The gauge thats reading 100, is it the same gauge thats been installed for the last 6 years or something new, or are you using a infrared gun? Do you have more than one gauge that reads 100?
If its just one gauge thats reading 100 maybe its a fault gauge, or if your using a infared gun sometimes when they are used on different materials they won't be accurate at all, I've noticed the accuracy being way off when using them on copper and galvinized.
If it is truely 100 degress when it used to be 160+ you should be able to keep your hand on a 100 degree pipe for quite awhile, at 160 I'd think you'd only be able to hold onto it for a second.
At 100 degrees trying to heat your house I would think the circulator pumps would run almost constantly, or the fan if your using forced air. Does everything seem normal on the house side of things?