Short answer: balancing underfloor heating means setting each loop on the manifold to the flow rate its design gives it, not one figure for all of them. A loop's flow rate follows from the room's heat loss in four steps: the room's heat loss, the heat the water carries, how much the water cools in the loop, and l/h = 0.86 × W / K. In the HeatAlgo reference house - 102.2 m², Warsaw, -20 °C, 15 cm pipe spacing, 40/35 °C flow and return - the eleven loops need from 0.61 l/min in the bathroom to 1.37 l/min in the hall, 10.03 l/min in total.
The article underfloor heating flow meter settings shows that every loop needs a different flow. Here we work out where each of those numbers comes from: from the room's heat loss to what the flow meter should read. Every figure was calculated by HeatAlgo's underfloor heating module (EN 1264-2 and EN 1264-3) for the reference house: tiles in the kitchen, bathroom, hall and plant room, engineered wood in the other rooms, 16×2 pipe, 45 mm of cement screed over the pipe, loops laid at 15 cm across the whole floor.
Balancing underfloor heating in four steps: the hall
The hall has 10.2 m² of tiled floor and one loop. This is how its setting comes about:
- The room's heat loss. The heat loss calculation gives the hall 552.9 W, or 54.2 W/m². That is the heat the floor has to give off into the room.
- The heat the water carries. The water also gives off heat downwards, through the insulation under the pipes. EN 1264-3 (Formula 13) therefore adds the floor's own heat loss from the calculation again under the pipe, 110 W in the hall, and multiplies the sum by 1 + Ro/Ru - the ratio of the thermal resistance above the pipe to the one below it. In the hall that is 4.5% more: (552.9 + 110) W × 1.045 = 692.5 W.
- How much the water cools in the loop. The manifold gives one flow temperature. The plant room (16 °C, 69.7 W/m²) needs 39.3 °C, so the module sets 40 °C for the whole house. The hall manages with 38.7 °C, so its loop may cool the water by more than the design 5 K: the water comes back from the hall at 32.7 °C, cooled by 7.27 K.
- The flow rate. 0.86 × 692.5 W / 7.27 K = 81.9 l/h, or 1.37 l/min. That is what the hall loop's flow meter should read.
The quick sum from the room's heat loss at 5 K on every loop gives the hall 1.58 l/min. The module gives less, because at 40 °C the water in the hall may cool by 7.27 K rather than 5 K - even though it carries more heat, with the downward loss added.
The flow rate of every loop: what the flow meters should read
The same sum for all eleven loops at a 40/35 °C flow and return. Where a room has several loops, its heat loss is shared equally, because the loops have the same area and floor covering.
| Loop | Heat loss | Floor's own loss, again underneath | Heat in the water | Water cools by | Flow meter |
|---|---|---|---|---|---|
| Living room, each of 3 loops (wood) | 446.6 W | 102.7 W | 576.5 W | 8.63 K | 0.96 l/min |
| Kitchen (tiles) | 380.5 W | 111.0 W | 514.2 W | 10 K | 0.74 l/min |
| Bedroom, each of 2 loops (wood) | 376.7 W | 95.0 W | 497.0 W | 9.18 K | 0.78 l/min |
| Room 1 and Room 2, 1 loop each (wood) | 449.5 W | 124.0 W | 600.5 W | 10 K | 0.86 l/min |
| Bathroom, 24 °C (tiles) | 311.3 W | 99.0 W | 424.9 W | 10 K | 0.61 l/min |
| Hall (tiles) | 552.9 W | 110.0 W | 692.5 W | 7.27 K | 1.37 l/min |
| Plant room, 16 °C (tiles) | 418.5 W | 69.0 W | 513.5 W | 6.27 K | 1.17 l/min |
Together the eleven loops need 10.03 l/min, or 0.60 m³/h through the manifold. The hall gets the most water - it loses the most heat per square metre of the rooms heated to 20 °C, and its water cools less than in the other rooms. The bathroom gets the least. The kitchen, both rooms and the bathroom cool the water by 10 K - the most HeatAlgo's module allows for one loop.
No loop here runs at the design 5 K. Set the flow temperature to exactly 39.3 °C and the plant room loop would cool the water by 5 K, and every other setting would move. That is why the flow meter table is worked out for the flow temperature the house actually runs at.
Why not 1.5 l/min on every loop
Eleven loops at 1.5 l/min make 16.5 l/min - 64% more than this house needs. The bathroom loop would get 2.5 times the water the design gives it, and its water would come back warmer. The circulating pump has to move all of that water, and the hall, which needs the water most, gets no more of it. What happens to the hall when its flow meter reads too little is in underfloor heating not working in one room.
How to set the flow meters on the manifold
- Run the heating at the design flow temperature and open every actuator (room thermostats turned all the way up).
- Set each flow meter to its loop's figure from the table. The flow meter reads l/min, so divide a design figure in l/h by 60.
- Go along the manifold a second time. Turning one loop down sends more water to the others, so the first settings move.
- Check the total. The flow meters together should give the manifold's flow, here 10.03 l/min. If a loop cannot reach its figure even with its flow meter fully open, the circulating pump gives too little or the loop's resistance is too high - that is one for your installer.
What the table does not tell you
The design flow is the value to set on the flow meter. HeatAlgo's module also gives each loop's pressure drop, but it does not work out valve presettings or size the circulating pump. These are calculations, not measurements, for the reference house with wet underfloor heating in a screed. Your house will give different numbers - the same steps, the same formula. Every input of the reference house is on the page the reference house, calculated in full.
Work out the flow meter table for your own house
In HeatAlgo's underfloor heating design tool every loop gets its heat, its cooling and its flow rate from the same four steps, and the Flow meters table lists them by manifold and port - ready to set on site. A free account shows the full result for one loop; the report gives the table for every loop.
Work out the flow rate of every loop