A pumped underfloor heating manifold does two jobs at once. It blends primary water down to a safe floor temperature, and it splits that water between loops that are never the same length. The second job is where most systems quietly fail. Pipework gets installed correctly, the screed goes down, the boiler or heat pump fires, and one room still sits two degrees below setpoint while another overheats by lunchtime.
Almost always the cause is the same: nobody balanced the manifold. The flow meters were left wherever the factory shipped them, so water took the path of least resistance and the longest loop got starved.
This guide covers the numbers. How to work out the flow each loop actually needs, what ΔT to design around, how to size the circulator, and the order in which you set the flow meters. If you want the background on what a manifold is and what it is made of, we cover that separately in what are underfloor heating manifolds.
What the Pump and Mixing Unit Actually Controls
On a pumped manifold the circulator and the blending valve work as one assembly. The blending valve mixes cooler return water back into the hot primary supply, and the circulator moves that blended water around the floor loops independently of the primary circuit.
That independence matters. A boiler wants to run at 60 °C to 80 °C. A screed floor must not. So the mixing unit acts as a hydraulic separator between two circuits running at very different temperatures, and the manifold’s flow meters then divide the blended water between zones.
Two variables come out of this arrangement, and you set both:
- Flow temperature, adjusted on the blending valve or its wax thermostatic element. This decides the average surface temperature of the floor.
- Flow rate per loop, adjusted on the flow meters. This decides how the available heat is shared out between rooms.
Get the first wrong and the whole floor is too cool or too warm. Get the second wrong and the rooms fight each other.
Step One: Fix the Design Flow Temperature
Balancing is pointless until the supply temperature is settled, because flow rate calculations depend on it.
For a screed floor with 16 mm pipe at 150 mm to 200 mm centres, the usual design window is 35 °C to 45 °C. Timber or floating floors need a little more, often 45 °C to 50 °C, because the deck resists heat transfer. Vinyl and engineered timber usually cap out lower on the surface side, which pulls the flow temperature back down again.
The ceiling on all of this is surface temperature, not water temperature. EN 1264 limits the floor surface to roughly 29 °C in occupied areas, with allowances up to 33 °C in bathrooms and 35 °C in narrow peripheral strips near glazing. Most flooring manufacturers then impose their own limit, commonly 27 °C for timber and vinyl. Whichever number is lowest wins.
Heat Pumps Change the Arithmetic
An air to water heat pump loses efficiency as flow temperature climbs. The gap between A7/W35 and A7/W45 operation is significant, often 20 percent or more of COP, so heat pump systems are designed around the lowest flow temperature the floor can live with.
This has a practical consequence people often miss. If the heat pump is already producing 35 °C water, a blending valve has nothing left to blend. The mixing unit becomes redundant and can be omitted, leaving just the circulator, the manifold and the flow meters. Keeping a mixing valve in a low temperature heat pump circuit adds resistance and gains you nothing.
Step Two: Calculate the Flow Rate Each Loop Needs
This is straightforward arithmetic, and it is the part that gets skipped.
Every loop carries a known heat output, taken from the room heat loss calculation. Convert that output into a volume of water using the design temperature difference between flow and return:
Flow (L/min) = (Heat output in kW × 14.34) ÷ ΔT in K
The 14.34 comes from the specific heat capacity of water converted to litres per minute. For the common design case of ΔT = 5 K, the shortcut is simply kW × 2.87.
Why 5 K? Underfloor heating runs a narrow ΔT deliberately. A wide ΔT means the far end of the loop is running much cooler than the start, and you get visible temperature striping across the floor. Domestic systems normally sit at 5 K to 7 K. Larger commercial floors sometimes open up to 10 K to reduce pump duty, accepting slightly less even surface temperature in return.
A worked example for a five loop manifold at ΔT 5 K:
- Living room, 900 W, 95 m loop, 2,58 L/min
- Kitchen, 650 W, 70 m loop, 1,87 L/min
- Bedroom, 520 W, 60 m loop, 1,49 L/min
- Bathroom, 380 W, 40 m loop, 1,09 L/min
- Hall, 300 W, 35 m loop, 0,86 L/min
Total output 2,75 kW. Total flow 7,89 L/min, which is 0,47 m³/h.
Notice the spread. The living room needs three times the flow of the hall, and it also has the longest, most resistant loop. Left unbalanced, water would do the opposite of what is required: it would pour through the short hall loop and trickle through the long living room loop. That single fact is the entire reason flow meters exist.
Most manifold flow meters cover 0,5 to 5 L/min, which comfortably brackets the whole table above. If a calculated loop flow lands below 0,5 L/min the loop is probably too short to control properly and should be combined with another, and anything above 5 L/min means the loop is trying to carry too much output and should be split.
Keep Loop Lengths Close
Balancing has limits. You can throttle a loop that is running too fast, but you cannot make a badly proportioned loop behave.
Practical rules that make a manifold balanceable in the first place:
- Maximum loop length around 100 m for 16 mm pipe, 120 m for 17 mm, 80 m for 12 mm.
- Keep loops on the same manifold within roughly 10 to 15 percent of each other in length.
- One loop per room wherever the room is small enough to allow it.
- Use oxygen barrier pipe to DIN 4726 with an EVOH layer, since corrosion debris in the loops changes resistance over time and ruins a balance you set years earlier.
Our floor heating pipe range is built to that oxygen barrier specification for exactly this reason.
Step Three: Size the Circulator
Two numbers define the pump: duty flow and head.
Duty flow is just the sum of the loop flows. In the example above, 0,47 m³/h.
Head is the resistance of the index circuit, meaning the most resistant loop, plus the manifold body, flow meters and blending valve. For a 95 m run of 16 × 2 mm pipe at 2,58 L/min, pipe resistance works out at roughly 1,2 m. Add manifold and mixing unit losses of around 0,6 m to 0,8 m and the requirement is close to 2 m.
That is modest. A standard Class A wet rotor circulator rated to 6 m covers it several times over, which is why the same pump body suits almost every domestic manifold. The mistake is leaving that pump on its highest fixed speed. Run it in proportional pressure mode at a low setting instead. Oversized pump speed produces noise in the flow meters, wastes electricity, and makes fine balancing almost impossible because the adjustment range gets compressed into the first quarter turn of each valve.
Step Four: The Balancing Procedure
Order matters here. Doing this out of sequence is why so many manifolds end up half balanced.
1. Purge and Pressure Test First
Flush each loop individually with all other loops isolated, until the water runs clear and no air comes through. Air in a loop reads as a false flow rate, so balancing an unpurged system is wasted effort.
Then pressure test before the screed goes down, typically at 6 bar for 24 hours, and leave the pipes pressurised through the pour so any damage from the screeding shows up immediately.
2. Wait Out the Screed Cure
Cement screed needs about 21 days before first heat up. Anhydrite needs roughly 7 days. Then follow the commissioning heat up curve: start at 20 °C to 25 °C for three days, raise to the maximum design flow temperature for a further four days, and only then bring the system into normal operation.
Balancing before the screed has cured gives you numbers that will not hold.
3. Open Everything and Get to Design Temperature
Remove the thermal actuators or force every zone to call, so all valves sit fully open. Bring the circuit up to design flow temperature and let it stabilise. Water viscosity falls as it warms, so a manifold balanced cold will not be balanced hot.
4. Leave the Index Circuit Fully Open
Identify the longest, most resistant loop. That is your index circuit and it stays wide open throughout. You are not adding resistance to it, you are removing flow from everything else until the index circuit finally gets its share.
5. Throttle the Rest, Then Go Round Again
Work through the remaining flow meters, setting each to its calculated figure. Every adjustment changes the pressure available to the others, so the first pass will never hold. Expect two or three complete passes before the readings settle.
Read the flow meters with the manifold in its final orientation. A vertical flow meter read at an angle gives a false figure, and that small error compounds across five or six loops.
6. Confirm With Temperature, Not Just Flow
Once flows are set, check the flow and return thermometers on the manifold. The measured ΔT should land near your design figure. A ΔT noticeably wider than design, say 10 K when you aimed for 5 K, means a loop is starved regardless of what its flow meter claims. A very narrow ΔT means you are pushing more water than the floor can use, which wastes pump energy and nothing else.
What Happens When All the Zones Close
This is the failure mode that gets designed in and then forgotten.
Thermal actuators are typically normally closed, and a wax element takes roughly three to five minutes to open or close fully. When every room reaches setpoint, every valve shuts. If the circulator keeps running against a closed manifold, pressure rises, the pump runs dry of useful flow, and you get noise and premature wear.
Two accepted solutions:
- Pump shutdown through an end switch on the wiring centre. Once the last zone closes, the base station drops the pump. This is the cleaner option and it saves energy.
- Differential pressure bypass, a spring loaded valve across flow and return that opens once a set pressure is exceeded. Simple, but it recirculates water for no benefit.
Whichever route you take, do not simply leave the circulator running with no bypass at all. It is a common shortcut on retrofit jobs and it shortens pump life noticeably.
Symptoms of a Manifold That Was Never Balanced
Worth knowing these, because clients describe them rather than diagnose them:
- One room consistently 1 °C to 3 °C below setpoint while neighbouring rooms overshoot.
- The far end of a large room feels cool even after hours of running.
- ΔT across the manifold reads 10 K or more when design was 5 K.
- The boiler or heat pump cycles short and often, because return temperature swings.
- Audible flow noise at the manifold, usually an oversized pump setting combined with nearly closed flow meters.
None of these need new hardware. They need an afternoon with the flow meters and the heat loss figures.
Maintenance That Preserves the Balance
Once set, a manifold balance holds for years. What disrupts it is worth watching.
Check flow meter readings at the start of each heating season and compare them against the commissioning figures, which is exactly why those figures should be written on a label inside the manifold cabinet. A loop that has drifted downward usually means debris or air, not a faulty valve.
Exercise the actuators. Wax elements that sit in one position all summer can stick, so a brief all zones call before the heating season starts is a sensible habit. Inspect the manifold body and compression fittings for weeping, particularly at the first fitting after the blending valve where thermal cycling is harshest. A stainless steel manifold body resists this far better than thin brass over a long service life.
Rebalance after any change to the building or the floor. New flooring over a zone changes its thermal resistance. A room converted from a study to a bedroom has a different heat demand. Extra insulation reduces output requirements across the board. In all three cases the original flow figures no longer match the loads.
Legom Manifolds, Actuators and Mixing Units
We manufacture the components this whole procedure depends on, from our facility in Jiaxing, Zhejiang.
Our manifold range covers stainless steel bodies in 1 inch and 1¼ inch, plus brass and nickel plated brass versions, in 2 to 13 port configurations with integral flow meters calibrated across the 0,5 to 5 L/min range.
Alongside those we produce thermal actuators in normally closed and normally open variants, the wax thermostatic elements inside them, room thermostats and wiring centres, and oxygen barrier floor heating pipe. For the blending side, our underfloor heating mixing valves handle the temperature reduction from primary to floor circuit.
If you are specifying manifold assemblies for a range or need a configuration built to your own drawing, our OEM and ODM service covers port counts, connection types, flow meter ranges and branded cabinets. Talk to us about your project and we will work through the specification with you.
Frequently Asked Questions
What ΔT should I design an underfloor heating manifold around?
5 K is the normal domestic figure and gives the most even surface temperature. 7 K is acceptable. Larger commercial floors sometimes use 10 K to keep pump duty and pipe sizes down, trading a little surface evenness for lower running cost.
Do I still need a mixing valve if I have a heat pump?
Usually not. A heat pump designed to deliver 35 °C flow is already producing floor temperature water, so a blending valve adds resistance without doing any useful work. You still need the circulator, the manifold and the flow meters.
Can I balance a manifold with the actuators fitted?
Only if every zone is calling for heat, which holds all the valves open. If some actuators are closed you are reading flow through a partially shut circuit and the figures mean nothing. Removing the actuator heads during balancing is simpler.
Why does my flow meter read zero when the pump is clearly running?
Air trapped in the loop, or the flow meter’s own indicator stuck after a long idle period. Purge the loop individually first. If the reading stays at zero with confirmed flow through the loop, the meter cartridge needs replacing.
How often should a manifold be rebalanced?
Not on a schedule. Rebalance after the screed has fully cured, after any flooring change, after room use or insulation changes, and any time comfort complaints appear in one zone only.
What flow rate is too low for an underfloor heating loop?
Below roughly 0,5 L/min you lose reliable control, because flow meters cannot resolve that range accurately and the loop’s ΔT becomes very wide. If the calculation gives a figure that low, the loop is too short and should be merged with an adjacent one.
“A manifold is not commissioned until the flow meters have been set against the heat loss figures. We ship products capable of accurate balancing, but the flow meter positions leaving our factory are a shipping default, not a setting. The complaint we hear most often is one cold room, and in almost every case the loops were never balanced at all. An hour with the design figures fixes what people assume needs new hardware.”
— Maggie Shen, Director of Legom
Reviewed by Maggie Shen, Director at Legom, on September 26, 2026. This article was reviewed for technical accuracy, including the flow rate formula and its worked example, the EN 1264 surface temperature limits, the screed curing and commissioning heat up sequence, the index circuit balancing method, and the zone closure protection options.