A float and numbers in a transparent manifold tube often become the verdict on the whole heating system: 'my neighbour has two, I only have one'. But the number means little without that circuit's target. A flowmeter shows the volume of water passing through a branch over time. It does not directly show room heat output, floor temperature or an overall system quality score.
Owners benefit from recognising a reading and recording conditions. Adjusting every circuit to general online advice is risky: improving one room may reduce water to another. The best reference is the particular home's circuit table with design flows.
What does l/min mean?
One litre per minute means one litre passes the measurement point in a minute. Converted to hours, one l/min corresponds to 60 l/h, and two l/min to 120 l/h. These are the same physical quantity in different units. If the design states litres per hour, divide by sixty.
For example, 90 l/h listed for a circuit means 1,5 l/min. The number 90 does not need to appear on the flowmeter scale. Unit errors can cause unnecessary concern, particularly when some documents come from design software and others from manifold instructions. Always write units beside values.
From flow to heat: another quantity is needed
Transferred heat also depends on the difference between supply and return water temperatures. A simplified water example can use P ≈ 69,7 × q × ΔT, with P in watts, q in l/min and ΔT in kelvins or a temperature difference in degrees Celsius. This is an approximate physical relationship, not a calibration formula for a particular manifold.
At 1,5 l/min and a 5 K temperature difference, the water transfers approximately 523 W. This is not yet the exact amount released only into the room: some heat travels into the structure or adjacent zones. The Danfoss guide also links flow setting to heat load and temperature difference.
Which part of the float should you read?
That depends on the particular flowmeter design. Some use a float edge, others another indicator or scale direction. Do not instruct everyone to read only the top or bottom. Consult the manifold instructions or obtain a clear demonstration at your unit from the installer.
Hold the camera approximately level with the scale when photographing. Pictures from above or the side may mislead. If the viewing window is coated, water is dirty or the float does not move as operating conditions change, interpret the reading cautiously. A dirty meter may show zero even when the pipe receives some heat under another operating condition.
Why may the numbers legitimately differ?
Rooms are not identical. Bathroom demand, living room glazing and bedroom temperature may require different circuit outputs. Active floor area and circuit quantity per room also differ. A lower flow may therefore be justified for one branch and a higher flow for another. Identical flowmeter readings do not automatically prove balancing.
Thermostat operation matters too. Once a room is warm, its actuator may close or limit flow according to the particular control logic. A low reading at that moment may be normal rather than a fault. Assessment needs to know whether the room calls for heat, whether the actuator is open and whether the heat source is heating rooms at all.
If one flowmeter shows zero
First photograph the whole manifold and record room temperature and the thermostat setpoint. Check control messages accessible to the user. Do not dismantle electric actuators or loosen pipe connections merely to see whether 'water comes out'. Hot, pressurised water requires a safe servicing procedure.
A specialist will check valve position, actuator, air, pump conditions and the meter itself. A zero reading does not prove that pipe is crushed in the screed. That conclusion follows only after simpler causes are excluded and appropriate tests performed. Dismantling floors must not be the first diagnostic step based on one poor photograph.
What changes when other circuits close?
Branches are linked through the shared manifold and pump. Depending on controls and balancing, closing one zone may change flows in others. Record operating status during measurements. A test with every zone open and a random evening comparison are not equivalent conditions.
The Uponor systems overview also highlights individual room control. Such a system cannot be assessed as pipework with every branch permanently open. If noise follows every thermostat action, check the overall logic and pump settings rather than trying to keep all floats fixed at one level.
How can you preserve a useful baseline?
What does one manifold photograph not show?
A photograph usually does not reveal whether the heat pump was producing domestic hot water, rooms had reached setpoints or a night schedule limited heating. Record time, outside and room temperatures and control mode alongside readings. If there is a clear fault code, provide it exactly rather than paraphrasing. This helps the specialist prioritise checks and avoid a false conclusion caused by temporarily stopped flow in a perfectly sound branch.
After commissioning, create a table of branch number, room, design flow, set flow and notes. Add manifold and control photographs. If readings change later, you will have a comparison rather than remembering whether 'it seemed higher before'. Record the reason for setting changes, not just the new number.
Read about whole-system relationships in the underfloor heating guide. When requesting help with uneven heating, include the circuit list and operating conditions, not just a cropped flowmeter. Litres per minute then become a useful diagnostic indicator rather than an unjustified competition with the neighbour's manifold.
Information sources
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