While working on a Francis Francis in our repair center, we found out it's awesome little secret: It comes with an internal PID! You can't adjust it yourself, and some of the newer models like the X7 aren't designed to be adjusted at all, but it will keep the boiler's temp within 1 degree of the set temperature -- unlike less sophisticated thermostats that have a varying range of about 20 degrees and, thus, require temperature surfing.
Josh and Gail cracked open an X7 to show its guts and glory!