I just installed a Honeywell Model RTH221B. It is working properly, excepting that it is showing the ambient temperature to be exactly 5 degrees above actual. I know this as I have 2 other digital thermometers that both show the same lower ambient temperature. Is there a way to adjust the thermostat to show actual ambient temperature?

Check the installation manual for an option "outdoor temperature offset" or something similar. Variance could be due to location - did you place the other instruments near the sensor to check?

Make sure you caulk or plug the hole in the wall where the wires come through. This can have a large effect on thermostats.

I don't believe there is any way to put an offset into that thermostat. Just figure out how to set it to get the results you want and deal with it. In other words, if you want the area to be 72 degrees, set it to 77 or something like that. I've seen them off by a degree or 2 before, but not 5. Buy one of the higher models like the 6000 series. You can adjust the temperature calibration on them.

EPM

HVAC1000 is right. I forgot about that detail.

Why I was thinking "outdoor ambient", I have no idea... (smacks self in forehead... )

You are forgiven.

Just a bit of additional info here... thermostat is on an interior wall in a hallway, so no outside air influence or direct sunlight.

@hvac1000... I did caulk the hole, but if I hadn't, it might have accounted for a lower temp not a higher one as any air would be coming up from the basement. When I bought my house I actually took the faceplates off all the outlets on outside walls and caulked around the openings. Too many people just do not realize how much heating / cooling loss happens right there.

@KC13... Other thermometers were placed right beside the thermostat, and since they agreed with one another I was fairly confident then that the thermostat was the one that was "off". ;-)

@EPMiller... I've come to that conclusion, yes... set the temps on the programs to reflect the ambient difference. Was just hoping someone knew of a way to actually adjust the thermostat itself.

In any case, thanks for the advice everyone!

Almost every thermostat either has an anticipator setting or a heat cycle rate setting. Read through the instructions and set thermostat to specific type of furnace. This setting will throw the reading of set temp to actuall temp way off if not set correctly.

I did caulk the hole, but if I hadn't, it might have accounted for a lower temp not a higher one as any air would be coming up from the basement

That was good that you caulked them all up but I have had cases where a leaking piece of sheetmetal duct called wall stack was causing the problem with heat in the wall spaces. While this is not a common occurrence I always suggest caulking the hole since that is what most thermostat manufactures recommend and it does solve many thermostat accuracy problems. To bad you have to live with the problem.

Is it possible for you to return the thermostat and get a replacement? Just a thought.

@mygirlsdad77... No anticipator setting on this thermostat, as it's the basic model in the line, and all other settings / jumpers are set correctly.

@hvac1000... Hadn't thought of that possibility, just another reason why it's a good idea to caulk all openings. ;-) I am OK living with the ambient temp. being off on the unit, as otherwise it's working just fine. In fact, I'm very happy with the new thermostat already. One of the thermometers I am using as a check keeps track of high / low temp. for each 24 hour period. It's telling me that the new thermostat is staying within less than a degree of the set temps. Very happy with that accuracy!