Underfloor heating design flow: inputs, result and limits
Which inputs are required?
Room design heat load QN,f comes from the heat-load calculation. AF is only the active heated floor area. Temperature drop σ is supply minus return. Ro is the upper partial thermal resistance from pipe level to the room; Ru is the lower partial resistance to the room or zone below. Derive both from the actual layer build-up. Water density at mean water temperature converts the standard mass flow to l/h.
What is returned?
Results are design heat flux qdes, downward heat flux qU, total circuit output, mass flow in kg/s and kg/h, and volume flow in l/h. The qU/qdes ratio shows how much the construction below pipe level increases flow.
Method and interpretation
DIN EN 1264-3 gives qdes = QN,f/AF. The resistances and temperatures give qU = qdes·Ro/Ru + (θi−θu)/Ru. With cW = 4190 J/(kgK), mH = AF·(qdes+qU)/(σ·cW), algebraically identical to equation (13).
Calculator limits
This calculation replaces neither the EN 12831 heat load nor the EN 1264-2 output characteristic and surface-temperature check. Pipe spacing, circuit length, pressure drop, pump selection, balancing, and controls are excluded. For the room governing supply temperature, the standard specifies σ ≤ 5 K.
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Source: Locally held DIN EN 1264-3:2021-08, sections 4.1.3.2 to 4.1.3.4, equations (5) and (13) to (15).