My programmable thermostat current temp reading is 65 but my temp is set to 68. I noticed it yesterday also (read 66), and this is not normally the case. The "current temp" setting is normally always the same as the "set temp" reading. Is something wrong?

Hi Demetrio
Furnaces turn on and off based on what they're told by your thermostat. The thermostat, in turn, will control the furnace based on:

1. The temperature AT the thermostat.
2. What the internal heat sensor thinks the temperature of the air is.

So, if your idea of 68 degrees is what the thermostat thinks is 71 degrees (thermostat reads three degrees high), then when the air by the thermostat gets to about 65 degrees (by your account), it'll think it's 68 degrees and shut the furnace off.

Also, if the thermostat is, say, near a source of local heat, like a lamp, a big piece of electronics, a fireplace, or that place that you habitually set down your coffee, then the air right next to the thermostat may be warmer than the rest of the room, and fool it into thinking it's at it's target.

The sensor is usually inside the housing for the thermostat. If it hasn't been dusted in a while, then it can be hard for air to get inside to affect the sensor, though this would generally cause it to lag on readouts and run the temperature higher than you wanted.

If you have a digital readout, and the system is saying , hey, target is 68, and the readout gets to 66 and the system cuts off the furnace.... my only other guess is that the unit may have features that are trying to prevent overshoot - i.e. by the time the temp at the thermostat actually gets to 68, most of the room is more like 73. That'd depend on your particular thermostat. At that point, you may want to call customer service with the manufacturer and tell them the issue. There may be ways to adjust the unit to get behavior closer to what you seek.

Good Luck!

DH

Well Brian, an air conditioner creates the same temperature drop across the evaporator at any thermostat setpoint, so there is no setting that will get it really cold really fast. If your return air is 88 degrees (in a hot house where the system has been off) the supply air temp is going to be near 68 degrees..... as the a/c runs, the air changes within the structure cause a reduction in the average return air temperature and slowly drops the return to 87, then 86 then 85 etc, etc and the corresponding supply air temp continues to drop from 68 to 67 to 66 to 65 etc etc. Eventually the supply air temp after the appropriate 'air changes' within the structure will drop to 55 degrees and possibly even lower to satisfy the average room temp at 75, 74, 73 etc. to a point.

There are SOME newer a/cs that have two speed compressors that may effect the supply air temperature drop under differing load conditions, but thats a different ball of wax.

So, if you want it cold quickly you are out of luck, just leave the t-stat set at 70 degrees and pay the electric bill when it comes in.....

You didn't include a model number, so it's hard to know for sure - but I'll take a stab at this one. Many of these t-stats have a programmable offset to reduce "hunting". This can be as little as 1 degree or more than 2 degrees.

This means if you set the offset to 1 degree and the room temperature to about 65 degrees, the furnace will heat the room until the t-stat registers a rising temperature of 66 degrees. The furnace shuts off and will not turn on again until the t-stat registers a falling temperature of 64 degrees. The average is 65 degrees, and prevents the furnace from starting and stopping numerous times instead of just once with the 1 degree offset.

You can further reduce the number of furnace start and stops by increasing the offset to 2 degrees. This would work like this when the t-stat is set to 65 degrees and the room is cool:

Room Temp Furnace
63 On
64 On
65 On
66 On
67 On
68 Off

The room is now warm and heat or furnace is off. The room begins to cool like below:

67 Off
66 Off
65 Off
64 Off
63 Off
62 On

With a 1 degree offset, the room temperature varies +/-1 degree (about 2 degrees total). With a 2 degree offset, the temperature about varies +/ -2 degrees (about 4 degrees total).

Also, when the thermostat shut the heat off, the heat that was made is delivered to the space instead of wasted by sending outdoors - so the room will get a little warmer even though no fuel is being burned. The same holds true at the other end. When the t-stat turn heat on, fuel is burned, but there's little heat until the source has reached the optimum temperature and then the heat is delivered to the space.

I hope this helps & good luck! Pleas rate my reply - thanks!

Dear friend! Any thermostat, it's calibrating to cut off at the end of 2nd beginning and cut in 2nd ending of temp.
Ex: In heat mode, If you set @ 68 degree, it will cut off 70 degree beginning and cut in at the end of 66 degree.
Hope it's helpful to you?

Depending on the outside temp. your heat pump might not keep up with the demand. It is not economical to run your heat pump in temperatures at or below freezing. Doing so could cause compressor failure and these are not cheap to replace. I would be running the aux. electric in low temps.

You did not make it clear if the thermostat is a programmable unit or not. I am wondering if the thermostat is not set on the correct heat anticipation setting. A heat anticipater is a small heater that helps the thermostat turn off before it over shoots the desired temperature setting. If the thermostat is set for a 5 degree setting, it will turn off at 63 degrees if set at 68. If the anticipater was set at 1 or 2 degrees, it will shut off the heater at 66 or 67. Newer thermostats will only allow the thermostat to cycle 6 times per hour so if you never reach the desired temp, you have to wait 10 minutes before it will allow a call for heat. Check the installation instructions to see if there is a setting where you can get the anticipator temperature closer to your set temp. Good luck.

if it is around 93-97 degrees outside,thats all the cool your going to get

Don't know of anyway the filter could cause this. If the filter was bad (clogged), it would cause a rise in temp of the air while in the heat mode but the system should still cut off when the set temp (65) was reached.
Some of these t'stat's are programmable for 7 individual days. You may have to set the temps you want and then "Copy" them to the other 6 days. Read your instructions that came with it or, sometimes there are some brief instructions on the inside of the cover, if it has one.

Most of these problems are the program settings on these way overcomplicated thermostats. You will need to re-read the instructions on setting the programs. Or bite the bullet and buy a cheaper and simpler one. If you installed it yourself, make sure the settings on the inside are correct, like gas/electric heat, and 24 volts, stuff like that.