Do it Yourself: Re: [DIY] Re: Inaccurate Thermostat Temperature

31 December 2014

Re: [DIY] Re: Inaccurate Thermostat Temperature

Thermostats do go bad. The temperatures where the thermostat turns off and back on again is called the differential setting. Most thermostats don't have a controllable adjustment for that. Its usually set several degrees apart from the set temperature to stop what is called hysteresis, I think that s the right word for it. That is a condition where the thermostat turns off and on repeatedly faster that the unit can respond.
Usually the wire used is about 18 gauge bell wire. Mostly just for the durability but a smaller gauge can be used. The amperage at that voltage is very slight. Bell wire is a solid copper wire. The voltage is usually low but can vary according to the standard of the AC unit/heating unit being used. Most AC/heat units are 24VAC but look on the AC unit to see if you can find the voltage for the thermostat. That voltage will determine what thermostat you get to replace the old one. There should be 3 wires used, a common, AC, and heat wire. I suppose its possible they could use 24VDC too. Just be sure to get the right thermostat and the repair is easy.
Steve

On 12/30/2014 4:46 PM, subprong subprong@gmail.com [DoIt_Yourself] wrote:

So, I think I've stumped the panel with that previous question.

Anywho, I think I need to replace the digital thermostat with an older manual thermostat that I already have.

As far as I can tell things seem to be straight forward with the wires and their corresponding letters (colors). The only snag that I can see is that I believe the RC and RH (power for cooling and heating) have a jumper wire. Now the gap on the current thermostat between those two terminals is shorter than the gap on the thermostat that I want to install. Meaning, the jumper wire will be too short for the new thermostat. Is there a certain type or gauge of wire that I need to get and cut as a jumper for the install on the thermostat?

On Mon, Dec 15, 2014 at 11:05 PM, subprong <subprong@gmail.com> wrote:

Digital thermostat for AC/HEAT (about 10 years old). It's reading the temp incorrectly. I don't know what the actual temp is but initially I estimated that the reading is at least 10 degrees higher than what it should be reading. ...and I think the gap has widened within recent days (as a matter of fact, as of the last reading it was apparently 110 degrees or "10" since there isn't room for a 3rd digit).

I've checked the batteries. They still have juice in them (plus the change battery indicator isn't showing) and there are no signs of leaked acid. I've "reset" it a few times but the temp climbs right back up to the incorrect reading. I see no signs of dirt or dust in the unit. This thermo's temp can be manually changed but the differential is only by 4 degrees up or down and I'm sure it's well above that mark.

My question. At this point, I'm going to assume the thermo has gone bad (but please tell me if there is something else I should look for). I don't care what the reading says. I'll keep it so long as the AC or Heat kicks on at the correct temperature. Meaning....please tell me whether this would be the case or not....

Let's say the actual temp inside is 70 degrees but the unit states that it is 80 degrees. Now, if I set the heat to kick on at a thermostat reading of 82 degrees (2 degrees above the thermo readin)....will the heat actually pump out at 82 degrees or will it pump out at 72 degrees (2 degrees above the actual temp)?

I guess I'm not sure if there is a secondary measure of temp in the central air unit or if all measures of temp are read and sent strictly by the thermostat.

...on a side note, is there a reason why thermostats don't allow you to start the heat or AC (not the fan) at any particular temperature (as oppose to only above or below the actual temp).