and error, but I hope an understanding of the function of the screw will help you arrive at a quick solution.
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Old 02-27-2010, 06:04 AM   #5
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-thanks, guys, and yes, markdoane, "magnetically latched switch" describes this thermostat exactly....

-the setting on the adj screw at this point is too far backed-out because it will not un-couple when the temperature rises in the trailer...I have to disconnect with my finger to turn the furnace off...

- my first step will be to try and find the sweet spot through adjustment and, hopefully, keep this original functioning properly... worst case I go buy a new one but I'm hoping to avoid that..
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Old 02-27-2010, 08:38 AM   #6
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Let me know if you have trouble and I will dig mine out. It's in a box somewhere off-site.
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Old 02-27-2010, 09:19 AM   #7
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-very generous offer, appreciated.... your explanation is so good I'm going to copy it for my 10 yr old's science class... a good math/science experiment..
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Old 02-27-2010, 09:50 AM   #8
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.... your explanation is so good I'm going to copy it for my 10 yr old's science class... a good math/science experiment..
Understand that the units used in my example are totally made up. My explanation should be just a starting point for his or her own experimentation.

For example, the magnetic force equation in the example (-1 gram/mm) is certainly not a linear function; it is at least an exponential function and may be more complex.
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Old 02-27-2010, 09:58 AM   #9
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understood... I don't think they're working with exponents yet..

I'm more interested in showing her the magnetic and mechanical workings of the bimetallic spring and adj screw... your made-up assumed values are clear enough that even I can understand it...
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