Why use a Delta-T based circulator? Delta-T works, it has to, it's part of the Universal Hydronics Formula which governs everything from pipe sizing to circulator selection that states:
GPM = BTUH ÷ (∆T x 500)
Basic math tells us that if we change the heat load required while the Delta-T and the 500 constant remain the same, the GPM required to deliver the heat has to change (variable speed pumping). The pump will automatically speed up if the heat load increases and slow down if it decreases.
Delta-P is not part of the equation!
At best, Delta-P is a guess (and even when guessed properly it's based on design conditions, not 20° - 55° where the majority of degree days occur). Delta-P can still over or under-pump a zone and does little to improve system comfort.
