Yet another A/C question...............

Problem (I think)
I have a 3000 S/F ranch with central A/C. The A/C is controlled by the same thermostat as my heat. It's the standard issue Honeywell t-stat, round with A/C and Fan switches at the 11:00 and 2:00 positions. Heres the thing thats strange. When I set the temp to say 80, the A/C comes on but doesn't shut off until the temps down around 70 - 72. This is according to the thermometer on the t-stat, as well as a digital thermometer I have on the other side of the room. This same t-stat is spot on in the winter months when used for heat. Why is there such a wide swing in tempurature on A/C but not with heat ???? When I pop the cover off the t-stat there looks to be an adjustment pointer on the bottom. what does this adjustment do ???? I'm stumped here..... Thanks for the help....
Brian
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This is Turtle.
Pull the ring cover and see if any wires are hanging up on the mercury bulb going to the subbase. If not change the thermostat for you got a bad cooling antipistator in the sub base and getting a long range on cooling.
TURTLE
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Brian wrote:

Possibilities: 1) The t-stat is out of calibration. What does the temperature rise back up to before it calls again for cooling? 2) The t-stat may have a worn-out bimetal (that's the ribbon spring looking thing behind the bulb assembly). No problem in heat mode because it wasn't bad during winter. 3) Short, causing current to flow through the heat anticipator or cooling compensator during call for cool. This will fool the t-stat into thinking its warmer inside than it is.
This is a typical problem in older Honeywell rounds when they pass about 20 years in age. How old is this stat?
In any case the t-stat seems to be the defective component. There are other possibilities, but most would cause erratic cycling which you didn't report. Given that this is a common fault for this t-stat odds are its due for replacement. HTH.
hvacrmedic
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It rises back up to the set temp before it comes back on.
Whats that slide adjustment thing on the bottom of the t-stat ????

Actually, this problem started at the end of last summer but I had forgotten about it. The heat was ok this winter though.

Just about 20 years old............. Now i'm noticing it's 12 degrees colder than it's set for and the A/C is still running........ Maybe it's time for a digital t-stat............. Are they any good ????

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wrote:

Yes, they are. Honeywell also makes a digital Round, but I wasn't that thrilled by it. Very hard to read... Electronic stats are pretty cheap these days..
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If it's a mercury switch based stat (not electronic), make sure it's level. There will be some pegs on the base that you put a level on.
As Turtle pointed out, that "pointer" is the anticipator...
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That pointer is all the way to the left......... Is that where it's suppose to be ????
Thanks..................
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Brian wrote:

That has no effect on cooling whatsoever. That's a heating element that's inline with the heating circuit only. It's has little numbers on it. 0.2, 0.4 etc. These are the current draw of the 24volt heating circuit, and it should be set to match that current. Anyway, not your present problem.
hvacrmedic

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That's the heat anticipator-from what I recall it's supposed to be set to match the current draw of the gas valve in the furnace
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I have an oil fired furnace........ Does it do anything for that ????
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Yes, but like hvacrmedic said, the anticipator is only for heat anyway. These stats HAVE to be level to work correctly.
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Is the digital t-stat under a duct or in the cold air stream? If so move it to another location and see if the same situation happens.
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