This is the site's home turf. Every hydronic calculation you will ever do rests on one line: $\dot{Q} = \rho_w c_w \dot{V} \Delta T$ — read aloud Q-dot equals rho-w c-w V-dot delta-T. $\dot{Q}$ is the heat the loop carries in kilowatts; $\dot{V}$ is the water flow rate in litres per second; $\Delta T$ is the supply-to-return temperature difference in kelvins; and $\rho_w c_w$ is water's density times its specific heat, $998.3 \times 4186.8 = 4.18 \times 10^{6}\ \mathrm{J/(m^3 \cdot K)}$. Against litres per second that is 4.18 kilowatts per kelvin.

American plant rooms know this as the 500 rule: BTU/hr = 500 × GPM × ΔT°F. Same physics, different bookkeeping — the 500 is 60 minutes times 8.33 pounds per gallon times 1 BTU per pound per degree. It is worth knowing where a memorised constant came from, because that is what tells you when it stops being true.

And it stops being true the moment you add antifreeze. Glycol is denser than water and holds distinctly less heat per kilogram, so the product $\rho c$ falls: 30 % propylene glycol runs about 3.94 instead of 4.18, and a 50 % ethylene mix drops to roughly 3.53 — sixteen per cent of your capacity gone at the same flow and the same ΔT. For a glycol loop the relation is written with the mix's own properties, $\dot{Q} = \rho c \dot{V} \Delta T$, and the properties come from the manufacturer's table at the loop's actual temperature.

One habit that pays for itself: the ΔT is the designer's lever. Doubling the design temperature drop halves the flow, which lets you drop a pipe size and shrink the pump — and pump power goes as roughly the cube of flow. That single line is why modern hydronic design keeps widening its ΔT.