Hydronic Outdoor Reset Ratio and Supply Target

The boiler outdoor-reset curve: reset ratio = (design supply - min supply) / (no-heat OA - design OA), and the supply target at any outdoor temperature = min supply + ratio x (no-heat OA - current OA), clamped between the min and design supply. A 180 F design supply at 0 F, resetting to an 80 F minimum at a 65 F no-heat point, is a 1.54 ratio, so at 30 F outdoors the boiler targets about 134 F -- lowering the water as it warms saves fuel and improves comfort. High-temp fin-tube runs a steep ratio; radiant floors run a shallow ratio off a low design supply. The boiler must still protect its return above the condensing/flue limit; the control manual and the building heat loss govern the curve.

reset_ratio = (supply_design_f - supply_min_f) / (oa_noheat_f - oa_design_f); supply_target_f = clamp( supply_min_f + reset_ratio x (oa_noheat_f - oa_current_f), supply_min_f, supply_design_f ).

Hydronic outdoor-reset control practice (Taco / tekmar / Caleffi Idronics), by name; the linear reset curve is standard control math.

The reset relation is a first-principles linear interpolation; the design and minimum supply temperatures and the outdoor design / no-heat points come from the building heat loss and the control manual.

Estimate. AHJ and licensed professional govern.

Field names used by the API: supply_design_f, supply_min_f, oa_noheat_f, oa_current_f, reset_ratio, supply_target_f

• Linear curve a straight reset line between the design point and the no-heat point; clamped to the min/design supplyoutdoor-reset control practice
• Return protection a non-condensing boiler must keep its return above the flue-condensation limit; the min supply and a bypass govern the low endboiler protection