Why is one underfloor heating zone colder than others?

Why should the problem be sought at the manifold?

Underfloor heating typically consists of several separate pipe loops, all receiving warm water from the same blending valve but interconnected within a shared distribution cabinet, or manifold. Each loop on the manifold passes through its own flow meter and often its own actuator, which is controlled by the thermostat in the specific room.

If only one loop remains cold while all other loops supplied by the same manifold are working normally, it's almost certain that water is reaching the manifold correctly — the problem lies either in the regulation of that one loop, its physical condition, or the floor covering over that loop, rather than the entire heating system.

This significantly narrows the search to one manifold cabinet and one specific pipe, making problem-finding much quicker than checking the entire system.

Flow Meter is Closed or Insufficiently Open

Each manifold loop typically has a transparent flow meter (rotameter) with a small float inside, indicating how much water is currently flowing through that specific loop. If the float doesn't move at all or is very low, that loop isn't receiving enough warm water.

A common reason is that the flow meter's adjustment screw has been accidentally fully or partially closed — for example, during previous maintenance, repair work, or balancing. It can also happen that all loops were initially given the same setting, but a longer loop requires a higher flow rate that was not accounted for.

• Open the manifold cabinet and check the flow meter for each loop — compare the reading of the cold loop with those of other, properly functioning loops.
• If the float is clearly lower than in other loops belonging to the same room type, carefully turn the adjustment screw open until the flow rate rises to a similar level.
• Do not open the flow meter completely at once — adjust in small increments and check after a few hours whether the room starts to warm up.

Hydraulic Balancing Between Loops

In an underfloor heating system, all loops connected to the manifold compete for the pressure generated by the same pump. If shorter loops are open more than necessary, they draw a larger share of the water flow, leaving the more distant or longer loop underserved, even if its own flow meter isn't completely closed.

This is a typical phenomenon in installations where balancing was not properly performed initially, or where a loop has been manually altered later without affecting others. The result is that one specific room systematically remains cooler, especially on the coldest days of the heating season when the entire system is operating under higher load.

Stuck Actuator or Faulty Thermostat Wiring

In many underfloor heating systems, the opening and closing of each loop is controlled by a small electric or thermal actuator, which is mounted directly on the manifold valve and receives commands from the respective room thermostat. If this actuator is jammed in the closed position, it will not allow water to pass into the loop at all, even if the thermostat sends the correct command.

Actuators can get stuck after a long period of inactivity — for example, if that loop was closed all summer — or simply due to a technical fault. Another common reason is a wiring issue: a loose connection, a broken thermostat, or incorrect programming, which prevents the thermostat from sending an opening command to the actuator.

• Feel the actuator — if it's distinctly cold and motionless, while other actuators are warm and yield slightly when pressed, it's likely stuck or faulty.
• Check if the corresponding room thermostat displays the correct target temperature and if the screen or indicator light shows that heating is demanded — if the thermostat shows no activity at all, the problem might be in the settings or batteries.
• If the actuator does not move even after manual pressing (some models allow manual testing), it is likely a technical fault requiring actuator replacement.

Air has Accumulated in this Single Loop

If air is trapped in one specific underfloor heating loop, it prevents warm water from circulating in that loop, even if the flow meter shows some movement and the actuator is properly open. Air typically collects at the highest point of the loop, which in underfloor heating is usually near the manifold itself, as the pipework within the floor is relatively flat.

This is particularly likely if the problem has occurred recently — for example, after the system was filled, serviced, a loop was disconnected and reconnected, or after a long shutdown. Unlike bleeding radiators (which we cover in more detail in the article "Air in the Radiator", /radiaatorisse-koguneb-ohk), bleeding an underfloor heating loop is slightly different because the pipes are within the floor and each loop usually has its own bleed valve or tap on the manifold.

• Locate the bleed valve for the specific loop on the manifold — it's usually at the end of the manifold or separately for each loop.
• Temporarily close other loops to direct more pressure specifically to the problematic loop, and carefully open the bleed valve until a steady stream of water without bubbles emerges.
• If air repeatedly returns within a short period, the problem might also be a small leak in the loop's pipework or a pressure issue with the expansion tank in the entire system, not just a single air bubble.

Loop Length Differences and Increased Flow Resistance

Each underfloor heating loop varies in length, depending on the size of the room it covers and the spacing of the installed pipe. The longer the loop, the greater the resistance to water flow within it, meaning a longer loop requires a higher flow rate compared to others to achieve a similar heat output.

If the manifold balancing did not initially account for one specific loop being, for instance, twice as long as an adjacent room's loop, that loop will systematically remain underheated — not due to a fault, but because of the initial setup. In such a case, increasing the flow rate for that particular loop helps, which in turn requires reducing the flow in shorter loops accordingly to keep the entire system balanced.

Partial Blockage or Damage in the Loop Pipework

If the loop has accumulated sediment, dirt, or rust over the years — especially if the system has never been flushed — the internal diameter of the pipe can narrow to the extent that water can no longer flow through in sufficient quantities, even if the flow meter and actuator are perfectly fine. You can read more about this in the article "Dirt and Sediment in the Heating System" (/kuttesusteemi-mustus-ja-sete).

Less commonly, but still possible, is physical damage to the pipe — for example, crushing during installation that didn't immediately cause a problem but has worsened over time. If you suspect physical damage, not just dirt, it's useful to read the article "Detecting the Location of Pipe Damage" (/torustiku-kahjustuse-asukoha-tuvastamine), where we explain what methods specialists use to find the exact location without breaking open the floor.

• With a blocked loop, it's typical for the flow rate to be low even when the flow meter is fully open and the actuator is working correctly.
• If other loops of the same age from the same system are still working well, but one specific one is clearly weaker, it's worth considering flushing that loop separately.
• If physical damage is suspected — for example, if the problem occurred suddenly after some renovation work in the room — it's advisable to call a specialist who can use a camera or pressure test to determine if it's a blockage or a leak.

Different Floor Coverings Hinder Heat Transfer

Even if water is flowing perfectly well in the loop and the settings are correct, one room's floor might feel cooler than others simply because it has a thicker or less thermally conductive floor covering. For example, a thick carpet, cork, or engineered wood flooring with a thick underlay will insulate the heat within the floor much more than thin ceramic tiles.

In such a case, it's not a technical fault at the manifold, but rather a floor construction issue. The solution is either to increase the target temperature of the thermostat in that room, change the floor covering material, or consciously accept that this room will remain slightly cooler than the others. Before assuming a technical fault, it's always worth checking if the "cold" room differs from others specifically in terms of its floor covering.

Step-by-Step Manifold Inspection Plan

If one loop remains cold, it's worth following the inspection plan below in order — this way, you'll quickly rule out the simplest causes before moving on to more complex ones.

• 1. Open the manifold cabinet and locate the loop for the problematic room; loops are usually marked with a label or room number on the manifold.
• 2. Check the flow meter — does the float move at all, and is its level similar to other functioning loops?
• 3. Feel the actuator — is it warm, and is the thermostat in the corresponding room sending a heating demand signal?
• 4. If there is flow but it's weak, open the bleed valve for that loop and check for air in the loop.
• 5. If no air comes out and the flow remains weak, compare the length and initial setting of this loop with others — it might require a higher flow rate.
• 6. If everything above seems fine, but the room is still cooler, check if the floor covering in that room differs from other rooms.
• 7. If none of the above explains the problem, it's likely a blockage, damage, or a faulty actuator/thermostat that requires specialist assessment.

When is it sensible to call a heating system specialist?

Some of the checks described above — observing the flow meter, feeling the actuator, opening the bleed valve — can be performed by most homeowners without special skills. However, if the problem remains unsolved after these steps, the cause is likely something that requires measuring instruments or a deeper understanding of the system.

• If the actuator or thermostat appears faulty, but you're unsure whether the problem is with the electronics or the wiring.
• If you suspect a blockage or physical damage in the pipe that cannot be assessed without special equipment.
• If the balancing of the entire manifold seems incorrect and the settings of multiple loops need to be re-evaluated simultaneously.
• If air repeatedly returns to one loop, which may indicate a hidden leak or an issue with the expansion tank.

How to prevent one zone from becoming cold in the future?

Most of the problems described in this article are preventable if the manifold and its settings are checked regularly, not just when a problem is already noticeable.

• Check the manifold's flow meters and actuators at the start of the heating season, before cold weather brings the system to full load.
• Have the system professionally balanced, especially if loop lengths vary significantly between rooms.
• Bleed the manifold loops after any maintenance, repairs, or extended shutdowns during which water has been added to the system or loops have been opened.
• Flush the heating system occasionally to prevent sediment accumulation in individual loops, especially in older systems.
• If you plan to change floor coverings, be aware that thicker and more insulating materials will affect the heating speed of that room compared to others.

Thermostat Location and Setting Can Worsen the Problem

Sometimes the fault isn't with the actuator or flow meter at all, but rather with how and where the room thermostat is installed. If the thermostat is located in a spot exposed to direct sunlight, near an external door, or in an area with air currents, such as near a ventilation opening, it might display a temperature different from the actual room temperature and switch off the heating too early.

In such a case, the person in the room feels that the floor is cooler than in other rooms, even though the thermostat actually "thinks" that the target temperature has already been reached. The problem isn't a physical fault at the manifold, but rather a measurement inaccuracy that causes the entire loop to close earlier than needed.

The same applies if the thermostat's target temperature is simply set lower than in other rooms — this might be a conscious choice, but if someone in the family perceives it as a "fault," it's worth checking the set value first before looking for a technical fault.

• Check the thermostat's location — is it near direct sunlight, a draft, or a heat source (such as a TV or oven) that might distort the measurement?
• Compare the thermostat's reading with a separate thermometer in the same room, preferably at the same height and location as the thermostat.
• Check the set target temperature — has it accidentally been set lower than in other rooms, for example, due to a forgotten setting from the previous season?

Common Mistakes When Diagnosing at Home

When trying to solve a problem oneself, mistakes are sometimes made that rather complicate subsequent diagnosis or even damage the system. It's useful to be aware of these errors before you even approach the manifold cabinet.

The most common mistake is to turn all flow meters at once and arbitrarily, hoping that one of them will "help." As a result, the knowledge of the original, correct setting is lost, and the balance of the entire system can become worse than before. Another common mistake is forcefully removing or pressurising an actuator without first checking that the pressure in that part of the loop has been released — this can cause a small leak exactly where there was no problem before.

A third mistake is drawing conclusions too quickly: if the room doesn't warm up immediately after adjustment, it's often assumed that the change didn't help, and a new, often opposite, change is made immediately. However, underfloor heating reacts slowly due to the thermal mass of the concrete floor, so it's necessary to wait long enough before taking the next step.

• Do not adjust multiple flow meters simultaneously — adjust one loop at a time and observe the result before moving on to the next.
• Before removing an actuator, ensure that the corresponding branch is depressurised to avoid unexpected water leakage.
• Allow each change at least several hours to a full day before deciding whether it helped or not.
• Write down the initial positions of the flow meters and actuators so you can return to the starting point if necessary.