Underfloor heating not warming up: manifold, pump, actuator, air or thermostat? How to find the cause

Short answer: if everything stays cold, look at the pump, the mixing valve or the control signal from the boiler or thermostat. If one loop stays cold, the problem is in that loop: a thermal actuator that does not open, a flow meter that has been closed, or air or dirt in the loop. Underfloor heating responds slowly: allow two to four hours before you notice a change in the floor, and judge by the temperature of the pipes at the manifold, not by your feet. You can do most of the investigation yourself at the manifold; only flushing and opening up the water side of the system are jobs for an installer.

First: what does the manifold tell you?

Put your hand on the flow pipe to the manifold (usually the top bar) and on the return (bottom bar). Then feel each loop separately, at the flow meters or the pipe connections.

Step 1: is heat arriving?

Is the room thermostat set higher than the room temperature and is the boiler firing? If not, start at the boiler: water pressure (1.5 to 2 bar when cold, top up otherwise), fault code on the display (look it up) and whether the thermostat is controlling the boiler (how to check this). If the boiler is firing but the flow to the manifold stays cold, there is often a zone valve or diverter valve in between that does not open, or the manifold is at the end of a long pipe run with air in it.

Step 2: is the pump on the manifold running?

A manifold with a mixing valve has its own pump. It is supplied with 230 V, usually via a pump switch or the zone control, which only lets the pump run when a loop is open. Feel whether the pump is vibrating or listen for a hum; a pump that gets warm but does not turn is seized. Check the plug and the fuse, and whether the pump switch (€69) is enabling the pump. On older models you can free a seized pump with a screwdriver via the bleed screw; a pump that seizes again afterwards is due for replacement. Pumps by brand are listed under pumps.

Step 3: is the mixing valve set correctly?

The mixing valve determines the temperature of the water going to the floor. If it is on the lowest setting, or the thermostatic head has fallen off, only return water circulates and the floor stays lukewarm. Set it to 40 to 45 °C for a sand-cement screed, a maximum of 35 to 40 °C with wood or laminate, and check the thermometer on the flow bar after two hours. If it stays low while the valve is open, the flow from the boiler is too cold (step 1) or the valve is stuck due to limescale.

Step 4: does the thermal actuator open?

With zone control, each loop has a thermal actuator that opens the valve when the room thermostat calls for heat. An actuator that does not open gives a cold loop while the rest is warm. Remove the actuator from the valve (bayonet or screw ring) and look: if the valve pin is up, the valve is open and the loop works; if you press the pin in and it does not come back, the valve is stuck. Use a voltage tester to check whether there is 230 V (or 24 V on a 24 V system) at the actuator when the thermostat calls for heat. No voltage: thermostat, wiring or control unit. Voltage present but the actuator stays closed: replace the actuator. The Honeywell MT-4 230 V (€21.95) and MT-4 24 V are the common types; check the voltage and the thread (M30 × 1.5) of your manifold.

Without an actuator, a loop is always open. If you want to test whether the controls are to blame, remove the actuator and wait two hours: if the loop now gets warm, the control is the cause.

Step 5: air in the loop

Air collects in the highest loop and blocks the flow; you can tell from a flow meter that stays at zero while the loop is open. Bleed via the automatic air vent on the manifold (loosen the cap one turn) and, if that does not help, loop by loop: close all other loops, set the pump to its highest setting and let the air escape through a hose on the drain cock of the return bar until only water comes out. Then top up the system to 1.5 to 2 bar. A manifold that keeps getting air in it points to a leak or an expansion vessel that has lost its charge, see Water pressure keeps dropping.

Step 6: dirt and flow meters

Check that the flow meters (the small glass gauges with a red ring) are open and show roughly the same reading: 1 to 2.5 litres per minute per loop is normal. A loop that barely flows even when fully open is clogged with sludge or magnetite, or was kinked during installation. Flushing with a flushing pump, possibly with a cleaning agent, is work for an installer; a magnetic filter in the return prevents it from happening again. Brownish-black water when draining confirms the diagnosis.

What to leave to a professional

If you do replace a part yourself, check the type on the manifold: a 24 V actuator on 230 V will fail immediately. Which manifold suits which installation is explained in Which underfloor heating manifold do I need?; the complete approach in Installing underfloor heating 2026. Parts and manifolds are listed in the underfloor heating category.

Frequently asked questions

How long does it take for underfloor heating to warm up?

Two to four hours with a sand-cement screed, less with a thin screed or dry system. After a night switched off it can take up to six hours. So judge how it is working at the manifold, not by the floor.

My floor gets lukewarm but never really warm. Is that normal?

Often, yes: properly working underfloor heating has a surface temperature of 24 to 28 °C, which feels lukewarm. If the room is still too cold, the flow temperature is too low or the pipe spacing is too wide for the room's heat loss.

The pump is running but the flow meters are at zero.

Then there is air in the manifold or all the thermal actuators are closed. Bleed first; then remove one actuator to see whether that loop starts to flow.

Can I simply leave out the thermal actuators?

Yes. All zones will then always be open and you control the heating with the room thermostat only. You lose room-by-room control, but a faulty actuator can never again cause a cold room.