Finding flow rates from the heat carried, sizing pipework, and selecting a pump from the index circuit resistance.
Flow rate from the heat carried
A pipe must carry enough hot water to deliver the heat. Mass flow rate:
m = Q / (c x deltaT)
where Q is heat-transfer power (kW), c is the specific heat capacity of water (about 4.18 kJ/kg.K) and deltaT is the system temperature drop (K).
Example: a circuit carrying 9 kW at a system deltaT of 11K: m = 9 / (4.18 x 11) = 0.196 kg/s, about 0.20 L/s (about 700 L/h).
Choosing the pipe
Use the pipe manufacturer’s charts and the design criteria for the material, temperature and circuit. Check velocity and pressure loss together. For example, if a design uses a 1.0 m/s velocity limit, a 0.20 L/s flow in an assumed 20 mm internal bore gives about 0.64 m/s. A 22 mm nominal copper route that passes the velocity check still needs its total resistance checked. Smaller 10 or 15 mm branches may suit individual emitters, but their actual bore, duty and length decide.
Sizing the pump (index circuit)
In a filled closed circuit, the static head gained on the downward path balances the rise on the upward path. The circulator overcomes resistance to flow; it does not lift the same water through the building height on every circuit. Height still matters when establishing the required static pressure and expansion-vessel settings.
The index circuit has the greatest calculated pressure loss at the required flow conditions. It is not always the longest run: pipe sizes, fittings, valves and the heat source all contribute. Select the pump for the total design flow at the required index-circuit head. Assess each pipe section at the flow it actually carries, then balance the branches to their own duties.
- Pipe friction comes from the relevant length and charted loss at that section’s flow.
- Include fitting losses, either directly or as equivalent lengths, without counting them twice.
- Add the applicable heat-source, valve and emitter resistances.
Example: an index effective length of 30 m at about 200 Pa/m gives 6000 Pa; adding 8000 Pa for the boiler and valves gives 14 kPa, roughly 1.4 m head (since 1 m head is about 9.81 kPa). Select a pump whose curve supplies the required total flow at that head. Read that duty from the curve, not from a maximum-head figure or a speed-setting number.
- Count the actual flow and return paths. Double a one-way distance only when the return follows the same route; otherwise measure each path separately. Use the appropriate loss rate for each pipe size and flow, then include its fittings and components.