Bleeding underfloor heating means removing trapped air from a wet, water-based UFH system. The first place to deal with small amounts of air is normally the manifold air vent. A stubborn airlock inside a floor loop may require that circuit to be isolated and purged with water through the manifold's fill and drain points.
The important distinction is that venting the manifold and fully purging a loop are not the same job. A simple vent can be suitable for a confident homeowner where the manufacturer instructions clearly identify the valve. A full loop purge involves the manifold valves, fill/drain connections and system pressure, so it should follow the exact manifold instructions or be carried out by a competent heating engineer.
What Does Bleeding Underfloor Heating Actually Mean?
A wet underfloor heating system circulates warm water through long pipe loops beneath the floor. Those loops connect to a manifold, which distributes water to the different rooms or zones.
Air can become trapped in the manifold or inside one of the loops. Because air does not transfer or circulate heat in the same way as water, a sufficiently large air pocket can restrict flow and leave a circuit partly or completely cold.
The term "bleeding underfloor heating" is often used for two different procedures:
Manifold Venting
Releasing accumulated air through a manual or automatic air vent on the manifold. This is the simpler procedure and deals with air that has reached the manifold.
Loop Purging / Flushing
Forcing water through one UFH circuit at a time using the manifold fill and drain points so trapped air is physically carried out of the pipe loop.
Manufacturer guidance from major UFH suppliers uses this one-loop-at-a-time approach when a system is initially filled or when a persistent airlock has to be removed. The exact valves and sequence vary by manifold, which is why the product instructions matter.
Signs There May Be Air in Your Underfloor Heating
Trapped air is one possible cause of poor UFH performance, but it is not the only one. Symptoms that make an airlock more plausible include:
Gurgling or Bubbling
Water and air moving through the manifold or pipe circuits can produce obvious gurgling, trickling or bubbling sounds.
One Loop Has Little or No Flow
A flow meter may stay at or near zero, or its reading may move erratically, even though that zone should be calling for heat.
One Zone or Part of a Floor Stays Cold
A circuit can remain cold while neighbouring zones heat normally, particularly after draining, filling or heating-system work.
Problem Started After Draining or Repairs
Opening the heating circuit for repairs or refilling can introduce air if the system is not properly purged afterwards.
Before You Bleed Underfloor Heating
Do not start opening valves simply because a room feels cold. First establish what type of system you have and whether bleeding is an appropriate next step.
| Check | Why It Matters | What to Do |
|---|---|---|
| System type | Only wet UFH contains water to bleed | If it is electric UFH, do not open plumbing valves |
| Manufacturer / manifold | Valve layouts and filling arrangements vary | Use the manifold or UFH manufacturer's instructions where available |
| System temperature | Hot water can cause scalding | Switch off the heating and allow the system to cool before opening a manual vent |
| Pressure | Very low or unstable pressure may indicate another fault | Note the cold reading; do not continually top up a system that keeps losing pressure |
| Visible leaks | A leak can cause pressure loss and introduce air | If water is escaping, investigate the leak rather than repeatedly bleeding the system |
| Flow settings | Flow meters may have design settings for each loop | Record settings before any adjustment and do not rebalance by guesswork |
How to Bleed Air from an Underfloor Heating Manifold
If the manufacturer instructions show a manual air vent and you are comfortable carrying out basic heating-system maintenance, small amounts of air can often be released at the manifold without performing a full circuit flush.
Switch the Heating Off and Let It Cool
Stop the heat source and circulation before opening a manual vent, unless the specific manufacturer instructions state otherwise for that system.
Locate the UFH Manifold and Identify the Air Vent
The vent is normally positioned at or near the end of a manifold rail. Use the manifold instructions to distinguish it from fill/drain valves, flow meters and isolation valves.
Protect the Area from Water
Place a cloth or suitable container beneath the vent. Even a normal bleed can release some heating-system water.
Open the Manual Vent Slowly
Use only the tool and method specified for the vent. Air may hiss or spit out first. Do not remove the vent body or force a fitting that does not move normally.
Close the Vent When the Discharge Is Steady
Once air has stopped and water is coming through without obvious bubbles, close the vent securely without overtightening it.
Check the Other Manifold Vent if Fitted
Some manifolds have venting on both flow and return rails. Follow the same manufacturer-specific approach where appropriate.
Recheck Cold System Pressure
Bleeding can reduce system pressure. Restore pressure only to the value specified by the boiler, heat-pump or heating-system manufacturer and only using the correct filling procedure.
When a Full UFH Loop Purge or Flush Is Needed
A stubborn airlock may need to be physically pushed out of the affected pipe circuit. Major UFH manufacturers describe this as a fill-and-purge procedure: each loop is dealt with individually so water is forced through that circuit and carries trapped air to the drain.
In general terms, the procedure involves:
- 1
Isolating the Manifold from the Primary Heating Circuit
The manifold is separated from the boiler or heat-pump side as required by the specific manifold design.
- 2
Closing the Other UFH Loops
Only one circuit is opened at a time so flushing water cannot take an easier route through neighbouring loops.
- 3
Using the Correct Fill and Drain Connections
Water is introduced through the designated fill point and discharged through the designated drain point using suitable hoses.
- 4
Purging One Circuit Until It Is Bubble-Free
The loop is flushed until the discharge becomes steady and free from obvious air, then that circuit is closed before moving to the next one.
- 5
Restoring the Manifold and Commissioning Settings
Fill/drain valves, loop valves, isolation valves, flow settings and system pressure must all be returned to the correct operating arrangement before the system is restarted.
What to Do After Bleeding Underfloor Heating
The job is not finished as soon as the hissing stops. Before judging whether the problem has been solved:
- check that all vents and fill/drain points used are closed correctly
- restore any loop valves or manifold isolation valves that were intentionally changed
- return flow meters to their recorded or commissioned settings if they were altered
- check the cold system pressure and follow the heat-source manufacturer's filling instructions if pressure needs restoring
- inspect the manifold and accessible joints for water leakage
- restart the heating and allow sufficient time for the floor to respond
- compare the affected zone with the other circuits rather than expecting an immediate floor-temperature change
What If the Underfloor Heating Is Still Not Working?
If venting or purging does not restore normal circulation, the fault may never have been trapped air. UFH problems often produce similar symptoms, so the pattern matters.
| Symptom | Air Is Possible? | Other Faults to Consider |
|---|---|---|
| Gurgling + poor flow in one loop | Yes, particularly after filling or repairs | Flow setting, blockage, pump performance |
| One zone cold, no unusual noise | Possible | Actuator, thermostat, wiring, closed valve, balancing |
| All UFH zones cold | Less likely as the only cause | Heat source, circulation pump, controls, power supply |
| Pressure keeps falling | Air may be present, but it does not explain ongoing water loss | Leak, PRV/expansion issue, system fault |
| Flow meter remains at zero | Yes | Closed/stuck valve, actuator, blockage, weak pump |
| Electric UFH is cold | No | Thermostat, sensor, wiring, mat or cable fault |
For broader diagnosis, see why underfloor heating is not working. For wet UFH pressure loss or suspected concealed water escape, see our underfloor heating leak detection service.
Why Does Air Keep Coming Back into Underfloor Heating?
A properly filled closed heating system should not require continual bleeding. If the same circuit repeatedly becomes noisy or loses flow, look for the reason rather than treating venting as routine maintenance.
Possible causes include:
- the system was not fully purged when first filled
- air was introduced after draining, repairs or component replacement
- system pressure is repeatedly becoming too low
- a water leak is causing pressure loss
- an automatic air vent is closed, blocked or not operating correctly
- the system is repeatedly being topped up with fresh water
- circulation, balancing or pump problems are being mistaken for an airlock
How Often Should You Bleed Underfloor Heating?
There is no universal schedule that says wet UFH must be bled every few months or every year. A properly commissioned system should not continually collect large amounts of air.
Bleeding or purging is more commonly required:
- during initial filling and commissioning
- after part of the heating system has been drained
- after pipework, manifold, pump or valve work
- when gurgling or bubbling suggests trapped air
- when a particular circuit loses flow and an airlock has been reasonably identified
If you find yourself bleeding the same UFH system repeatedly, that is a reason to investigate the system rather than simply bleed it more often.
When to Call an Underfloor Heating Engineer
Professional diagnosis is advisable when:
- you cannot identify the manifold valves or do not have the manufacturer instructions
- a simple manifold bleed has not restored flow
- the system requires a full circuit purge and you are not familiar with the manifold
- pressure repeatedly falls after bleeding or refilling
- there is visible water around the manifold or suspected leakage below the floor
- a flow meter remains at zero despite the circuit being open
- the same zone repeatedly develops air or poor circulation
- the manifold pump, actuators, wiring centre or thermostats may be involved
- work would involve opening or servicing a gas boiler
Fix & Repair Engineers provides wet and electric underfloor heating repairs, servicing and fault diagnosis across London and the Home Counties. For location-specific service information, see our UFH pages for London, Essex, Kent, Suffolk and Hertfordshire.