Lesson E1 made the case for the hot-water loop; this lesson designs one. Four decisions shape every hot-water system — the ΔT, the pumps, the balancing, and the storage — and each one made lazily becomes a permanent operating cost. The good news: hot-water design is honest arithmetic all the way down, and the arithmetic is here.
Decision one: the ΔT — the master variable
Choose the temperature drop across the users and you have chosen almost everything else. The duty equation (lesson A10 with water on both sides):
flow (m³/h) ≈ duty (kcal/h) ÷ (1000 × ΔT)
A 500,000 kcal/h load at ΔT = 10 °C needs 50 m³/h of circulation; at ΔT = 25 °C, 20 m³/h. Wide ΔT means smaller pipes, smaller pumps, and pumping power falling faster than linearly (it scales roughly with the cube of flow through a given circuit). The price of wide ΔT: users at the return end see cooler water, so heat-transfer surfaces must grow (lesson A11 — LMTD shrinks). The working compromise in process systems is 20–30 °C; tight-uniformity duties argue it down, pumping economics argue it up, and the argument should happen on paper, once, at design.
Decision two: pumps and the loop's shape
The pump is the loop's boiler-equivalent: sized for flow (from the ΔT decision) against the circuit's resistance, on the pump curve where efficiency lives — not at its edge. Two disciplines: NPSH respect — hot water is always near flashing; the pump wants the coolest, highest-static-head point of the loop (pump into the heater on the return side, classically), or it cavitates every summer afternoon (the E1 site check). Variable speed where loads swing — a VFD trimming pump speed to hold loop ΔP converts the cube law from enemy to ally; on systems whose demand halves at night, it is routinely the fastest-payback item on the drawing.
Decision three: balancing — reverse return beats valves
Multiple users in parallel all want their share of flow; left alone, the nearest user gorges and the farthest starves — the universal disease of piped networks. Two cures combine: reverse return layout, where the return line is routed so every user sees the same total pipe length (first fed, last returned), making the network self-balancing by geometry; and balancing valves (or per-user flow controllers) to trim what geometry cannot. Reverse return costs some metres of pipe once; permanent throttling at every branch costs pump head forever — buy the geometry when the layout allows it.
Decision four: storage that actually stratifies
Where demand is peaky (lesson D9's charts), a buffer tank rides the peaks — but only a stratified tank does it well. Hot water floats on cold in stable layers if you let it: tall tank, gentle flows in and out through diffusers, hot connections at the top, cold at the bottom. Preserved stratification means the process draws true-hot water until the tank is genuinely spent, and the generator sees true-cold return (maximising its recovery — condensing sources and heat pumps, lesson F6, live or die on cold return). One crude mixing inlet destroys the whole effect and turns the asset into a lukewarm barrel. Tank connections are the cheapest place in the system to be careful.
- Measure the real ΔT across your loop at working load. If design said 25 and the pipes say 8, flow is triple the need — the pump is burning the difference (cube law). Rebalance before resizing anything.
- Walk parallel users feeling return legs: uniform warm = balanced; one hot and one cold = the network disease. Check for reverse-return possibility before buying balancing valves.
- Buffer tanks: thermometers top, middle, bottom. Three similar readings = destroyed stratification — inspect the diffusers, not the insulation.
- Summer cavitation on the loop pump = NPSH margin gone; check cushion pressure (E1) and suction layout before condemning the pump.
- Flow = duty ÷ (1000 × ΔT). The ΔT decision sizes pipes, pumps and surfaces — make it once, on paper, at 20–30 °C unless the product argues.
- Pumping power follows the cube of flow: half the flow, an eighth the power. VFDs collect this.
- Reverse return balances by geometry, forever, free. Throttle only what geometry can't reach.
- Storage works only stratified: tall, gentle, hot-top cold-bottom, diffused.
Stratification was the household engineer's secret first: the classic Indian overhead "geyser and drum" bathroom, like the English airing-cupboard cylinder, worked because nobody stirred it — hot stayed up, cold stayed down, and a modest heater served a big family from a small tank. Solar water heating rediscovered the same physics at panel scale, and district heating industrialised it into thousand-cubic-metre stratified accumulators. The principle never changed: heat floats — design so it may.