Maintenance and air venting of floor heating system

We are still preparing products.

But you can look at other categories.

Maintenance and air venting of underfloor heating system – complete practical guide

Underfloor heating is now a standard of comfortable living. Quiet operation, even heat from the floor upwards, absence of visible bodies – these are the reasons why more and more people choose it. However, precisely because the entire system is hidden in the floor construction, many owners underestimate its maintenance. When the water circuit of the underfloor heating is not properly vented, or when the heated water in the system becomes clogged, problems appear late – and the remedy can be painful.

This article will explain in detail what and when to check on the underfloor heating system, how to properly vent individual circuits, what indicates uneven heating of the floor, and which chemical protection of the system to choose. I write from a practical perspective – after years of solving complaints, service visits, and handing over new systems, I have seen almost everything that can go wrong. And most of the problems could have been avoided.

Why air in the underfloor heating system is a bigger problem than in radiators

In standard radiator systems, venting is simple – each radiator has a vent valve, air rises up and you release it. In underfloor heating, the situation is more complicated for several reasons.

First, the underfloor heating pipes are laid horizontally in the screed, almost in one plane. Air does not naturally tend to move to one point – it spreads out across the loops and creates air pockets in various places. Second, the length of one loop can be 60 to 120 meters (depending on the pipe diameter and the area of the loop). Air in such a long pipe can significantly restrict the flow and thus the performance of the given zone. Third, most underfloor heating manifolds have flow regulators set to relatively low flow – and an air pocket can reduce it almost to zero.

From practice: a customer calls saying that only half of the living room is "working", the other half is cold. We arrive for service, measure the temperatures at the manifold outlets – one loop has ΔT (supply-return temperature difference) of 8 °C, the other 0.5 °C. A classic symptom of an air pocket in the loop.

Where air in the underfloor heating system actually comes from

Air enters the hydraulic circuit in several ways, and it is important to understand them so you know when venting is necessary.

1. Initial filling of the system

When filling a new or drained system, air is present in every corner of the distribution. If the filling is not done properly (slowly, from the bottom up, opening individual loops one at a time), air remains trapped in the loops. This is by far the most common source of problems in new installations.

2. Diffusion through pipe walls

PEX pipes (cross-linked polyethylene) without an oxygen barrier (EVOH layer) allow oxygen to pass through the walls. This oxygen dissolves in the water, but gradually forms microbubbles. Therefore, in modern underfloor heating systems, only pipes with an oxygen barrier according to DIN 4726 should be used. If not, air – more precisely oxygen – will keep increasing continuously.

3. Services, repairs, water topping up

Whenever the system is opened, water is added, or parts are replaced, air gets inside. After each such intervention, venting is a necessity, not an option.

4. Pressure changes in the system

When the pressure in the expansion tank drops or the system operates below the minimum pressure (usually 0.8–1.0 bar cold), air can enter the system through automatic vents in the reverse direction. Therefore, it is important to maintain the correct pressure in the system – more on that below.

How to recognize that underfloor heating needs venting

The symptoms are not always obvious right away. Sometimes the problem develops slowly over the entire season. Here are the key signals you should not ignore:

Cold floor area despite the system otherwise working – part of the room is warm, part is cold. A typical sign of an air blockage in one loop.
Noise in the distribution – knocking, bubbling or humming during water flow in the circuits. Air causes turbulent flow.
Low or fluctuating pressure on the manometer – pressure should be 1.0–1.5 bar cold, 1.5–2.0 bar hot (at operating temperature). If it drops without an obvious reason, the system is either losing water (leak) or has a problem with the expansion tank.
Pump running "in vain" – the pump is running, but does not reach the set speed and heating is not noticeable. Air in the pump prevents it from creating pressure.
Uneven performance across seasons – the system worked well last winter, now some zones lag behind.
Rapid increase in supply temperature with slow increase in return temperature – flow in the circuit is restricted, water overheats on the supply side.

Venting procedure – step by step

Proper venting of the underfloor heating system is not complicated, but you need to proceed systematically. Opening and closing a valve quickly will not solve the problem.

Step 1: Preparation – check pressure and system condition

Before bleeding, check the pressure in the cold state on the manometer (the system should be unheated). The correct value is 1.0 to 1.5 bar. If it is lower, add water via the filling valve to the correct level. The pump should be turned off, the boiler inactive.

Step 2: Isolate one circuit – close the others

On the floor heating manifold, close all circuits except the first one you want to bleed. On both the supply and return side. This concentrates the entire pump flow into one loop – the flow rate increases and the air is "pushed" toward the manifold.

Step 3: Run the pump at maximum speed

Turn on the circulation pump at the highest speed. Let it run for 3–5 minutes. If you have an automatic air vent on the manifold, the air escapes by itself. If not, open the manual air vent valve on the manifold (it is usually at the end, above the manifold) – hold a container or cloth under the valve, slowly release the air until clean water without bubbles starts to flow.

Step 4: Move to the next circuit

Close the first circuit (or leave it open if you don't have enough flow available otherwise) and open the second one. Repeat the process. Systematically go through all circuits – with a 6-circuit manifold, expect 30–45 minutes of work.

Step 5: Check the pressure after bleeding

After bleeding the entire system, check the pressure again. Bleeding may slightly reduce it – add water to the correct level. Then start the system in normal mode and after 24 hours, check the temperatures of individual circuits (the presence of air can be verified by touching the return pipes of the manifold – they should be appropriately warm).

Automatic air vents on the manifold – yes or no?

Most modern floor heating manifolds have space for an automatic air vent (AOV) on the supply side. It is a small float valve that automatically opens and releases air when air is present.

I recommend using them, but with one important warning: after the initial system bleeding (and thus when the system is stabilized and the pressure is stable), it is advisable to close the AOV or replace it with a blind plug. Why? Because automatic air vents can, under certain circumstances (pressure drop, rapid pressure changes), suck air in instead of releasing it. In a stabilized system, they are unnecessary and sometimes counterproductive.

In practice: when handing over the system to the customer, we explain that the AOV is in the "open" position during the first season, when the system naturally bleeds. After that, we close it.

Pressure in the floor heating system – what is normal and what is not

Pressure is one of the most important operating parameters to monitor. Most home users don't even look at it until there is a problem.

Rules for monitoring pressure:

Cold state (system unheated): 1.0 – 1.5 bar. This is the reference value when adding water.
Operating state (system heated to 35–45 °C): 1.5 – 2.5 bar. The pressure increase is caused by thermal expansion of water.
The safety valve opens at 3 bar (in most systems). If it opens regularly, the system is overfilled with water or the expansion tank is not functioning properly.
Pressure drops over days/weeks without opening the system: most commonly a leak, worn membrane in the expansion tank, or a faulty air vent.

Expansion tank – an underestimated maintenance component

The expansion tank is a pressure tank that compensates for the volume of water during thermal expansion when heating. Most systems have a membrane expansion tank with pre-pressure on the air side.

The pre-pressure should be set to a value equal to or 0.2–0.3 bar lower than the static filling pressure (e.g., you fill the system to 1.2 bar → tank pre-pressure = 1.0 bar). If the pre-pressure drops (membrane ages, air leaks), the tank loses its function and the pressure in the system fluctuates unnaturally.

Check the pre-pressure of the expansion tank every 2 years using a standard car tire pressure gauge (same Schrader valve). Complete replacement of the tank is usually necessary after 10–15 years of operation.

Regular system maintenance – what and when to check

Maintenance of floor heating is much less demanding than in a radiator system, but it is not zero. Here is an overview of recommended intervals:

Before each heating season (autumn – September/October)

Check the pressure in the system in the cold state.
Visually inspect the manifold – leaks, corrosion of connections, condition of flow meters.
Test actuators on each circuit – each should open and close in response to a temperature signal.
Check the flow meter settings – they may shift during the season.
Verify the function of the regulator and thermostats in each room.

Every 2 years

Bleed all circuits (even if there are no obvious problems).
Check the pre-pressure of the expansion tank.
Analyze the water quality in the system (pH, inhibitor content, hardness).
Relubricate or verify the function of ball valves on the manifold.

Every 5 years

Flush the system – cleaning from deposits and sludge.
Replace or verify the condition of the circulation pump.
Check the condition of flow meters and replace seals.

Water quality and chemical protection of the system

A topic that is systematically neglected in practice – and then people wonder why after 10 years the manifold is corroded and the pipes have sludge.

The water circulating in the floor heating system should meet several parameters:

pH: 7.0 – 8.5 – neutral to slightly alkaline. Acidic water corrodes metal parts (pump, manifold, control valves). Water that is too alkaline can cause scaling.
Hardness: ideally below 3 °dH (soft water) – hard water forms limescale at temperatures above 60 °C. With floor heating at temperatures of 30–45 °C, the risk is lower, but not zero.
Chlorides: below 50 mg/l – chlorides cause pitting corrosion on stainless steel (connections, fittings).
Oxygen content: as low as possible – therefore PEX pipes with EVOH barrier and a closed system without regular topping up with fresh water.

To protect the system, corrosion inhibitors and disinfecting additives are used. They are dosed once every 2–3 years at a concentration according to the water volume in the system (the volume can be found in the project documentation or calculated). The inhibitor must not be compatible with PEX pipes and with the pump material – always check the compatibility list from the manufacturer.

If the system will not be in operation for several months (renovation, summer break), I do not recommend draining it. A closed system with an inhibitor is much safer than an empty one, into which oxygen and moisture can enter.

Sludge and deposits – system flushing

Sludge in the floor heating system is a dark brown or black sediment that forms due to corrosion of metal parts (fine iron shavings from machining), mineral precipitation from water, and biological processes (microorganisms in stagnant water). Sludge settles in the lowest parts of the system – paradoxically, precisely in the horizontal loops of the floor heating.

System flushing is done as follows:

Drain the old water from the system (open the drain valve on the manifold or boiler).
Fill the system with clean water and let the pump run for 15–20 minutes at full flow.
Drain this water (it will be dark, with sludge).
Repeat the process 2–3 times until the water runs clean.
If the contamination is severe, use a chemical cleaning agent (flushing agent) according to the manufacturer's instructions.
After flushing, fill with clean water, add a corrosion inhibitor, bleed the system, and set the pressure.

Flow setting at the manifold – hydraulic balancing

Bleeding the system will solve the air problem. But if the circuits are hydraulically unbalanced – that is, one circuit has too much and another too little flow – that is not enough. Hydraulic balancing is a separate operation that ensures the proper thermal performance of the entire system.

Modern manifolds have flow meters (flow indicators with a numerical scale) on the return side. Each circuit should have a flow rate set according to the project – usually between 1.5 – 3.0 l/min for a 16 mm pipe at a spacing of 15 cm.

Practical balancing procedure:

Start the system with a supply-return temperature difference of 5–10 °C.
Measure the return temperature of each circuit (contact or infrared thermometer).
Circuits with higher return temperatures likely have too high a flow (lower ΔT) – reduce the flow meter.
Circuits with lower return temperatures – increase the flow meter.
Goal: ΔT of 5–8 °C on each circuit at the same supply water temperature.

Pipes, fixing and insulation – how installation quality affects maintenance

Many problems with air or uneven performance that we encounter during service calls have their root cause even in the installation phase. Improperly laid pipes, poor fixing or missing separation film – all of this affects the entire lifespan of the system.

During system installation, for example, a separation film with a grid is placed under the pipes, which not only separates the insulation layer from the screed, but also facilitates the accurate placement of the pipe at the correct spacing. Films are joined using metallic tape for joining films, ensuring a continuous surface without gaps or movement during concreting.

The pipes must be properly fixed so that they do not float or shift during the screed pouring. For this, pipe fixing strips or pipe clamps are used. Poorly fixed pipes can form local air pockets in the bends, which are difficult to bleed. For proper shaping of bends during directional changes, PEX pipe fixing bends help prevent pipe kinking and ensure a uniform minimum bend radius.

For more information on installation, I recommend the article Installation of floor heating step by step – from insulation to pouring and Fixing of floor heating pipes – clamps, strips and bends in this Knowledge Center.

Winter and summer break – what to do with the system during it

Floor heating systems that are also used for cooling in summer are operated almost year-round – only the medium temperature is changed. But systems that are used exclusively for heating stand from spring to autumn.

During the summer break, the following applies:

Do not drain the system – an empty pipe is more prone to corrosion and when refilling, air and "fresh" oxygen from the water can get into it.
Maintain minimum pressure – 0.8–1.0 bar even in summer. Check once a month.
If your system is connected to a boiler, turn off the boiler, but let the circulation pump run occasionally (once a week for 10 minutes), to prevent it from seizing and to prevent the water from overheating in a closed space.
Close the actuators on the manifold during long periods of inactivity – this will save their mechanics.

Connection to another heat source – heat pump and floor heating

Floor heating is most commonly combined with a heat pump, which operates with low supply temperatures (35–45 °C). This is an ideal case. However, several special operational requirements for the hydraulic part arise:

Pressure conditions must be carefully set, as heat pumps are more sensitive to hydraulic interruptions than gas boilers.
The flow should not drop below the minimum flow for the heat pump (usually 15–25 l/min at a power of 6–12 kW) – this is achieved by a hydraulic balancer or mixer.
The expansion tank must be sized for the entire system volume, including the internal water of the heat pump.
System air venting with a heat pump is the same as with a boiler – there is no difference.

More about this combination can be found in the article Floor heating and cooling – what must the floor composition meet?

Most common maintenance errors we see in practice

Over years of service experience, I have identified several recurring mistakes that homeowners make:

Air venting "while hot" with a running boiler – water under pressure flows out faster, and air does not have time to be captured. The correct way is to vent when the water is cold and not heated, with the pump running.
Opening all circuits at once during venting – the flow is divided and too weak to push out air. One circuit, full flow – only this works.
Not monitoring pressure after refilling water – an overfilled system (pressure above 2.5 bar when cold) can cause the safety valve to open and pressure to drop again, which looks like a continuing leak.
Ignoring sludge during venting – during venting, not only air but also sludge and water come out of the valve. Homeowners see dark water and block the valve. The correct way is to let it flow until it runs clear.
Using unconditioned tap water for refilling – hard water with chlorides damages the system from the inside. If you refill, use softened water or distillate, always with an inhibitor.

More about installation errors (not only maintenance) is covered in the article Common mistakes in laying floor heating pipes and how to avoid them.

Frequently asked questions (FAQ)

How can I determine which floor heating circuit is air-blocked if the manifold is not accessible?

The best way is to measure the return pipes on the manifold with an IR thermometer or contact thermometer during operation. A circuit whose return is as warm as the supply (i.e., ΔT close to zero or under 1 °C) is either blocked by air or has a closed flow meter. If the flow meters are open, the problem is almost always air. The second method is by touch – with a heated system, compare the temperature of individual return outlets by hand.

Can I vent the floor heating system myself, without a service technician?

Yes, venting is a common household maintenance task that any capable homeowner can handle. You only need a bucket, a cloth, a screwdriver or wrench for the vent valve, and several hours of patience. It is important to proceed systematically – one circuit at a time. If problems persist after two venting cycles, I recommend calling a technician, as the issue may be hydraulic (balancing) or mechanical (damaged pump).

How often should I vent the floor heating system in regular operation?

In a properly installed system with oxygen barriers in the pipes and a closed expansion tank, venting once every 2 years as a preventive measure is sufficient. If symptoms appear (cold areas, noise, pressure drop), vent immediately without waiting for the regular interval. For new systems, I recommend the first venting after 2–4 weeks of operation – the system stabilizes and releases the remaining air from the initial filling.

My pressure is dropping, but I don't see any visible leak. What should I do?

A pressure drop without visible leaks is most commonly caused by three issues: (1) a leak where water escapes as vapor and not as liquid (e.g., on hot pipes in a floor slab – rare in floor heating), (2) a faulty membrane in the expansion tank – the membrane is perforated, air enters the water part and escapes through the AOV, (3) the automatic air vent draws in air during pressure drop and releases it during overpressure – a cycle that looks like a "leak". Solution: check the pre-charge of the expansion tank, replace or close the automatic air vents, and monitor the pressure for 48 hours.

Do I need to add corrosion inhibitors every time I vent the system?

Not automatically. If you only drain a small amount of water (only a few deciliters during venting), the inhibitor concentration in the system does not change significantly. Add inhibitor during flushes or when you add more than 10 % of the system's total volume. The rule is: once every 2–3 years, check the inhibitor concentration using a test kit (available from inhibitor manufacturers), and add it according to the result.

Can I completely turn off the system in summer and not perform any maintenance?

You can operate it in off mode (boiler/heat pump in summer mode, thermostat at minimum), but keep the system filled and pressurized. Completely draining is justified only in long-term unoccupied situations with a risk of freezing in winter, or during reconstruction. In any other case, a closed system with an inhibitor and correct pressure is much healthier than an empty one. Once a month, run the pump for at least 10 minutes – this protects the pump bearings from seizing.

Conclusion – investment in maintenance pays off

Floor heating is a system designed for decades of operation. Quality pipes with EVOH barriers, properly dimensioned manifolds, and reliable pumps – all of these can last 30 years or more if the system is properly maintained. On the contrary, a neglected system without venting and water quality checks can run into problems within 5–8 years.

The rule is simple: check the pressure and manifold condition once a year, vent and check the expansion tank every 2 years, and have the system flushed to "rejuvenate" it every 5 years. That's all. No major investment, no professional license. Just systematic care and a bit of attention to the system that keeps your floor warm every winter.

If you are interested in learning more about proper sizing and component selection, I recommend further articles in this Knowledge Center: What pipe diameter and spacing do I need for floor heating?, How to choose an insulation board for floor heating – thickness, material, and requirements, or System board vs. grid foil – what is better for your subfloor?

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us – we're happy to help.