Short answer: there is no single flow meter setting for every loop. On the fictional Cedar House ground floor at a 43/38 °C flow and return, the design gives 0.53 to 1.41 l/min 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: l/h = 0.86 × W / K.
"Is there a rule of thumb for adjusting the flow meters?" asks one heat pump owner on a UK self-build forum. The replies suggest trial and error from a reference room, or "1.5 on everything". Both skip the fact that each loop's flow can be worked out.
Why every loop has a different flow
The water in a loop carries heat for the room, plus what the floor gives off downwards. 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 flow temperature: it needs 56 W/m² at 20 cm spacing, so the water has to be at 43 °C. That water gives the kitchen and the bedroom more than they need, so the design lets them cool it by 10 K, and the living room loops by only 6.54 K. That is why the living room loops get the most water.
Loop flows - Cedar House ground floor, 43/38 °C
"1.5 on everything"
0/1/C living room643 W · 6.54 K1.41 l/min
0/1/D living room624 W · 6.54 K1.37 l/min
0/1/A living room538 W · 6.54 K1.18 l/min
0/1/B living room521 W · 6.54 K1.14 l/min
0/3/A kitchen446 W · 10 K0.64 l/min
0/4/A bedroom434 W · 10 K0.62 l/min
0/4/B bedroom434 W · 10 K0.62 l/min
0/3/B kitchen366 W · 10 K0.53 l/min
- living room - sets the flow temperature, water cools 6.54 K
- kitchen and bedroom - water cools 10 K
Balancing: work out your own loop's flow rate
The formula fits on a scrap of paper: l/h = 0.86 × W / K, and l/min is that divided by 60. W is the heat the water in the loop carries, K how much the water cools in the loop. For loop 0/1/C: 0.86 × 643 / 6.54 = 84.6 l/h, or 1.41 l/min. A good design gives both numbers for every loop.
| Loop | Loop [m] | Heat in the water [W] | Cooling [K] | Flow [l/min] |
|---|---|---|---|---|
| 0/3/A (kitchen) | 53.8 | 446 | 10 | 0.64 |
| 0/3/B (kitchen) | 31.7 | 366 | 10 | 0.53 |
| 0/1/A (living room) | 49.1 | 538 | 6.54 | 1.18 |
| 0/1/B (living room) | 42.5 | 521 | 6.54 | 1.14 |
| 0/1/C (living room) | 56.1 | 643 | 6.54 | 1.41 |
| 0/1/D (living room) | 50.7 | 624 | 6.54 | 1.37 |
| 0/4/A (bedroom) | 49.7 | 434 | 10 | 0.62 |
| 0/4/B (bedroom) | 51.0 | 434 | 10 | 0.62 |
| Total | 384.5 | 4007 | - | 7.51 |
Loop length does not set the flow. Kitchen loop 0/3/A is 53.8 m long, almost as long as living room loop 0/1/C, yet it gets less than half its water.
What "1.5 on everything" does
Eight loops at 1.5 l/min make 12 l/min, where the design needs 7.51 l/min. 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 circulating pump can deliver it: that depends on pressure losses in the loops and valves, which HeatAlgo does not calculate. These are calculations for a fictional floor with wet underfloor heating in a screed, not measurements. Your loops will give other numbers - same steps, same formula. The Cedar House floor is described in the guide where to put the underfloor heating manifold.
Work out your loop flows