Underfloor Heating Not Working: Fault-Finding
Underfloor heating faults are usually one of a handful of things — an air lock, a stuck actuator, a control calling incorrectly, or a balancing problem. Work through it in order and you find it faster than by guesswork.
Wet underfloor heating hides its workings under the floor, which makes a fault feel harder to find than it is. In practice UFH problems fall into a short list: no heat at all, one cold zone among warm ones, slow warm-up, or heat that never reaches temperature. Each points to a different part of the system.
This guide is a logical fault-finding order for wet UFH: start at the controls and heat source, move to the manifold and actuators, then to the loops and balancing. Following it in sequence stops you stripping a manifold when the real problem was a thermostat that was never calling for heat.
Key takeaways
- Work in order: heat source and controls first, then manifold and actuators, then the loops.
- No heat anywhere usually points to the heat source, pump, or a control not calling — not the loops.
- One cold zone is typically a stuck actuator, a closed loop, or an air lock in that circuit.
- Air in a loop is common on a poorly commissioned system — venting and re-balancing at the manifold often cures it.
- Slow or weak warm-up can be a balancing problem or a flow temperature set too low at the blending valve.
No heat at all: heat source and controls
If nothing is warming, start upstream, not at the floor. Confirm the heat source is running and reaching temperature, the system pump and the UFH circulating pump are running, and the manifold is getting hot flow. A cold manifold flow bar means the problem is before the UFH, not in it.
Then check the controls are actually calling for heat: the room thermostats are set above room temperature, the wiring centre is powered, and it is switching the actuators and the pump/heat source demand. A UFH system that never gets a call to heat behaves exactly like a broken one.
Start upstream
Most 'UFH not working' calls turn out to be a heat source not firing or a control not calling — not the loops. Confirm hot flow is reaching the manifold before you touch anything downstream.
One cold zone among warm ones
If most zones heat but one stays cold, the fault is in that circuit. The usual suspects are a stuck or failed actuator on that loop, a loop isolated at the manifold flow or return valve, or an air lock trapping the flow. Lift the actuator and check the loop flows manually — many actuators have a manual override to test this.
A working actuator with the loop open but still no heat points to air or a blockage in that circuit. Vent the loop at the manifold and check its flow meter is showing flow. A single dead loop with no flow reading is either air-locked or physically obstructed.
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Air locks and flow problems
Air is the most common UFH fault, especially on a system that was filled but never properly vented and balanced. Trapped air stops the water circulating in a loop, so it reads no flow and the floor above stays cold. Vent each loop in turn at the manifold, running one loop at a time to push the air through, then re-check the flow meters.
While you're there, confirm the loops are balanced — each flow meter set to the design flow for that circuit. An unbalanced manifold sends most of the water through the short loops and starves the long ones, which shows up as some zones warm and others weak.
Weak or slow heat: flow temperature and balancing
If the whole floor heats but never really gets warm, look at the flow temperature and the balancing rather than a single component. UFH runs at a low flow temperature, but if the blending valve on the manifold is set too low the floor won't deliver enough output for the room's heat loss.
Equally, a floor that warms unevenly or slowly is often a balancing issue — the loops aren't set to their design flows, so heat is distributed unevenly. Set the blending valve to the design temperature and balance the loops at the flow meters before you conclude anything is faulty.
Recommission before you condemn
A lot of 'faulty' UFH is simply never commissioned — unbalanced loops, air not vented, blending valve wrong. Vent, balance and set the flow temperature, and record it on a commissioning certificate so the settings are captured.
From guidance to action
Frequently asked questions
My underfloor heating has no heat at all — where do I start?
Upstream, not at the floor. Confirm the heat source is running and hot flow is reaching the manifold, the pumps are running, and the thermostats are actually calling for heat and switching the wiring centre. Most 'no heat' faults are before the loops.
Why is one underfloor zone cold when the others are warm?
That circuit has a problem: a stuck or failed actuator, a loop closed at the manifold, or an air lock. Check the actuator (many have a manual override), confirm the loop is open, and vent it if the flow meter shows no flow.
How do I get air out of an underfloor heating loop?
Vent at the manifold, running one loop at a time so the water pushes the air through that circuit, then re-check each flow meter. Air is the most common UFH fault, especially on systems that were never properly commissioned.
The floor heats but never gets warm enough — why?
Usually the blending valve is set too low for the floor's output, or the loops aren't balanced to their design flows. Set the flow temperature to design and balance the loops at the flow meters before assuming a component has failed.
Related guides
Underfloor Heating Installation Guide
Wet vs electric UFH, screed and pipe spacing, commissioning and handover documentation.
Balancing a Central Heating System
How to balance radiators using lockshield valves and the temperature-drop method to get even heat across a system.
Radiator Cold at the Bottom: Causes and Fixes
Why a radiator goes cold at the bottom (sludge), how to confirm it, and how to flush, powerflush and protect the system.
