Why is my underfloor heating uneven after installing a new distributor

Floor heating is a system that requires precise hydraulic balancing. If uneven floor heating occurs after the installation of a new stainless steel manifold, it is usually due to an error in flow setting or missing filtration, not a material defect. This article explains the most common technical causes of this problem and provides specific steps to resolve it without the need to dismantle the entire system.

Cause 1: Missing balancing of flow control valves

The most common reason for uneven heating is incorrect setting of flow control valves on individual loops, which have different pipe lengths and hydraulic resistance. Without proper balancing, water naturally flows where there is the least resistance, meaning short loops heat up quickly and intensely, while long loops remain cold due to insufficient flow. Proper calibration requires opening the valves on longer loops to maximum and partially closing the valves on shorter loops to achieve the same temperature at the output of all loops.

In practice, we dealt with a case where the customer had a 10-meter short loop with the valve fully open at 10 turns, while the 80-meter loop had the valve only at 2 turns; the result was that the first loop was hot and the second was cold. After adjusting the setting to maintain a flow ratio between the shortest and longest loop of approximately 1:1 to 1:1.5 according to the manufacturer's recommendation, the temperatures balanced out.

Cause 2: Clogged filters before the manifold

If the system is new and you have just installed a new stainless steel 4-way manifold for floor heating, it is essential to check the mechanical filters located on the manifold's inlet nozzles. These filters are designed to catch impurities from the installation, but if they are clogged, they will limit the overall water flow into the system and cause the most distant loops to receive minimal heat.

• The filters are located directly under the manifold and must be regularly cleaned during the first weeks of operation.
• A clogged filter increases hydraulic resistance in the system and forces the pump to work against a barrier, reducing pressure in distant loops.
• We recommend cleaning the filter screens at least once a month during the heating startup phase, when the most impurities are released from new pipes.

In our last project, we found that after starting the boiler, one loop was completely cold because a piece of PVC tape was stuck in the filter mesh. After cleaning it, the flow immediately returned to normal and the loop began to heat evenly.

Cause 3: Air blockages in the loops

Air trapped in the pipes acts as a hydraulic barrier that prevents the circulation of hot water, similar to trying to drink through a straw that is clogged in your mouth. If the system is not completely de-aerated during filling, air will settle at the highest points of the loops and create a local block that prevents flow. This is a common occurrence in new installations where stainless steel 6-way manifold for floor heating with multiple branches are used.

To resolve this issue, you must gradually open the valves on each loop and slowly let water flow through the bleed valve until only clean water without bubbles comes out. During this process, it is important to monitor the system pressure to ensure it does not drop below the critical level, which could cause further air to be drawn back into the pipes.

Cause 4: Incorrect pipe diameter or loop lengths

Another technical cause can be the design of the project, where loops with extremely different lengths or diameters were used, leading to one loop having a hydraulic resistance that prevents it from balancing with the others. If the maximum loop length is, for example, 120 meters and the minimum is only 20 meters, even with perfect valve settings, it will be difficult to achieve the same temperature because the pressure loss is disproportionately high.

• The recommended maximum length of a single loop for floor heating is 100 to 120 meters with a 16 mm diameter.
• The difference in loop lengths should not exceed 10 to 15 meters to ensure natural balance.
• For systems with a large number of loops, for example when using stainless steel 5-way manifold for floor heating, it is advisable to use collectors with adjustable flow.

In one case, we had a project where one loop was 110 meters and the other only 15 meters, which caused the long loop to always be colder. The solution was to divide the long loop into two shorter loops and connect them to separate manifold valves, achieving a uniform temperature across the entire area.

Cause 5: Low water inlet temperature or weak pump

The final factor can be the heat source itself, where the boiler supplies water at a low temperature or the circulation pump does not have sufficient power to overcome the hydraulic resistance of the entire system. If the inlet water temperature is below 30 °C, the floor will heat up very slowly and the differences between loops will be more noticeable, especially if the insulated spaces are of different sizes.

Check the boiler thermostat settings and ensure that the circulation pump is running at a sufficient speed to overcome the resistance of all circuits simultaneously. In some cases, increasing the incoming water temperature by 2 to 3 degrees is enough to achieve the desired floor temperature in the colder circuits.

Cause 6: Unbalanced Installation or Missing Thermal Insulation

The last possible cause is the physical construction of the installation, where some circuits are laid in areas with higher heat loss or are not insulated in places where they should be protected. If the distributor is mounted outside the heated area and the pipes leading to it are not insulated, part of the heat is lost even before it reaches the circuits, causing local cooling.

• Check whether all connections and valves on the distributor are properly insulated with thermal tape or foam.
• Ensure that the pipe is correctly embedded in concrete or the base layer without air gaps.
• Verify that there is no "dead" section in the system that could function as a thermal bridge.

In one of our installations, we found that one circuit was laid directly under a load-bearing beam where the thermal insulation was insufficient, causing this part of the floor to be always 2 °C cooler. Additional insulation in these areas immediately eliminated the problem.

Cause 7: Missing Functional Control of Thermostatic Heads

Modern stainless steel distributors are often equipped with thermostatic heads that should automatically regulate the flow according to the room temperature.

Common Questions

Why does my floor heating not heat evenly right after turning it on?

This is a common phenomenon caused by the system needing time to thermodynamically balance, especially if the concrete screed is thick or if there are air blockages in the system. Removing air and adjusting the flow valves should resolve this issue within a few days.

Can faulty distributor material be the cause?

Stainless steel distributors of the brand Hepworth, such as stainless steel distributor 2-way for floor heating or other models, are made of high-quality steel and do not have a tendency to corrode or change shape, which could affect the flow. The problem is almost always in the setting, not in the material.

How often should I check the filters on the distributor?

During the first two weeks of operation of a new system, you should check and clean the filters every 3 days, as this is when the most dirt is released. Afterwards, a monthly check or after each seasonal shutdown of the system is sufficient.

Proper diagnosis and setting of flow valves together with thorough system air removal are key to achieving even floor heating after installing a new stainless steel distributor. If these steps do not help, we recommend contacting a hydraulics specialist who will check the overall hydraulic balance of your system.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us - we are happy to help.