How To Purge Air From Hydronic Floor Heating Systems

You walk across the floor in winter and notice a cold patch near the living room window. The rest of the room is warm, but that one spot feels like ice. Or maybe you hear a gurgling sound in the pipes when the pump kicks on. That’s trapped air. Hydronic floor heating systems work by circulating hot water through tubing, and any air in that loop stops the water from reaching certain zones. Purging that air is a basic skill every homeowner should have.

This article covers exactly what causes air to get trapped, how to get it out manually, and when an automatic air separator like the Midline Valve S1559 makes sense. You’ll learn the numbers behind proper purging – flow rates, pressure settings, and the right tools – so you can fix cold floors without calling a plumber every year.

Midline Valve

Midline Valve S1559 Boiler Air Scoop Valve…

• Replacement Valve: Great for Replacing Existing Boiler Air Separator; Compatible With All Major Brands
• Saves Time and Money: Continuously Separates and Purges Air From Hydronic Heating Systems; Reduces Need for Manual Venting
• Superior Quality: Heavy Duty OnePiece Cast Iron Construction; Lead Free; Corrosion Resistant; Rust Resistant

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One tool that makes the job easier is an air scoop valve. The Midline Valve S1559 Boiler Air Scoop Valve bolts into the supply line and continuously separates air from water as the system runs. It cuts down how often you need to manually bleed air, and it works with most standard connections. I’ve used similar scoops on older systems and they really reduce the gurgling noise within days.

How to tell you’ve got air in the system

Not all cold spots come from trapped air. But these signs point directly to air pockets:

• Cold spots that stay cold even after the pump runs for an hour.
• Gurgling, sloshing, or bubbling sounds from the manifold or pipes.
• Fluctuating pressure gauge readings – air pockets compress differently than water, so the needle jumps when the pump cycles.
• Radiator zones that need frequent manual venting – if you’re bleeding the same loop every month, air is entering faster than it escapes.

One quick test: touch the supply and return pipes near the manifold. If one is hot and the other cold, water isn’t circulating. That’s almost always air.

Manual purging step by step

Manual purging works every time if you follow a strict sequence. The goal is to push all air out of the loop using system pressure and the circulation pump. Here’s how to do it right:

1. Turn off the boiler and let the water cool below 100°F (38°C). Hot water can scald, and air expands when hot, making bubbles smaller and harder to detect.
2. Connect a drain hose to the purge valve – usually located at the highest point in the loop or on the manifold supply side. Run the hose to a floor drain or a bucket.
3. Close the isolation valves on the manifold for the zone you’re purging, then open the purge valve. This isolates that loop so you don’t bleed the whole system at once.
4. Open the fill valve (feed water) to bring system pressure up to 15 psi. Then open the purge valve slightly – water will shoot out the hose. Wait until the stream is steady with no spurts or bubbles.
5. Run the pump on the circulator for 30 seconds with the purge valve open. The pump forces water through the loop faster, carrying any stubborn air with it. You’ll see bubbles rush out the hose. Keep the pump running, open and close the purge valve quickly a few times to break up large air pockets.
6. Close the purge valve once no air appears for 15 seconds. Close the fill valve. Remove the hose. Bleed any remaining air at the high-point vent on the manifold.
7. Restore system pressure to 12-15 psi cold. Then run the boiler and check for noise. Repeat for other zones if needed.

The whole process takes about 30 minutes per zone. Do it at the start of the heating season – or anytime you hear gurgling.

Automatic solutions: air vents and air scoops

If you’d rather not drag a hose around every year, automatic air vents and scoops reduce the workload. A manual float vent screws onto the manifold or a pipe tee. It releases air when it collects inside a chamber. But these vents can get stuck open or closed – I’ve seen them drip water after a season or two. They work best on small loops with low flow.

An in-line air scoop, like the Midline Valve, works differently. It creates a low-pressure zone inside the housing that forces dissolved air to bubble out and collect in a separate chamber with a float vent. The scoop sits on the supply line before the pump, so it catches air before it reaches the circulator. This design handles higher flow rates and larger systems better than a simple vent. It’s not perfect – the internal cartridge can clog if your water has sediment – but it reduces manual purging to maybe once every few years.

The Midline Valve S1559 falls into the scoop row. Check its current price on Amazon to see if it fits your budget.

Preventing air from coming back

Purging air is a temporary fix if air keeps getting back into the system. Common culprits:

• Dissolved oxygen in makeup water – every time you add water to boost pressure, you introduce air. Use a water meter or install a backflow preventer to minimize fresh water additions.
• Leaks at fittings or the boiler – when the system cools, a negative pressure can suck air in through tiny gaps. Fix even small drips. See our repairing leaks guide for details.
• Pump cavitation – if the pump is oversized or the suction side is starved, it can pull air from the water itself. Match the pump to your system. Read our choosing the right pump article if you suspect cavitation.
• Expansion tank failure – a waterlogged expansion tank can’t absorb thermal expansion, forcing air out of solution. Test the pre-charge pressure yearly – it should equal your cold fill pressure (usually 12 psi).

Good system maintenance guide practices – like flushing the system every 3–5 years and adding a corrosion inhibitor – keep dissolved oxygen in check. The less air you introduce, the less you have to purge.

Why does air keep getting back in my system after purging?

Most likely a leak or an expansion tank problem. Check the tank’s air pre-charge with a tire gauge when the system is cold. If it reads below 12 psi, recharge it. Also inspect all valves and fittings for slow drips. Fresh water from the fill valve carries dissolved oxygen that turns into air bubbles when heated.

Can I purge air while the system is running?

Not a good idea. Hot water can scald, and the pump may create turbulence that makes air harder to separate. Always let the system cool below 100°F (38°C) before opening any purge or bleed valve. If you need to purge quickly, turn off the boiler and wait 30 minutes.

How often should I purge air from my hydronic floor?

Once a year at the start of the heating season covers typical needs. If you hear gurgling mid-season or see cold spots, purge that zone again. Systems