Specific heat capacity, cp, is the exchange rate between heat and temperature for a flowing fluid, and it sits at the centre of the sensible heat-transfer relation Q = m·cp·ΔT: a hydronic loop delivering load Q at a supply-to-return difference ΔT must carry the mass flow m = Q/(cp·ΔT). Water’s cp collapses the relation into the two shortcuts hydronic designers use daily — in SI, L/s = kW/(4.186 × ΔT); in US units, GPM = BTU/h/(500 × ΔT°F).
Both levers around cp are design choices. The ΔT is picked — commonly 5–8 K for chilled water and 10–20 K for hot water — and a wider ΔT means proportionally less flow for the same load. The fluid is picked too: switching from water to 30% or 50% glycol changes the specific heat, so the same load and ΔT demand a different mass flow, which is why a flow calculation names its working fluid rather than assuming water.
The mass flow is only the middle of the chain: dividing by the fluid’s density converts it to the volumetric flow the pipes and pumps are sized on — litres per second, cubic metres per hour, or US GPM — after which the velocity check (typically held between 1 and 3 m/s to balance pumping energy against erosion and noise) picks the pipe diameter.