Short answer: To bleed air from water underfloor heating, you purge each loop on its own: close every loop except one, push mains water in through the flow bar and out of a drain hose on the return bar until the flow meter float sits still and no bubbles come out, then move to the next loop. A 100m loop of 16mm pipe holds about 11.3 litres, so at mains flow each loop clears in a few minutes. Afterwards, set the cold system pressure to the heat source maker's figure, often 1 to 1.5 bar on a boiler system.
Air is the most common reason a loop gurgles, heats partway or stays cold after a refill. The manifold's automatic air vent only removes air that reaches the manifold bar. Air trapped out in the loops needs water pushed through it fast enough to carry it back.
This guide goes further than the quick purge in our cold underfloor heating guide. It covers how to spot air, where it comes from, how long to purge each loop, what to do if you have no hose point, and how to stop air coming back.
How do I know there is air in my underfloor heating?
The clearest sign is a flow meter float that jumps or flickers instead of sitting steady. One manifold maker's instructions treat erratic float movement as proof that air is still in the loop. Other signs are noise and uneven heat.
| What you notice | What it usually means |
|---|---|
| Float bouncing or flickering | Air passing the meter |
| Gurgling or trickling sound | Air pocket in a loop |
| Loop warm near manifold only | Air blocking flow further out |
| Pressure falls days after a fill | Air venting out, or a leak |
| Pump noisy, little flow | Air at the pump |
If the pressure keeps falling after you have purged properly, the cause is more likely a leak than air. Our guide to underfloor heating pressure dropping covers how to find it.
Where does the air come from?
Most air gets in when the system is filled or topped up. Fresh mains water also carries dissolved air that comes out of solution as the water heats, so a new system often needs venting again after its first few heat-ups.
- First fill and refills. Every drain-down and refill brings in new air.
- Frequent top-ups. Each top-up adds fresh water and more dissolved air. If you top up often, find out why the pressure is dropping.
- Low pressure at high points. If the pressure at the top of the system falls too low, automatic air vents can draw air in instead of letting it out.
- Pipe without an oxygen barrier. Oxygen passing through the pipe wall feeds corrosion in the metal parts of the system. The PERT-EVOH pipes we sell are oxygen tight to DIN 4726.
Why won't the manifold air vent clear it?
An automatic air vent only lets out air that rises into the manifold bar. Air trapped in a loop stays there until the water moving through that loop is fast enough to carry it back to the manifold.
On a working system, the pump shares its flow between all the open loops, so each loop may not move fast enough to shift a stubborn pocket. Loops with closed actuators get no flow at all. That is why purging works best one loop at a time, with all the flow going through a single loop.
Check the vent itself too. A vent that drips or sticks shut should be replaced. Isolate the manifold with its ball valves before you change it.
How do I purge underfloor heating loops with mains water?
You purge with mains water by filling through the flow bar and draining from the return bar, one loop at a time. This method follows a UK manifold maker's instructions. You need a hose from a mains tap, a drain hose and a bucket or drain.
- Switch off the heat source and the UFH pump, and let the system cool.
- Close the manifold ball valves so you are not pushing mains water into the boiler or heat pump.
- Lift the thermal actuators off the manifold valves. On most manifolds this lets the valve underneath open. Some makers' normally closed actuators arrive with a clip that holds them open for filling, which is removed before commissioning.
- Connect the mains hose to the fill valve on the flow bar, and a drain hose to the drain valve on the return bar.
- Close every loop except the first one. Open its flow meter and its return valve.
- Open both valves and turn on the mains. Water runs from the flow bar, round the loop and out of the return bar.
- Keep it running until the float is steady and the water from the drain hose comes out with no bubbles.
- Close that loop, open the next, and repeat until every loop is done.
- Vent the flow bar with its manual or automatic vent, then close the drain and the fill.
- Refit the actuators, open the ball valves and set the system pressure.
On a sealed system, mains water should only enter through an approved filling loop or fill point. If you are not sure how your system is filled, ask the installer or a heating engineer.
How long should I purge each loop?
Purge each loop until the float stops moving and the water runs clear of bubbles, which usually means passing several times the loop's own volume. Loop volume depends on the pipe's inside diameter, worked out from the outside size minus twice the wall.
| Pipe | Water per metre | Per 100m loop |
|---|---|---|
| 12 x 1.6mm | 0.061 litres | 6.1 litres |
| 16 x 2.0mm | 0.113 litres | 11.3 litres |
| 20 x 2.0mm | 0.201 litres | 20.1 litres |
Worked example. A ground floor has 6 loops of 16mm pipe, averaging 85m each.
- Each loop holds 85 x 0.113 = about 9.6 litres.
- All six loops hold about 58 litres, before the manifold, pump set and primary pipework.
- If your hose delivers 10 litres a minute, one loop's volume passes in under a minute. Running each loop for 3 or 4 minutes passes several volumes and is a sensible minimum.
- Six loops take about 20 to 25 minutes of purging in total.
Run longer on any loop where bubbles keep appearing. Long loops and loops with many tight bends trap air more easily.
Can I purge the loops without a hose?
Yes, you can often shift a small air pocket with the pump alone by closing the other loops, so all the pump's flow goes through the problem loop. A heating maker's troubleshooting guide recommends this, and suggests flushing with mains water only if it doesn't work.
- With the system running, close the flow meters or valves on every loop except the one with air. Turning the other room thermostats down also closes their actuators.
- Leave the pump running on a higher setting for 10 to 15 minutes.
- Watch the float. When it settles, open the other loops again.
- Check the system pressure and top up if needed.
Don't leave the pump running with every loop shut, as that leaves it with no flow path. If the manifold has a bypass, check it is set to the pump maker's guidance.
What pressure should the system be after bleeding?
Set the cold pressure to the heat source maker's figure. One boiler maker's filling guide gives a normal range of 1 to 1.5 bar. Heat pump systems vary, so follow the heat pump's instructions.
The manifold gauge and the boiler gauge can read differently. Water pressure rises by about 0.1 bar for every metre of height, so a manifold 3m below a loft boiler reads about 0.3 bar more than the boiler gauge. Use the gauge on the heat source for the target, and the manifold gauge to watch for changes.
Purging pushes water out, so the pressure will usually fall afterwards. Top up slowly, then run the heating and check the pressure again once the water is hot and has cooled.
How do I stop air coming back?
Air keeps coming back when fresh water keeps going in. Stop the top-ups and you stop most of the air.
- Fix the cause of pressure loss rather than topping up week after week.
- Vent again after the first heat-ups of a new or refilled system, as dissolved air comes out of hot water.
- Keep the pressure in range so the high points stay above atmospheric pressure.
- Use oxygen barrier pipe on any new loop or repair.
- Dose the system with inhibitor after a refill, following the inhibitor maker's instructions.
- Consider a manifold with fill, drain and vents on both bars, which makes purging much quicker next time.
Once every loop has steady flow, check the flow rates are where they should be. Our guide to balancing underfloor heating loops explains how.
Products for this job
Prices include VAT, October 2026.
- Manifold Auto Air Vent End Unit: £10
- Manifold Flow Meter: £10
- Manifold Pressure Gauge End Unit: £16
- Manifold Ball Valve Set 1": from £22
- Auto-Balancing Stainless Steel UFH Manifold, with fill, drain and vents on both bars: from £62 (2 port)
- 5-Layer PERT-EVOH Oxygen Barrier Pipe 16mm: £129 (200m)
- All manifolds and components
Related guides: Underfloor heating not getting warm and How to install water underfloor heating.
Planning new loops that purge easily? Design it free with MaxHeatUFH ProLoop. Add your rooms and floor type, and get a loop design with balanced loop lengths, plus a full kit list with prices. Start your design
Frequently asked questions
Can you bleed underfloor heating like a radiator?
Not in the same way. There is no bleed screw on each loop. The manifold vents release air that reaches the bar, but air in a loop has to be purged out by pushing water through that loop on its own.
How do I know when a loop is fully purged?
The flow meter float sits still and the water from the drain hose runs with no bubbles. If the float still flickers, keep going.
Why is there still air after I purged the system?
New water gives off dissolved air as it heats. Run the heating for a few days, vent the manifold again and check the pressure. If air keeps appearing, look for a leak or a vent drawing air in.
Do I need to remove the actuators to purge?
On most manifolds, yes, or at least make sure they are powered open. A closed actuator shuts its loop, so no water can flow through it while you purge.
Should the heating be on when I purge?
No. Switch off the heat source and pump and let the system cool before purging with mains water. You can use the pump-only method with the system running.
How often should underfloor heating be bled?
A sound sealed system should rarely need it. Purge after a fill or drain-down, and whenever a loop gurgles or heats only partway. If you need to vent often, something is letting air or water in.
Sources: Wunda manifold instructions, Grant UFH troubleshooting, Danfoss TWA thermal actuator data sheet, Worcester Bosch system filling guide