Short answer: balancing radiant floor loops means setting each loop on the manifold to the flow its design gives it, not one gpm for all of them. A loop's flow follows from the room's heat loss in four steps: the room's heat loss, the heat the water carries, the temperature drop in the loop, and gpm = Btu/h ÷ (500 × ΔT °F). In the HeatAlgo reference house - 1,100 ft², a -4 °F design day, PEX at 6 in on center, 104/95 °F water - the eleven loops need from 0.16 gpm in the bathroom to 0.36 gpm in the hall, 2.65 gpm in total.
The article what radiant floor flow meters should read 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 radiant floor module (EN 1264-2 and EN 1264-3) for the reference house: tile in the kitchen, bathroom, hall and mechanical room, engineered wood in the other rooms, 16 mm PEX, 1.8 in of cement screed over the tubing, loops laid at 6 in on center across the whole floor.
Balancing radiant floor loops in four steps: the hall
The hall has 110 ft² of tile floor and one loop. This is how its setting comes about:
- The room's heat loss. The heat loss calculation gives the hall 1,887 Btu/h. That is the heat the floor has to put into the room.
- The heat the water carries. The water also loses heat downward, through the insulation under the tubing. EN 1264-3 (Formula 13) therefore adds the floor's own heat loss from the calculation again under the tubing, 375 Btu/h in the hall, and multiplies the sum by 1 + Ro/Ru - the ratio of the thermal resistance above the tubing to the one below it. In the hall that is 4.5% more: 2,363 Btu/h in the water.
- The temperature drop in the loop. The manifold gives one supply water temperature. The mechanical room (61 °F, 22.1 Btu/h·ft²) needs 102.8 °F, so the module sets 104 °F for the whole house. The hall manages with 101.7 °F, so its loop may cool the water by more than the design 9 °F: the water comes back from the hall at 90.9 °F, a 13.1 °F drop.
- The flow. 2,363 Btu/h ÷ (500 × 13.1 °F) = 0.36 gpm. That is what the hall loop's flow meter should read.
The gpm of every loop: what the flow meters should read
The same sum for all eleven loops with 104/95 °F water. Where a room has several loops, its heat loss is shared equally, because the loops have the same area and flooring.
| Loop | Heat loss | Floor's own loss, again underneath | Heat in the water | Temperature drop | Flow meter |
|---|---|---|---|---|---|
| Living room, each of 3 loops (wood) | 1,524 Btu/h | 350 Btu/h | 1,967 Btu/h | 15.5 °F | 0.25 gpm |
| Kitchen (tile) | 1,298 Btu/h | 379 Btu/h | 1,755 Btu/h | 18 °F | 0.19 gpm |
| Bedroom, each of 2 loops (wood) | 1,285 Btu/h | 324 Btu/h | 1,696 Btu/h | 16.5 °F | 0.21 gpm |
| Room 1 and Room 2, 1 loop each (wood) | 1,534 Btu/h | 423 Btu/h | 2,049 Btu/h | 18 °F | 0.23 gpm |
| Bathroom, 75 °F (tile) | 1,062 Btu/h | 338 Btu/h | 1,450 Btu/h | 18 °F | 0.16 gpm |
| Hall (tile) | 1,887 Btu/h | 375 Btu/h | 2,363 Btu/h | 13.1 °F | 0.36 gpm |
| Mechanical room, 61 °F (tile) | 1,428 Btu/h | 235 Btu/h | 1,752 Btu/h | 11.3 °F | 0.31 gpm |
Together the eleven loops need 2.65 gpm through the manifold. The hall gets the most water - it loses the most heat per square foot of the rooms heated to 68 °F, and its water cools less than in the other rooms. The bathroom gets the least. The kitchen, both rooms and the bathroom have an 18 °F drop - the most HeatAlgo's module allows for one loop.
No loop here runs at the design 9 °F drop. Set the supply water to exactly 102.8 °F and the mechanical room loop would drop 9 °F, and every other setting would move. That is why the flow meter table is worked out for the water temperature the house actually runs at.
Why not the same gpm on every loop
Eleven loops at 0.40 gpm make 4.40 gpm - 66% 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 circulator 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 radiant floor heating not heating one room.
How to set the flow meters on the manifold
- Run the system at the design supply water temperature and open every actuator (thermostats turned all the way up).
- Set each flow meter to its loop's gpm from the table.
- 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 2.65 gpm. If a loop cannot reach its figure even with its flow meter fully open, the circulator 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 circulator. These are calculations, not measurements, for the reference house with hydronic radiant floor heating in a screed, to EN 1264 - not to any US design manual. Your house will give different numbers - the same steps, the same formula.
Work out the flow meter table for your own house
In HeatAlgo's radiant floor heating design tool every loop gets its heat, its temperature drop and its flow 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 of every loopA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.