Part of the room is warm, part is cold, something is gurgling in the manifold cabinet, and the flowmeter float is at zero. This is what an air-bound underfloor heating system most often looks like, especially after the system is filled for the first time or after work on the manifold.
You bleed underfloor heating at the manifold by flushing the loops one by one: with the boiler switched off and cooled down, the circulation pump switched off, and the valves to the boiler closed, you let water in through the valve on the supply manifold and release it through the valve on the return manifold until no more bubbles come out of the hose. If the manifold is the highest point in the system, it is often enough to leave one loop open and let the pump push the air out through the air vent.
In this guide, we show both methods, a safe way to fill the system, and what to do when the air comes back.
Table of contents
- How do you bleed underfloor heating step by step?
- Bleeding underfloor heating yourself: from the tap or with a flushing station?
- Underfloor heating air bleeding with the pump: on or off?
- How to recognize air in underfloor heating?
- Manifold: which bar is the supply and which is the return?
- Unusual systems: without an air vent, with an RTL valve, supplied from a radiator
- What pressure should be set after bleeding underfloor heating?
- Why does underfloor heating become airlocked again?
- When should you call an installer?
- Frequently asked questions
How do you bleed underfloor heating step by step?
You bleed underfloor heating by flushing the loops one by one through the manifold: water enters through the valve on the supply manifold and exits through the hose connected to the valve on the return manifold, while the boiler and circulation pump are switched off. This allows you to bleed the underfloor heating in any system, even when the manifold is located below the loops.
What should you prepare before bleeding the system?
You need two hoses that fit the manifold valves, a bucket, a towel, and a bleed-key wrench. Place a towel or sheet of plastic on the floor under the manifold cabinet, as some water may leak when connecting and disconnecting the hoses. Also check whether the system is filled with glycol-based fluid or inhibitor: such fluid must not be poured into the drain, and work with it should be carried out by an installer.
Step-by-step procedure
- Turn off the boiler and circulation pump and wait for the system to cool down.
- Take a photo of the flowmeter and control-head settings so you can restore them after the work without guessing.
- Close the manifold's isolation valves from the boiler. This prevents mains pressure from reaching the boiler and opening the safety valve.
- Close all loops on the supply and return manifolds.
- Connect the hoses: the supply hose to the filling valve on the supply manifold, and the drain hose to the drain valve on the return manifold. Lead the end of the drain hose over a drain or into a bucket.
- Open the drain valve, then slowly open the supply valve.
- Open one loop on both sides of the manifold.
- Flush until a steady stream without bubbles or noise flows from the hose. A long loop needs more time than a short one.
- Close the loop and move on to the next one, repeating steps 7 and 8.
- Close the supply, then the drain valve.
- Disconnect the hoses, open the loops and isolation valves, and restore the settings from the photo.
- Bleed the manifolds using the air vents, top up the pressure through the boiler's filling valve to the value specified in the manual, check the boiler pressure gauge, and turn on the heating.
Supply or return?
You let water in through the supply manifold and discharge it through the return manifold, so the loop is flushed in the direction of normal operation. The flow meters and valve inserts then operate as designed. If the filling valve is located on only one manifold, follow the manifold manufacturer's instructions.
Most common mistakes when flushing loops
- Flushing with all loops open: water takes the shortest route, while air remains in the longer loops.
- Valves to the boiler left open: mains pressure opens the safety valve.
- Filling too quickly, which carries air along instead of pushing it out.
- Flushing a hot system.
- Leaving the hose connected to the tap after finishing the work.
- Failing to bleed the manifolds at the end.
Bleeding underfloor heating yourself: from the tap or with a flushing station?
You can bleed underfloor heating yourself using a hose from the tap, but only through a connection point with backflow protection compliant with PN-EN 1717. Heating-system water must not flow back into the drinking water supply, and a hose connected to the tap could siphon it in the event of a temporary drop in mains pressure.
A regular shut-off valve does not provide this protection. Disconnect the hose from the tap immediately after use; permanent system-filling points should be installed by a professional.
Tap water is also a risk to the system itself. Each portion of fresh water introduces oxygen, which accelerates corrosion of steel components, as well as calcium and magnesium compounds, which settle as scale on the boiler's heat exchanger. A single flush is not a problem, but frequent topping up and repeated flushing from the mains will gradually reduce the boiler's efficiency and shorten the pump's service life. Therefore, the less fresh water enters the system, the better, and the requirements for heating water quality should be checked in the boiler manual.
An alternative is closed-circuit flushing: water circulates from the tank through the loop and back to the tank, while air escapes at the open surface. This introduces less fresh water into the system, and bubbles are easy to monitor.
| Method | Advantages | Limitations |
|---|---|---|
| Hose from the tap through a protected connection point | Inexpensive, high flow rate | Lots of fresh water, requires drinking water protection |
| Flushing station with a tank | Closed circuit, easy bubble monitoring | Equipment cost or rental |
| System circulation pump | No additional equipment | Works only when the manifold is the highest point |
Underfloor heating air bleeding with the pump: on or off?
When flushing with water through the manifold, the circulation pump and boiler must be switched off. Switch on the pump only when the manifold is the highest point in the system and one loop is open.
What does it mean for the manifold to be at the highest point? The manifold bars with air vents must be higher than all the loop pipes, as in a typical wall-mounted cabinet above the floor on the same storey. Air always rises, so the pump pushes bubbles out of the loops, and they collect in the manifold bar and escape through the air vent. If the manifold is lower, for example in a basement boiler room while the loops are on the ground floor, the air will become trapped at the highest point of the loop, and the pump will not push it down to the manifold.
With this method, one open loop receives the pump’s entire flow, which pushes the air toward the air vent on the manifold bar. During bleeding, set the pump to its highest speed, and if it has a separate bleeding function, you can use it according to the instructions. However, do not leave the pump running with all loops closed unless the system has a bypass, because the pump will then be operating against a closed valve. Also watch out for air in the pump itself: if it is noisy or does not pump, first perform central heating circulation pump bleeding
| Situation | Method | Circulation pump |
|---|---|---|
| Manifold at the highest point, several cold loops | Forcing through with the pump | Switched on, one loop open |
| Manifold in the boiler room, loops higher up | Flushing through the manifold | Switched off |
| New system or after draining | Flushing through the manifold | Switched off |
| After service work on the manifold | Flushing through the manifold | Switched off |
| The pump did not help after two attempts | Flushing through the manifold | Switched off |
| System with glycol or inhibitor | Installer | Not applicable |
How to recognize air in underfloor heating?
You can tell that underfloor heating contains air if some loops are cold or heat in strips, you hear gurgling in the manifold or floor, and the loop flow meter shows zero even though the pump is running.
- Some loops are cold or the floor heats in strips.
- Gurgling and noise in the manifold or under the floor.
- The loop flow meter remains at zero or the float jumps.
- The pressure drops when the pump starts or after bleeding.
- Pump operating noisily.
If the pressure on the pressure gauge remains too low, start by topping up the water. We describe the symptoms in the article about what happens when there is too little water in the central heating system. If the floor still heats poorly after bleeding, the cause is usually the flow or loop settings, not air.
Manifold: which bar is the supply and which is the return?
The supply manifold is the one connected to the pipe from the boiler or pump group and is warmer when the heating is running. It usually has flow meters, while the return manifold has inserts for actuators or thermostatic heads, but the arrangement may be reversed, so if in doubt, follow the manifold’s instructions.
| Element | Where is | Role in bleeding the system |
|---|---|---|
| Supply manifold | Usually the upper one, with flow meters | This is where you let the water in |
| Return manifold | Usually the lower one, with actuator inserts | This is where you release the water |
| Isolation valves | At the boiler connection | Closed during flushing |
| Filling and drain valve | At the end of the manifold | Hose connection |
| Air vent | At the end of the manifold | Removes air collected in the manifold |
| Loop valve or flow meter | On each loop | Open one loop at a time |
An air vent on the manifold removes only the air that reaches the manifold. Air must be pushed out of the loop by the flow. You can see what manifolds, flow meters, and air vents look like in the category manifolds and pump groups for underfloor heating.
Unusual systems: without an air vent, with an RTL valve, supplied from a radiator
In unusual systems, the principle is the same: the air must be pushed by the flow to a place where it can be released, namely a drain valve or the nearest air vent.
How do you bleed underfloor heating without an air vent?
Without an air vent, remove the air by flushing the loop through the manifold drain valves or through the nearest air vent in the system, for example by the boiler. A permanent solution is an automatic air vent on the manifold, installed by a professional.
How do you bleed underfloor heating with an RTL valve?
A single loop with an RTL valve, which limits the temperature of its return flow, is bled after setting the valve to maximum. Air is released through the air vent in the valve box, if it has one, or the loop is flushed after shutting it off. Always check the sequence in the valve manual.
Underfloor heating supplied from a radiator system or located on another floor
Air moves to the highest point of the system, which is why radiators are bled from the lowest to the highest point. Air released from the lower radiators moves upward through the system, so if you started at the top, you would have to return there. Starting at the bottom, you collect it along the way and eventually remove it at the highest point, in accordance with the instructions described in the guide how to bleed a radiator step by step. If the manifold is lower than the loops, the pump will not push the air upward, and the loops must be flushed.
What pressure should be set after bleeding underfloor heating?
After bleeding the system, top up the pressure to the value specified in the boiler manual. For a single-family home, this is approximately 1.0 to 1.5 bar when cold, and the pressure rises slightly once the system heats up.
Pressure that is too low encourages air to be drawn in through the air vents at the highest points, while pressure that is too high opens the safety valve. After bleeding the system, the pressure usually drops, so check the pressure gauge again after a few hours of operation.
| Time | What to check | What to do |
|---|---|---|
| Before bleeding | Pressure gauge reading | Record the value |
| After flushing and opening the valves to the boiler, while cold | Value from the boiler manual | Top up through the boiler filling valve |
| After heating up | Pressure increase | Normal if below the safety valve threshold |
| After 2 to 3 days of heating | Pressure drop | Bleed the manifolds and top up |
Why does underfloor heating become airlocked again?
Fresh water releases dissolved gases during the first heating cycle because their solubility decreases as the temperature rises. Therefore, bleed the manifolds again after 2 to 3 days of operation and check the pressure. If air returns regularly, the cause is a fault:
- low system pressure,
- an incorrectly pressurized or damaged diaphragm expansion vessel, which we describe how to check in the guide how to pressurize a diaphragm expansion vessel in a central heating system,
- a leak and frequent topping up with fresh water,
- a faulty automatic air vent,
- in older systems, pipes without an oxygen-diffusion barrier (oxygen ingress and corrosion).
As a routine, it is enough to check the pressure and bleed the manifolds before each heating season.
When should you call an installer?
Call an installer when the procedure cannot be performed independently or does not produce the desired result:
- the system is filled with glycol or inhibitor,
- the manifold has no isolation or drain valves,
- something is leaking at the manifold,
- the pressure drops despite topping up,
- the loop is still cold after flushing and the flow meter shows zero,
- there is no safe filling point connected to the drinking-water supply.
Frequently asked questions
Can underfloor heating be bled without professional help?
Yes, if the manifold has isolation and drain valves and the system contains neither glycol nor inhibitor. Two hoses are sufficient; flush one loop at a time.
When bleeding underfloor heating, is water fed in through the supply and discharged through the return?
Yes, water is fed in through the supply manifold and discharged through the return manifold. Flush one loop at a time.
Should the circulation pump be switched on while bleeding underfloor heating?
Not when flushing with water: the pump and boiler must be switched off. Switch on the pump only when the manifold is at the highest point and one loop is open.
Is it normal for underfloor heating to become airlocked?
Yes, but only during the first few days of heating after filling with fresh water. If air returns regularly, the usual causes are low pressure, a leak, or a faulty diaphragm expansion vessel.
How often should underfloor heating be bled?
Once before the season and after each refill, plus again 2 to 3 days after startup. Needing to do it more often indicates a fault.
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