How to balance underfloor heating loops and set flow meters

Water takes the easiest path. On an unbalanced manifold, a 28m WC loop and a 92m kitchen loop see the same pump pressure, so the short loop gets far more flow than it needs and the long loop runs cool. The result is the classic complaint: some rooms too warm, one room never quite warm enough.

This guide shows how to calculate the flow for each loop, how to set flow meters, how auto-balancing manifolds change the job, how to balance without flow meters and how to set the pump. It assumes the system is already filled and purged of air. Commissioning instructions say every flow meter should be fully open while you fill.

Why do underfloor heating loops need balancing?

Loops need balancing because they are different lengths and heat different amounts of floor. Pressure loss rises with loop length and flow, so without adjustment the shortest loops take most of the water.

Good design keeps loops on one manifold as close in length as the rooms allow. Balancing then fine-tunes the differences that are left. It also matters for the heat source: a heat pump in particular needs a steady total flow, and a few starved loops can upset its flow-return temperature difference.

Balancing is not the fix for a loop that is far too long. Danfoss gives 250mbar as the maximum pressure loss for a circuit, and keeping 16mm loops to 100m or less stays well within reach of a standard UFH pump. Our guide to how many manifold ports you need covers loop lengths.

How much flow does each loop need?

Flow in L/min = loop heat output in W ÷ (69.8 × ΔT), where ΔT is the flow minus return temperature in °C. The 69.8 comes from the heat capacity of water, 4,190J/kgK, divided by 60 seconds.

The EN 1264 design method in the Danfoss handbook uses a 5°C spread, and Uponor's design tables use 8°C. Use the figure your designer or heat source maker gives. A smaller ΔT means more flow for the same heat.

Worked example: the 8-loop ground floor from our manifold ports guide, with every room needing 50W/m² and an 8°C ΔT (558W per L/min).

Total flow is about 6.5 L/min, or 390 L/h. Where every room needs the same W/m², the target flow is roughly in proportion to the pipe in each room's floor, so longer loops get more. Where rooms differ, use each room's own heat loss. See heat loss and UFH output for working that out.

What do the flow meters on a manifold show?

A flow meter shows how much water is going round one loop, usually in litres per minute. On the MaxHeatUFH auto-balancing manifold, each circuit has a flow meter with shut-off reading 0 to 4 L/min.

• Reading it: the indicator moves up the scale as flow rises. Read it with the pump running and the loop's actuator fully open.
• No movement: the loop is closed, the actuator has not opened, or air is blocking it.
• Cloudy or stuck: dirt in the system can foul the glass. A single replacement flow meter costs £10 inc VAT, October 2026.
• What it does not show: temperature. Use thermometers on the flow and return to check the ΔT.

Where the adjustment sits varies by manifold. On many, you set flow on the return valve, and Danfoss describes converting the design flow from litres per hour to litres per minute (÷ 60) for setting.

How do I balance a manifold with flow meters?

Open every loop, find the loop that struggles most to reach its target, leave it fully open and turn the others down. Then repeat, because each change shifts the flow in the rest.

1. Write down the target flow for every loop, as in the table above.
2. Open every loop. Turn all thermostats up so every actuator opens, and wait a few minutes for thermal actuators to open fully.
3. Set the pump to a middle setting to start with.
4. Open every flow meter or balancing valve fully and read the flows.
5. Find the index loop. This is the loop furthest below its target, usually the longest. Leave it fully open.
6. Turn down the loops most over target first, usually the shortest, until each reads its target.
7. Read every loop again and adjust. Two or three rounds are normal.
8. Adjust the pump. If the index loop still cannot reach its target, raise the pump setting. If every loop is over target with the index loop open, lower it.
9. Check temperatures after a few hours of heating. Each loop's return should be close to the design ΔT below the flow.
10. Lock and record the settings, and leave a copy by the manifold.

Do this once the floor is cured and in normal use, not during the first screed heat-up. Our guide to commissioning underfloor heating covers the heat-up schedule.

Do I need to balance an auto-balancing manifold?

You set it once, but you don't chase the flows. On an auto-balancing manifold you preset each return valve to the loop's design flow with the setting key, and the pressure-independent valve then holds that flow as other zones open and close.

Prices inc VAT, October 2026. The second row is the one people miss. On a standard manifold, when the kitchen zone closes, the pump pressure pushes more water through the loops still open. An auto-balancing manifold keeps each loop at its set flow, which gives steadier heat and suits a heat pump. Check the flow meters after presetting to confirm every loop is running.

How do I balance loops without flow meters?

Balance by temperature. With the system heating steadily, measure the return pipe temperature of each loop at the manifold and turn down the loops whose returns are warmest.

1. Run the heating for several hours with every loop open, so the floor is at a steady temperature.
2. Measure each return pipe just before the manifold with a pipe thermometer or contact probe.
3. A loop with a return much warmer than the others has too much flow. Close its return valve a little.
4. A loop with a return much cooler than the others has too little flow. Leave it open and turn the others down.
5. Wait an hour or more between rounds. A screed floor responds slowly.

This takes longer than using flow meters, but it works. A 1" ball valve set with temperature gauges (£26 inc VAT, October 2026) shows the overall flow and return temperature at the manifold, which tells you the system ΔT at a glance.

What pump setting should I use for underfloor heating?

Use the lowest setting that lets the index loop reach its target flow. A higher setting wastes electricity and can make the short loops noisy, and a lower one starves the longest loop.

• Constant pressure: holds the same pressure as zones open and close. A common choice on UFH manifolds.
• Proportional pressure: lowers the pressure as demand falls. Useful where many zones switch on and off.
• Constant curve: runs at a fixed speed. Simple, but flows change as zones close.
• AUTOADAPT: the Grundfos UPM3 Auto sets itself to the system.
• PWM: the Grundfos UPM3 Flex can take its speed from a heat pump or boiler that has a PWM output.

The UFH pumps in our packs give 6 to 7.5m of head. A 250mbar circuit loss is about 2.5m of head, so a well-designed floor rarely needs the top setting. If yours does, look for a loop that is too long, a part-closed valve or air. Our guide to choosing a UFH pump compares the models.

How do I know the system is balanced?

Every room reaches its set temperature in a similar time, and the loop returns sit close to each other in temperature. These signs point to a balancing problem.

If a loop shows no flow at all, it is not a balancing problem. Work through underfloor heating not getting warm or bleeding air from underfloor heating first.

Products for this job

• Auto-Balancing Stainless Steel UFH Manifold
• Manifold Flow Meter
• Manifold Ball Valve Set 1", with temperature gauges
• Blending Valve Pump Pack with Grundfos UPM3 Auto
• Non-Mixing Pump Set with Grundfos UPM3 Flex
• Manifold Auto Air Vent End Unit
• All manifolds

Related guides: How many manifold ports do I need?, Heat loss and UFH output and Commissioning underfloor heating.

Frequently asked questions

What should my underfloor heating flow meters be set to?

Each loop's own target: heat output in W ÷ (69.8 × ΔT). At an 8°C ΔT, a 600W loop needs about 1.1 L/min and a 300W loop about 0.5 L/min. There is no single setting for every loop.

Should all UFH flow meters read the same?

No. Loops that heat more floor need more flow. Setting them all the same starves the long loops and overfeeds the short ones.

Which loop do I balance first?

Leave the loop that struggles most to reach its target, usually the longest, fully open. Then turn the others down, starting with those furthest over target.

Do I balance underfloor heating with the heating on?

Set flows with the pump running and every loop open. Then check the return temperatures once the floor has been heating steadily for a few hours.

Why is my underfloor heating manifold whistling?

Usually the pump setting is too high and water is being forced through a loop that is throttled hard. Lower the pump setting and rebalance.

Can I balance underfloor heating myself?

Yes, if you can work safely around the manifold and read the flow meters. It needs patience more than tools. Electrical work on the wiring centre or pump should be done by a qualified electrician.

How often should UFH loops be rebalanced?

Once at commissioning, then only if you change the floor covering, add a loop or notice rooms heating unevenly. Check the flow meters each year for dirt or stuck indicators.

Sources: Danfoss underfloor heating handbook, Uponor UFH installation guide, IMI TA UFH commissioning instruction