Air in your underfloor heating, and how to get it out

How air shows itself

Underfloor heating (vloerverwarming) is one long loop of pipe per area, buried in the floor and fed from a manifold (verdeler) in a cupboard or the meterkast. Look for:

• one room or one patch of floor that stays cold while the rest warms up
• gurgling or a trickling sound at the manifold or the pump
• a pump that hums louder than it used to, or rattles
• boiler pressure that has dropped since you last looked

A floor takes hours to warm through, so give it time. If nothing in the house warms up, start at the boiler with my guide to boiler pressure and fault codes.

What you’re looking at: the manifold and the pump group

The manifold has two bars, one for the warm flow and one for the cooler return, and every loop connects to both. On the flow bar sit small sight glasses, one per loop: the flow meters. On the return bar are the loop valves, often with a white cap or an electric actuator. At the end of each bar is a small bleed valve, sometimes with a hose tail.

Many manifolds also have a pump group (pompgroep): a pump, a mixing valve and a thermometer, because a floor runs far cooler than radiators. Milieu Centraal puts the water at about 40 degrees, sometimes 30, with 45 as the maximum. The mixing valve blends return water into the flow to get there. Fitting and replacing pump groups is part of my pipework.

Bleeding at the manifold

This is the safe do-it-yourself part. You need a cloth, a cup and, if the bleed valves have a hose tail, a short piece of hose.

1. Switch the heating off at the thermostat and turn the pump off if it has a switch or a plug. Leave it a while so air rises to the top of the bars.
2. Open the bleed valve on the flow bar a quarter turn, cloth underneath. Let it hiss until water comes out in a steady stream, then close it.
3. Do the same on the return bar.
4. Turn everything back on, wait ten minutes and bleed both bars again; a running pump carries air up from the loops.

Air in the loops themselves is the stubborn part. Remeha’s advice is that bubbles in the pipes under the floor can’t be removed with a standard bleed valve; each loop has to be flushed through.

Flushing one loop at a time

Remeha describes this for homeowners whose manifold has a fill point on the flow side and a drain point on the return side. In short:

1. Close every radiator valve in the house and every loop valve on the manifold.
2. Run a hose from a tap to the fill point, and a second from the drain point to a drain or a bucket.
3. Open the first loop only. Open the tap and the fill point fully, and use the drain point to hold the pressure at about 1.5 bar.
4. Let water push through that loop for about five minutes, until it runs without bubbles.
5. Close it, open the next loop, and repeat for every loop.
6. Close the fill and drain points, check the manifold’s bleed valve briefly for air, and open every radiator and loop again.

It works, but it puts a lot of fresh water into a closed system, and it’s easy to overshoot the pressure. No fill and drain points, or rather not? Send me a photo and I’ll do it.

Check the pressure afterwards

Bleeding lets water out of a closed system, so the reading on the boiler falls. Intergas says to check that it’s still between 1 and 2 bar with the system cold, and to top up and bleed again if it isn’t. Other makes give their own range in the manual. Topping up with the vulkraan (filling valve) is in my pressure guide.

Still cold? Valves, flow meters and balancing

If the air is out and one loop stays cold, look at the manifold again.

• The loop valve is shut. A failed actuator, or a valve someone closed, keeps that loop cold. Open it by hand and watch the flow meter.
• The flow meter shows nothing. No flow means a closed valve or a blocked loop, not air.
• The far rooms are lukewarm. The loops are different lengths, and water takes the shortest one. Setting each flow meter so every loop gets its share is balancing (waterzijdig inregelen). That’s part of my radiator work.
• The pump doesn’t run. It only runs when the thermostat asks for heat. Silent with the heating on, it may have seized or lost its power.

One more thing, from the Intergas installation manual: a floor pump that isn’t separated from the boiler properly can push water round the boiler when it shouldn’t and upset the hot water, so Intergas asks for a check valve or a two-way valve. I look for that when a floor has never quite worked.

When air keeps coming back

Bled it twice and the gurgle is back within weeks? Air is getting in. Flamco, the expansion vessel maker, points out that a system whose cold pressure drops too low, overnight for example, can draw air in at bleed valves and valve glands. The usual reason the pressure sags like that is an expansion vessel that has lost its charge, or a small leak.

Black, smelly water at the bleed valve is sludge, and a loop that stays cold after flushing may be silted up. That needs a proper look, not more bleeding.

Get it looked at

Send me a message on WhatsApp with photos of the manifold and pump group, the boiler’s pressure reading, your postcode and which rooms stay cold. My rate is €114.95 incl. BTW per hour, with no call-out charge. Materials are extra, and I explain the cost before I start. I work by appointment, 08:00-16:00, seven days a week.

If you smell gas, leave the property and call 0800 9009 from outside. If a CO alarm goes off or you suspect carbon monoxide, get everyone outside and call 112.