If you don’t read past the first paragraph of this article, the one takeaway I want you to have is that a hot water boiler does not determine the system's delta-T (ΔT). Instead, the boiler heats the return water up to the operating set point. The system ΔT is the weighted average of the loads along the type of control valves (2-way vs. 3-way), and how the boilers are piped and pumped.
Let’s walk through a quick history review of hot water boilers and system piping. With early systems, simplicity was the key factor using constant speed pump(s) and constant volume systems with 3-way control valves. Early boilers were noncondensing and had simple on-off or low-high-low fire controls, where the constant flow provided a large flywheel for the boiler operating controls. These early systems consumed significant pumping energy if the heating system operated year-round. As energy became more important in system design, engineers realized that most of the pumping energy was in the distribution system because it had higher head. Engineers developed a constant flow primary and variable flow secondary pumping arrangement with a hydraulic decoupled bypass, allowing the two loops to operate independently. The variable flow secondary with 2-way control valves allowed the flow to be varied with the load and the energy to be reduced from varying the secondary flow. The constant flow primary pumping didn’t fundamentally change how the boiler operated and worked universally with either a high- or low-mass boiler. The primary pump head is typically low, so the constant volume pumping energy isn’t a significant energy penalty. Existing constant volume systems could also be easily retrofitted to primary-secondary with the addition of primary pumps and a bypass; the existing system pump(s) would serve as the secondary pump(s). Prior to variable-speed drives (VSDs) being widely available and cost-effective, a pump without a VSD with a system of 2-way valves simply “rode” the pump curve as flow was reduced. This reduced power by approximately the square of the flow. With a VSD, the pump power reduction is approximately the cube of the flow. With noncondensing boilers, the boiler efficiency is correlated to the boiler leaving water temperature and stack temperature independent of the return water temperature. Therefore, how the boiler is piped, and the type of control valves used on the loads has a negligible effect on boiler efficiency.