"If your boiler makes your domestic hot water through a device called a
tank less coil then if the controller is working correctly it will
prevent the circulator pump from operating until the temperature
setting gets up to the minimum for domestic hot water. On a call for
heat the system will rise to the (high) limit setting which should be
at least 20 degrees higher than the low setting. The differential
setting is for the low setting only."
from and many more words you need of advice see source:
http://toad.net/~jsmeenen/hydronic.html