The thermostat is about 15 years old. I can't get my heat up to the set temperature. I have it set at 72 degree all the time, morning and night and it slowly falls during the night. I have had both a boiler technician and an HVAC person and both say everything seems to be working fine. I am wondering if it is the thermostat. Can you suggest anything that I can try. The heating system comes on and goes off supplying heat, like it always has, but unless the outside temperature is in the 40s I can't the temp up to 72.

You did not say what type of system you have, as the type of heat can make a difference and the age of the system, not just the age of the thermostat.

If the spot where the thermostat is getting to temp, then it will go out (even if other parts of the house cold) so checking that temp area would be good.

Assuming the boiler controls are operating correctly, and the thermostat is telling the system to shut down, then it has to be that.

This appears to be by the number a programmable unit, so an error in programming that is telling it to lower temp at night, may be happening.

After a manual cycle your tstat may be defaulting to it's program. You never said what the actual temperature was but sometimes if the set temp is to close to the actual temp, you get this kind of symptom, not enough heat. Check your program settings and adjust as needed. Raise your set temp to 77, and see if that makes a difference.

Please clarify for me your post. Are you saying the set point and actual temperature on your display is not the same? What are those actual temp numbers? Or do you mean parts of your home is colder than others?

Possibility it's not the TStat, or boiler but the actual delivery systems. Baseboard, or radiator?

"but unless the outside temperature is in the 40s I can't the temp up to 72."
That in itself is a clue. A lot depends on the insulation in the house, windows, air leaks and other things that make a house lose heat. As the ambient temperature drops at night, the heater has to work harder. A set point of 72 makes it even worse. If the outside temp is 22* and you have a set at 72*, you have a 50* differential, a lot to overcome. Then, with poor wall, floor or roof insulation, it will be almost impossible. We can't determine the real issue without knowing all the ambient factors in your house. If all the technicians say everything is OK, it is time to look at other house issues. Where do you live? Age of house, insulation R factors?