Short answer: there is no single setting for every loop. On the fictional Cedar House main floor at 109.4 °F supply and 100.4 °F return (43/38 °C), the design gives 0.14 to 0.37 gpm per loop, nearly three times more in a living room loop than in the small kitchen loop. Each loop's flow follows from the heat its water carries and how much the water is meant to cool in the loop: gpm = Btu/h ÷ (500 × ΔT °F).
A homeowner on HeatingHelp posted under the title "Uponor manifold balancing": one loop was running at 1.3 gpm, the rest at 0.6, and a bedroom stayed cold. Replies in threads like that tend to suggest turning valves until the rooms feel right, or setting every loop the same. Both skip the fact that each loop's flow can be worked out before anyone touches a valve.
Why every loop has a different flow
The water in a loop carries heat for the room, plus what the floor gives off downward. The more heat, and the less the water is meant to cool, the more water has to flow. That is how HeatAlgo calculates it, to EN 1264-3.
On this floor the living room sets the supply water temperature: it needs 17.8 Btu/h·ft² (56 W/m²) at 20 cm (7.87 in, about 8 in) spacing, so the water has to be at 109.4 °F. That water gives the kitchen and the bedroom more than they need, so the design lets them cool it by 18 °F (10 K), and the living room loops by only 11.77 °F (6.54 K). That is why the living room loops get the most water.
Work out your own loop's flow
The US formula fits on a scrap of paper: gpm = Btu/h ÷ (500 × ΔT °F). Btu/h is the heat the water in the loop carries, ΔT how much the water cools between supply and return. The 500 is water's weight per gallon times 60 minutes times its specific heat.
For living room loop 0/1/C: 2,194 Btu/h ÷ (500 × 11.77 °F) = 0.37 gpm.
It is the same formula HeatAlgo uses in metric, l/h = 0.86 × W ÷ K, only in different units. Substitute Btu/h = 3.412 × W and ΔT °F = 1.8 × K: gpm = 3.412 × W ÷ (500 × 1.8 × K) = 0.003791 × W ÷ K. One gpm is 227.1 l/h, so that is 0.861 × W ÷ K in l/h. For loop 0/1/C in metric: 0.86 × 643 ÷ 6.54 = 84.6 l/h, which is 0.372 gpm. A good design gives both numbers, the heat and the temperature drop, for every loop.
| Loop | Length | Heat in the water | Temperature drop | Flow |
|---|---|---|---|---|
| 0/3/A (kitchen) | 176.5 ft | 1,522 Btu/h | 18 °F | 0.17 gpm |
| 0/3/B (kitchen) | 104.0 ft | 1,249 Btu/h | 18 °F | 0.14 gpm |
| 0/1/A (living room) | 161.1 ft | 1,836 Btu/h | 11.77 °F | 0.31 gpm |
| 0/1/B (living room) | 139.4 ft | 1,778 Btu/h | 11.77 °F | 0.30 gpm |
| 0/1/C (living room) | 184.1 ft | 2,194 Btu/h | 11.77 °F | 0.37 gpm |
| 0/1/D (living room) | 166.3 ft | 2,129 Btu/h | 11.77 °F | 0.36 gpm |
| 0/4/A (bedroom) | 163.1 ft | 1,481 Btu/h | 18 °F | 0.16 gpm |
| 0/4/B (bedroom) | 167.3 ft | 1,481 Btu/h | 18 °F | 0.16 gpm |
| Total | 1,261.5 ft | 13,672 Btu/h | - | 1.98 gpm |
Calculated in metric (W, K, l/min) and converted; tile floor, 16 mm (5/8 in OD, the size of 1/2-inch PEX) tubing.
Loop length does not set the flow. Kitchen loop 0/3/A is 176.5 ft long, almost as long as living room loop 0/1/C, yet it gets less than half its water.
What "the same on every loop" does
Set all eight loops to 0.40 gpm (1.5 l/min, a figure that circulates on forums) and the manifold passes 3.17 gpm, where the design needs 1.98 gpm. The kitchen and bedroom loops then get 2.3 to 2.8 times the water the design gives them, and their water comes back warmer. Equal settings are not wrong in themselves; they are a guess where a calculation is possible.
What this number does not show
The design flow is the value to set on the flow meter. It does not say whether the circulator can deliver it: that depends on pressure drop in the loops and valves, which HeatAlgo does not calculate. The loop heat figures come from each room's heat loss. These are calculations for a fictional floor with tubing embedded in a slab, not measurements; HeatAlgo does not calculate staple-up, joist-bay or plate systems. Your loops will give other numbers - same steps, same formula. The Cedar House floor is described in the guide to radiant floor heating manifold location, and why loops have the lengths they do is in radiant floor tubing spacing and loop length.
Work out your loop flowsA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.