How to fill and purge an underfloor heating system

Filling and purging is the first job after the pipe is connected to the manifold, and it decides how the system runs for years. Air left in a loop stops it heating, makes noise and lets the pump run dry. Filling all loops at once almost always leaves air behind, because water takes the shortest loop and leaves the long ones half full.

This guide covers a first fill on a new system, step by step, with the valve settings published by manifold makers. If an existing system has air in it, our guide to bleeding air from underfloor heating covers the fixes.

What do I need to fill an underfloor heating system?

You need a clean water supply with the right backflow protection, two hoses, a bucket or drain, and the manifold's own fill and drain valves. Have the inhibitor and a pressure test pump ready too.

• Fill and drain valves on the manifold end units, one on the flow bar and one on the return bar.
• Two hoses: one in from the water supply, one out to a drain or bucket.
• A filling point with backflow protection (see below), or a test pump with a tank of clean water.
• The manifold air vent, fitted at the high point of each bar.
• Inhibitor suited to the system, and a test kit to check it later.
• A pressure gauge on the manifold or test pump.

Label every loop at the manifold before you start, so you know which room you are filling.

How do I set the manifold valves before filling?

Isolate the manifold from the rest of the system, close every loop, and open the flow meters fully. Then connect the hoses so water enters the flow bar and leaves from the return bar.

1. Close the main ball valves. Both manufacturers close the manifold's flow and return ball valves first. That stops dirt and air going into the boiler, heat pump or mixing unit.
2. Close every return valve. Use the protective caps or remove the actuators and turn the valves shut.
3. Open all flow meters fully. One manufacturer's manual requires every flow meter to be completely open during filling.
4. Connect the fill hose to the flow bar. Water goes in through the flow side fill valve.
5. Connect the drain hose to the return bar. Open the return side drain valve as the outlet, and run the hose into a bucket or drain where you can see the water.

One manufacturer's instruction is to fill and flush only in the direction of flow. Water goes in at the flow bar, out along each loop and back through the return bar, the same way it travels when the pump runs.

How do I purge each loop?

Open one loop's return valve fully, run water through that loop until the outlet runs clear and steady with no air, then close it and open the next. Do every loop on its own.

1. Open loop 1 only. Fully open its return valve. All other loops stay shut.
2. Open the supply slowly. Both makers warn that opening valves quickly causes pressure surges. One manufacturer asks you to avoid a pressure difference of more than 1 bar, and another manufacturer keeps the system at no more than 3 bar while flushing.
3. Watch the outlet. You will see spurts of air and water at first. Keep going until the flow is smooth and clear. Holding the hose end under water in the bucket makes bubbles easy to see.
4. Close loop 1. Shut its return valve.
5. Repeat for each loop. Long loops take longer than short ones, so judge by the outlet, not the clock.
6. Remove the hoses. Close the fill and drain valves, take the hoses off and refit the caps.

A new system may also carry swarf, flux and debris from the install. Running each loop until the water is clear is also its first flush.

How do I fill and vent the rest of the system?

With every loop purged, open all the return valves and the main ball valves, then fill the primary pipework and pump pack and vent the high points. The system should sit at the pressure set for your heat source.

• Open the loops. Open every return valve, refit the actuators and check each flow meter shows flow later when the pump runs.
• Open the ball valves. Let the manifold connect back to the pump pack and heat source.
• Vent the pump and mixing unit. Vent the pump and any air vents on the pump pack, then run the pump and let the manifold air vent clear the last pockets.
• Set the cold pressure. On a sealed system, one manufacturer says the cold pressure should ideally be 0.2 bar above the expansion vessel's air charge. Typical charges are 0.5 bar for bungalows and 1.0 bar for two-storey houses, which gives about 1.2 bar cold for a two-storey house.

Gas boiler work must be done by a Gas Safe registered engineer. Heat pump systems are filled to the heat pump maker's instructions, and an MCS certified installer is needed if you want a grant.

How must a filling loop be connected?

Through a backflow device approved for the job, and disconnected after use. A permanent connection between the mains and a heating system is not allowed.

One manufacturer's summary of the Water Supply (Water Fittings) Regulations 1999 says that for a domestic sealed heating circuit treated as fluid category 3, a double check valve to BS EN 13959, WRAS approved, on the fill point may be acceptable. After filling, close the valves and remove the flexible hose. Disconnecting one end is acceptable if the break is between the hose and the backflow device. The water supplier can ask for more protection.

Should I add inhibitor to underfloor heating?

Yes. BS 7593 sets out how to clean, flush and protect wet heating systems, and UFH is a wet heating system like any other.

A trade press summary of BS 7593:2019 covers the main points:

• Clean and flush using one of three recognised methods: power flushing, mains pressure, or gravity with the circulation pump.
• A filter protects components but is not a substitute for cleaning and flushing.
• Check the chemical levels and water cleanliness after commissioning, after any work that dilutes the water, and every year.
• Consider a biocide for all systems, especially those running below 60°C, which includes every UFH system.
• Re-dose inhibitor every five years, or use a lab test to confirm protection.

Our guide to inhibitor, biocide and antifreeze for UFH covers which products and doses to use.

When do I pressure test after filling?

On a new system, straight after the loops are purged and before any screed or floor covering goes on. Air left in the loops makes the gauge harder to read, so purge first.

One manufacturer's guide raises the system to 6 bar and holds it for 60 minutes, then keeps it at 6 bar while the screed is laid. Check the manifold's own limit: one manufacturer's stainless steel manifold allows up to 10 bar at 20°C with the return valves closed during the test. Our guide to pressure testing underfloor heating has the full method and records. After testing, set the flow rates as in our guide to balancing underfloor heating loops.

Why does air keep coming back after filling?

Usually because the system is losing pressure, a loop was never fully purged, or the water still has dissolved air that comes out as it first heats up. Some venting in the first few days is normal.

• Pressure falling: find the leak before you top up again. Each top-up brings in fresh air and dilutes the inhibitor.
• One cold loop: purge that loop again on its own.
• Gurgling after heat-up: the manifold air vent should clear it. Check its cap is open.

Products for this job

• Manifold Auto Air Vent End Unit
• Hydraulic Pressure Test Pump, 0 to 50 bar
• Manifold Flow Meter
• All manifolds and components

Frequently asked questions

Should I fill underfloor heating loops one at a time?

Yes. Water takes the easiest path, so filling all loops together leaves air in the longer ones. Open one loop, purge it until the outlet runs clear, close it and move on.

Which way do I fill an underfloor heating manifold?

In through the flow bar fill valve and out through the return bar drain valve, in the direction of flow. Manifold makers say to fill and flush only in the direction of flow.

Should the flow meters be open or closed when filling?

Fully open. One manifold maker requires all flow meters to be completely open while filling and flushing.

What pressure should underfloor heating be at after filling?

It depends on the expansion vessel charge and the heat source. On a sealed system, aim for about 0.2 bar above the vessel's air charge when cold. For a two-storey house that is often about 1.2 bar.

How long does it take to purge an underfloor heating loop?

Until the water runs clear with no air, which depends on the loop length and the water pressure. Judge by the outlet, not the time.

Can I leave the filling loop connected?

No. The filling hose must be removed after use. A permanent connection between the mains and a heating system is not allowed under the water fittings rules.

Do I need to add inhibitor to underfloor heating?

Yes. BS 7593 covers UFH like any wet heating system. Add inhibitor after flushing, consider a biocide because UFH runs below 60°C, and check the levels every year.

Sources: PHPI: six things to know about BS 7593:2019