Maintaining underfloor heating: bleeding air, flushing loops and annual checks

Most underfloor heating faults that get blamed on the floor turn out to be at the manifold: air, a stuck actuator, a blending valve set too low, or low pressure. A short check once a year, ideally before the heating season, catches most of them early. This guide covers what to check, how to do the simple jobs yourself, and when to call a professional.

If your heating has already stopped warming up, start with why underfloor heating is not getting warm.

What needs maintenance and what doesn't?

The pipe in a screed or board has no joints in the floor and no moving parts. Plastic UFH pipe with an oxygen barrier, as we sell, is made for a long life. The boiler or heat pump needs its own service by the right engineer. Any gas work must be done by a Gas Safe registered engineer.

How do I check underfloor heating pressure?

Most wet UFH is part of a sealed heating system, with a pressure gauge on the boiler and often one on the manifold. Read the gauge with the heating off and cold.

• Your boiler or heat pump instructions give the correct figure. Which? says many boilers should read around 1 to 1.5 bar when off.
• Pressure rises a little when the system is hot. That is normal.
• If the pressure is low, top up through the filling loop as the boiler instructions describe.
• If you have to top up more than occasionally, there is a leak or a weeping valve somewhere. Find it rather than keep topping up, because fresh water brings in oxygen and dilutes the inhibitor.

A manifold pressure gauge end unit lets you read the pressure at the manifold. If you suspect a leak in the loops, a loop-by-loop pressure test will find which one.

How do I bleed air from underfloor heating?

Air collects at the high points of the manifold. Signs include gurgling, a flow meter that flickers or reads low, and a loop that is warm at one end and cool at the other.

1. Turn the heating off and let the pump stop. Air is easier to release when the water is still.
2. Check the automatic air vents. Make sure any cap that should be loose is loose, and that the vent isn't dripping. A vent that drips or sticks should be replaced. Isolate the manifold with its ball valves first, then fit a new auto air vent end unit.
3. Open any manual vent slowly with its key, with a cloth underneath. Close it when water comes out with no air.
4. Check the pressure and top up if it has dropped.
5. Run the heating for a while, then repeat if needed.

Air trapped in a loop itself won't always reach the vents. The fix is to purge that loop through the fill and drain valves, one loop at a time, with the others closed. This is the same method used at commissioning.

How do I check the flow meters?

With all the thermostats turned up, so every loop is open, look at each flow meter and compare it with the reading on the commissioning record. You are looking for change, not a perfect number.

• A loop reads zero: its actuator may be closed, its valve may be shut, or there may be air in it.
• A loop reads much lower than before: air, sludge or a partly stuck actuator.
• Can't read it: sight glasses can cloud up. A cloudy meter may be fine, so check the loop is warm. A replacement flow meter can be fitted with the manifold isolated.

Don't change balancing settings unless you know why. If the loops need re-balancing, see how a manifold works for a simple method.

How do I check the actuators?

Thermal actuators are the small heads on the manifold that open each loop when its room calls for heat. They are the most common part to fail.

1. Turn one room's thermostat up so it calls for heat.
2. Check the wiring centre shows that zone calling. Wiring centres such as the Heatmiser UH8 have indicator lights.
3. Wait. Thermal actuators typically take 2 to 5 minutes to open fully.
4. Check the actuator has opened. Many have an indicator showing open or closed. The loop pipe should start to warm.
5. Turn the thermostat down and check it closes again.

If one won't open, the valve pin underneath may be stuck. With the power off, unscrew the actuator and press the pin. It should spring back. If the pin moves freely but the actuator doesn't open, try a known good 2-wire 230V actuator in its place. Actuators and wiring centres run at mains voltage. Wiring faults are a job for a qualified electrician.

Why does inhibitor and water quality matter?

Heating water slowly corrodes steel and iron parts, such as the boiler heat exchanger, pump and any radiators. That corrosion makes black sludge, called magnetite, which can block narrow passages, flow meters and pipes. Inhibitor is a chemical added to the water to slow corrosion and scale.

BS 7593:2019, the code of practice for heating water treatment, says systems should be cleaned before inhibitor is added and that inhibitor levels should be checked through the system's life. In practice, that is usually done with a test kit at the annual service.

• Low-temperature UFH water can grow bacteria and fungus. Water treatment makers recommend a biocide for underfloor circuits.
• If the system contains aluminium, use an inhibitor that is suitable for it.
• A permanent magnetic filter on the return to the boiler catches sludge before it builds up.
• Top up inhibitor after any drain-down or big top-up.

Signs of poor water include black water from a vent, slow or cloudy flow meters, cold spots and a noisy boiler.

How do I flush or powerflush underfloor heating loops?

If loops are blocked with sludge, they need cleaning. Underfloor loops are long and narrow, so they are cleaned best one at a time.

1. Isolate the manifold from the boiler or heat pump, and isolate the pump set.
2. Close every loop except the one being cleaned.
3. Connect the flushing supply to the fill and drain valves on the manifold, so water goes through that loop only.
4. Flush until the water runs clear and free of air, using a cleaning chemical that is suitable for plastic UFH pipe.
5. Close that loop and move on to the next.
6. When all loops are done, refill, bleed, and add inhibitor and biocide to BS 7593.

Powerflushing pushes water through at high flow. Keep the pressure within the pipe and manifold ratings. Our auto-balancing manifold, for example, is rated to a maximum of 6 bar. If a loop still won't pass water after flushing, with its actuator off and its valve open, a heating engineer can pressure test it on its own before anyone thinks about lifting floors.

How should I look after the UFH pump?

The pump on the manifold pump set circulates water round the loops. Modern pumps such as the Grundfos UPM3 and Wilo A-rated pumps need no routine service, but it is worth checking:

• Running: the pump should run when any zone is calling and stop when none is.
• Noise: a hum with no flow can mean a seized pump. A rattle can mean air in the pump.
• Settings: keep a note of the speed or mode set at commissioning. Many pumps have status lights.
• After the summer: pumps that stand still for months can stick. Some controls and pumps exercise the pump during idle periods. Check yours restarts at the start of the heating season.

A seized pump can sometimes be freed using the pump maker's procedure. Replacement Grundfos UPM3 pump cables and Wilo pump cables are available. Pump wiring should be done by a qualified electrician. On a mixing pump pack, also check the blending valve setting. A valve turned too low gives lukewarm floors.

Does electric underfloor heating need maintenance?

No routine maintenance. There are no moving parts, and the cable is sealed in the floor. Keep the thermostat's times and settings right, and don't cover large areas of heated floor with thick rugs or flat-bottomed furniture.

If an RCD trips, or the floor stops heating, a qualified electrician can test the heating cable. Maker instructions typically ask for two tests:

• Resistance: measured with a meter and compared with the figure for that mat or cable, within the maker's tolerance.
• Insulation resistance: measured with an insulation tester, often at 500V DC, to check that no current is leaking to earth.

Comparing these with the readings taken when the floor was installed shows whether the cable is damaged. The floor probe can be tested too. A 10K probe reads about 10kΩ at 25°C, and more when colder. Keep the installation test readings with your paperwork. See how to install an electric underfloor heating mat for when they are taken.

When should I call a professional?

• Pressure keeps dropping, or there is water where there shouldn't be.
• One or more loops stay cold after bleeding and checking the actuators.
• Black water, sludge or a noisy boiler.
• Any electrical fault, tripping RCD or burning smell.
• Anything involving the gas boiler: use a Gas Safe registered engineer.
• The system has never been balanced or commissioned properly.

MaxHeatUFH can put you in touch with vetted, approved installers. Spares ordered by 12pm are delivered the next working day across the UK.

Products for this job

• Manifold Auto Air Vent End Unit
• Manifold Pressure Gauge End Unit
• Manifold Flow Meter
• 2-Wire Thermal Actuator 230V
• Manifold Ball Valve Set 1"
• Hydraulic Pressure Test Pump
• All manifolds and spares

Frequently asked questions

How often should underfloor heating be serviced?

Check the manifold, pressure and water treatment once a year, ideally alongside the boiler or heat pump service and before winter.

Do you need to bleed underfloor heating?

Sometimes. Automatic vents release most air on their own. Bleed manually if you hear gurgling or a loop goes cool.

Can underfloor heating pipes get blocked?

Yes, with sludge from corrosion elsewhere in the system, or with bacterial growth. Clean water with inhibitor and biocide prevents it. Blocked loops are flushed one at a time.

Should I turn underfloor heating off in summer?

You can. Check the pump and actuators work when you switch back on in autumn. See warm-up time for running patterns.

Can I powerflush underfloor heating?

Yes, loop by loop through the manifold, within the pipe and manifold pressure ratings and with a cleaner that suits plastic pipe.

Does electric underfloor heating wear out?

There are no moving parts to wear. Faults are usually from damage during installation or later drilling, which an electrician can diagnose with resistance tests.

Sources: Fernox: BS 7593:2019 summary, Which?: boiler pressure, Wunda manifold instructions, UFH manifold fault-finding guide, Grant UK UFH troubleshooting, ProWarm electric mat instructions, Heatmiser TA230 actuator data