Why Underfloor Heating Rooms Heat Unevenly (and How to Fix It)

Anyone who has lived with an underfloor heating system knows the frustration of uneven room temperatures. One room feels perfectly warm while another, fed by the same manifold, never quite reaches comfort. This is one of the most common complaints in hydronic underfloor heating, and it is not a fault of the heat source. It is a limitation of how traditional zone control works. Understanding why it happens, and how variable-flow control fixes it, is the key to a genuinely comfortable home.

Why Underfloor Heating Rooms Heat Unevenly

Conventional underfloor heating control relies on fixed water temperature and fixed flow, switching each loop fully on or fully off through a standard on-off thermal actuator on the manifold valve. This on-off approach causes the room temperature to swing up and down rather than holding steady.

The deeper issue is flow imbalance between loops. Even when every loop on a manifold is laid with exactly the same pipe length, the loop closest to the supply main has the least resistance and therefore the highest flow, making it the warmest. The loop furthest from the supply main has the most resistance, the lowest flow, and ends up the coolest. In real installations, pipe lengths between loops are rarely identical either, which compounds the problem further.

The result is a manifold where some rooms run hot and others run cold, no matter how carefully the system was specified. Manual balancing valves can help at commissioning, but they are set once and tend to drift out of balance over time as conditions change. Solving the problem properly requires continuous, proportional control of the flow to each loop.

The Fix: Variable Water Temperature and Variable Flow

The solution that the industry is moving toward is variable water temperature combined with variable flow. Instead of switching valves fully on and off, advanced proportional actuator technology keeps every loop valve continuously open and regulates room temperature by adjusting the supply water temperature.

By monitoring the trend of indoor temperature change, the system holds each loop valve at a proportional, partially open position, ensuring every zone receives the right amount of warm water flow. The high-flow loop near the supply can be throttled back while the low-flow distant loop is opened wider, evening out the temperature across every room served by the manifold. This achieves true variable-temperature, variable-flow control, where balance is managed continuously rather than through crude on-off cycling.

This kind of control is only possible with an actuator that can hold a proportional position rather than just open or close. That is the difference between a standard on-off actuator and a modulating one. For a fuller explanation of the different actuator types, see our guide on thermal actuator types and their benefits.

How a Proportional Actuator Balances Every Loop

A proportional, or modulating, actuator accepts a 0 to 10V control signal, which lets a compatible controller position the manifold valve anywhere across its travel rather than only fully open or fully closed. This is what makes loop-by-loop flow balancing possible.

The Legom 920080PL is a variable-flow linear actuator built for exactly this purpose. Because each valve can hold a proportional position, the nearest and furthest loops on a manifold can be balanced against each other automatically and continuously, not just once at commissioning. For a detailed look at this specific product and its operating modes, you can read our dedicated article on the Proportional Thermal Actuator (0-10V). The table below summarizes its key specifications.

Specification	920080PL
Control signal	0–10V DC (proportional)
Voltage	24V DC 50/60Hz
Control direction	Normally Closed
Control modes	Energy-saving & comfort, heating, off
Power consumption	2W
Force	120 ± 10N
Stroke	≥4mm
Working temperature	0°C – 60°C
Response time	3–5 minutes
Adapter	M30 × 1.5mm
Protection grade	IP54
Housing material	PA66+30%GF
Standard	EN 60730

Design Features That Make Balancing Practical

Proportional control only solves the uneven-heating problem if it is practical to install and operate. The 920080PL includes several design features aimed at exactly that.

Multi-Stage Stroke with LED Color Identification

The actuator controls the wax element stroke in multiple stages, with each stage identified by a different LED color. This gives a clear visual indication of how far each valve is open. For the installer, it makes commissioning and flow adjustment far more straightforward, because the open position of each loop valve can be seen directly rather than estimated. It also helps with ongoing diagnostics, since the state of every loop is visible at a glance inside the manifold cabinet.

Manual Opening for Easier Installation

The 920080PL combines manual and electrothermal control in one unit. During first installation, before power is connected, the manual control can open the actuator stroke in advance. This overcomes the spring force of the manifold valve, making the actuator far easier and less strenuous to fit. Fighting the valve spring during installation is a small but real source of wasted time, and the manual function removes it. Once the system is running normally, the manual control is closed and actuation is handled electrothermally through the wax element.

Plug-In Structure and Upgraded Sealing

The power cable connects to the base through a plug-in structure, which simplifies installation and replacement. The actuator also features upgraded sealing protection, with sealing rings fitted between the hand wheel and handle, between the movable seat and the base, and between the base and the power cable. This improves resistance to moisture and dust, supporting the IP54 protection grade and contributing to a longer, more reliable service life in the damp conditions often found inside manifold cabinets.

“The uneven-room complaint is the one we hear most from underfloor heating installers, and for years the only answer was painstaking manual flow balancing at commissioning, which drifts out of balance over time anyway. What makes proportional control different is that the balancing becomes continuous and visible. The LED stroke stages let an installer actually see what each valve is doing, and the manual opening feature solves the small but real frustration of fighting the valve spring during installation. These are not abstract features. They come directly from watching where installers lose time and where systems lose comfort.”
— Maggie Shen, Founder of Legom

The Role of Control Logic

Good hardware is only half the picture. A high-quality climate solution also depends on the control logic behind it. The essence of good control logic is dynamic balancing and system optimization: continuously monitoring indoor and outdoor conditions such as temperature and humidity, and making precise adjustments through feedback.

Strong control logic does not pursue a single goal in isolation. It balances multiple objectives at once, most importantly the trade-off between comfort and energy efficiency. Modern systems are increasingly moving toward predictive and adaptive control, anticipating environmental changes ahead of time and adjusting accordingly. A proportional actuator like the 920080PL is the hardware that makes this intelligent logic useful, because predictive strategies only work if the valve can respond with fine, proportional movements rather than blunt on-off switching.

Frequently Asked Questions

Why does one room in my underfloor heating stay colder than the others?

The most common cause is flow imbalance between loops on the manifold. The loop closest to the supply main has the lowest resistance and the highest flow, so it heats fastest, while the loop furthest away has the highest resistance and the lowest flow, leaving that room cooler. Differences in pipe length between loops make this worse. With standard on-off actuators, there is no continuous way to correct it. Proportional actuators that hold each valve at a balanced position are the most effective fix, as they even out the flow across all loops automatically.

What is the difference between an on-off and a proportional thermal actuator?

An on-off actuator only opens or closes a manifold valve fully, switching the loop entirely on or off. A proportional, or modulating, actuator accepts a 0 to 10V control signal that lets a controller position the valve at any point across its travel. This allows the flow through each loop to be finely adjusted and balanced against the others, rather than relying on full-on or full-off cycling. The result is more even room temperatures and smoother, more efficient operation, particularly on manifolds where loops differ in length or distance from the supply.

Can I fix uneven heating without replacing my whole system?

In many cases, yes. If your manifold uses standard M30 × 1.5mm valve heads, the actuators can often be upgraded to proportional models without replacing the manifold or pipework, provided you also have a controller capable of sending a 0 to 10V signal. This is far less invasive than re-laying pipe loops. The best approach is to have an installer assess your existing manifold and controller, as the upgrade path depends on whether your current control system can support proportional actuation.

Does proportional control save energy as well as improving comfort?

Yes. Because a proportional actuator delivers only the flow needed to maintain the target temperature, rather than cycling fully on and overshooting, it reduces wasted energy. It also allows the system to run at lower, more efficient supply water temperatures, which is particularly beneficial when paired with a heat pump. The combination of even temperatures and reduced overshoot means the system works less hard to keep every room comfortable, which lowers running costs over time.

Is the 920080PL compatible with standard manifolds?

Yes. The 920080PL uses the standard M30 × 1.5mm adapter thread used by most manifold manufacturers, so it fits standard hydronic underfloor heating manifolds without additional adapters. It operates at 24V DC and requires a controller capable of providing a 0 to 10V proportional signal to use its variable-flow capability. For systems using simple on-off thermostats, a conventional actuator may be more appropriate, but for advanced controllers that support proportional or predictive control, the 920080PL unlocks loop-by-loop flow balancing.

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Reviewed and updated by the LEGOM Technical Team on June 26, 2026. This article explains why hydronic underfloor heating rooms heat unevenly, how variable water temperature and variable flow control solves it, and how a proportional linear actuator with LED stroke indication and manual opening enables continuous loop-by-loop flow balancing.