A leak in a hydronic heating system is different from a plumbing leak in one important respect: most of the pipework is buried, so you rarely see the water. What you see instead is system pressure falling, and by the time a damp patch appears on the floor the leak has usually been running for some time.
This guide covers how to recognise a leak in an underfloor heating or hydronic system, how to narrow down where it is before anyone starts lifting flooring, what repair options exist, and the specification and commissioning decisions that prevent most leaks from happening at all.
The Signs
Hydronic leaks announce themselves in a specific order, and recognising the early signs saves considerable damage.
Falling system pressure is the primary indicator and usually the first. A sealed heating system holds its pressure. If you find yourself topping up repeatedly, water is leaving the circuit somewhere.
A zone that behaves oddly, warming unevenly or not reaching temperature while others are fine, can indicate a loop losing water or drawing air in through the same defect.
A persistent damp patch on the floor, particularly one that reappears after drying and is warm to the touch, points to a buried leak in that area.
Unexplained increases in water consumption where the system is topped up automatically rather than manually, since the loss is being replaced without anyone noticing.
Damp or musty smell from a floor void or adjacent room, or visible mould developing where it did not before.
Recurring air in the system, requiring venting far more often than a settled system should. A small leak can draw air in as well as let water out, particularly on the suction side of the pump.
Do not simply keep topping up. A system that needs pressure added every few weeks has a defect, and repeatedly refilling it makes matters worse rather than better. Fresh water introduces dissolved oxygen every time, which accelerates internal corrosion of pumps, valves, and heat exchangers. So a slow leak left unaddressed causes damage in two places at once: at the leak itself, and throughout the system from the water used to compensate for it.
Narrowing Down the Location
Before any specialist equipment or floor lifting, a systematic process eliminates most of the possibilities. Work through it in order.
1. Check the accessible components first
The majority of hydronic leaks are not in buried pipe at all. They are at connections, and connections are visible.
Inspect the manifold thoroughly: branch unions, the flow meters, the pump and mixing group, air vents, and the fill and drain points. Also check the heat source connections, the expansion vessel, and any valves in the plant area.
Dry everything, then return after the system has run for a while and look again. A weep that is invisible when the surface is already damp becomes obvious against dry brass.
2. Rule out the expansion vessel
A frequently missed cause of apparent pressure loss. A failed expansion vessel with a ruptured diaphragm or lost air charge cannot absorb thermal expansion, so pressure rises during heating and is relieved through the safety valve. The result looks exactly like a leak: pressure keeps falling and needs topping up.
Checking the vessel before searching for a leak has saved a great many unnecessary investigations.
3. Isolate the loops at the manifold
This is the step that identifies which circuit is affected without lifting anything, and it is worth doing before calling anyone.
Close every loop at the manifold, pressurise the system, and observe. If pressure now holds, the leak is in one of the loops rather than in the plant. Open one loop at a time and watch which one causes pressure to start dropping again.
That isolates the problem to a single circuit, which typically serves one room or one area, and it reduces the search from an entire building to a defined space.
4. Then bring in detection equipment
With the affected loop identified, specialist methods locate the leak within it.
Detection Methods for Buried Pipework
| Method | How it works | Best for |
|---|---|---|
| Thermal imaging | Detects temperature anomalies at the surface | Warm water escaping under a floor |
| Acoustic listening | Detects the sound of escaping pressurised water | Pressurised leaks, quiet conditions |
| Tracer gas | Hydrogen or helium mix introduced, detected at surface | Very small leaks, unpressurised systems |
| Moisture meter | Measures moisture content in the floor structure | Confirming and mapping a damp area |
| Endoscopic camera | Visual inspection through a small access hole | Voids, ducts, behind panels |
Thermal imaging is particularly effective on underfloor heating because the escaping water is warm. Run the system, then scan the floor: the leak shows as a heat signature that does not follow the pipe layout. Its limitation is that it detects surface temperature, so a leak beneath a thick insulating covering may not register clearly.
Tracer gas is the most sensitive method available. The system is drained and charged with a safe hydrogen and nitrogen mixture, which escapes through even a very small defect and rises through the floor structure to be detected at the surface. It works where thermal imaging cannot, and it locates leaks precisely.
The combination of thermal imaging to identify the area and tracer gas to pinpoint the defect is what avoids exploratory excavation.
Detection costs less than guessing. Engaging someone with thermal imaging and tracer gas equipment costs money, but the alternative is lifting floor covering across an area until the leak appears. On a tiled or timber floor, exploratory work frequently costs more than the detection would have, and it damages a surface that then needs replacing whether or not the leak was found there.
Repair Options
What is practical depends on where the leak is.
At the manifold or in accessible pipework
Straightforward. A weeping union may need reseating or a new olive. A failed component such as a flow meter, air vent, or valve is replaced. Take care not to overtighten brass fittings, since a cracked fitting creates a worse leak than the one being fixed.
In buried pipework: local repair
Once located, the floor is opened at that point only, the damaged section cut out, and a repair coupling fitted.
The consideration here is that a joint has now been introduced beneath the floor, and joints are the most likely place for future leaks. Use a coupling rated for burial in screed, record its position, and be aware that this is a compromise rather than a return to the original condition.
In buried pipework: re-route
Rather than joining the damaged loop, the circuit is abandoned and a new one run from the manifold, sometimes through a different route or above the floor in a discreet position.
More disruptive initially, but it restores an unjointed run from manifold to manifold, which is what the original design intended.
When the leak is corrosion rather than damage
This distinction determines whether a repair will hold.
A leak caused by a nail, screw, or impact during construction is a single defect at a known point, and repairing it resolves the problem.
A leak caused by internal corrosion means the condition affecting that point affects the whole circuit. Repairing one pinhole in a system that is corroding internally buys time rather than fixing anything, and further leaks should be expected.
Establishing which you have matters before deciding how much to invest in the repair.
Why Corrosion Happens, and the Oxygen Barrier
This is the failure mode most owners have never heard of, and it is entirely preventable at specification.
Plastic pipe is permeable to oxygen. Without a barrier layer, oxygen diffuses continuously through the pipe wall into the circulating water. That oxygen then attacks every metal component it reaches: pump bodies, valve internals, the heat exchanger, and steel components anywhere in the system.
The insidious part is that the pipe itself is unaffected. The plastic is fine. What corrodes is everything else, and the damage appears years later in expensive components far from its cause, so the connection is rarely made.
Floor heating pipe with an oxygen barrier to DIN 4726 prevents this. The barrier is a co-extruded layer, typically EVOH, that limits oxygen permeation to a specified maximum.
This is one of the specifications that cannot be corrected later. Once the screed is poured, the pipe in it is the pipe you have for the life of the floor.
Preventing Leaks: What Actually Works
Most underfloor heating leaks trace back to decisions made during installation, which means most are preventable.
No joints beneath the floor
The single most important principle. Each loop should run continuously from the manifold, around its circuit, and back to the manifold, with no joint anywhere in between.
Joints are where leaks begin, and a joint buried in screed cannot be inspected, maintained, or accessed without breaking the floor. Loop lengths should be planned so that a single coil covers each circuit rather than requiring a join.
Pressure test before the screed goes down
The most valuable half hour in the whole installation, and the step most often shortened when a project runs late.
The system is pressurised and held while the screed is poured, so any damage during pouring shows immediately as a pressure drop, while the pipe is still exposed and repair costs almost nothing. A defect found at this stage is trivial. The same defect found after the screed has cured and the floor covering is laid is a major exercise.
Keep the pressure test record. Many warranties require it, and it establishes that the system was sound at that point.
Protect the pipe during construction
A high proportion of buried leaks are caused during the build rather than by any pipe failure: a nail through a loop, a screw into a circuit near a wall, an impact from equipment before the screed is poured.
Recording the pipe layout before covering it, with photographs and a marked drawing, prevents later trades from drilling into a circuit and gives anyone diagnosing a future leak a map to work from.
Specify components properly
Oxygen barrier pipe to DIN 4726. Manifold and valve materials suited to the water chemistry. Pressure ratings with margin above operating conditions. Our guide to PEX pipe versus copper pipe covers the material considerations.
Control system pressure
Excessive pressure stresses every joint and component continuously. Where incoming mains pressure is high or variable, a pressure limiting valve addresses the cause rather than the symptoms.
Monitor pressure as routine
Check the system pressure gauge periodically and note the reading. A slow decline detected early is a small repair; the same leak found after months of running has usually caused damage to the floor structure as well.
Leaks in Other Parts of the System
At the heat source
Inspect around the boiler or heat pump for drips, spray marks, or corrosion staining. Check the pressure relief valve discharge: a valve that discharges frequently may be responding to a system pressure problem rather than a fault of its own.
Radiator circuits
Valve spindles and unions are the usual points. Thermostatic radiator valves and lockshield valves both develop weeps as seals age, and both are visible on inspection.
Air handling and condensate
Not a hydronic leak but frequently mistaken for one. A blocked condensate drain on an air conditioning or heat pump unit causes water to back up and overflow, producing damp that looks like a pipework problem. Check the condensate route before assuming a leak.
“The step people skip is the pressure test before the screed goes in, and it is the difference between a five minute repair and a five thousand euro one. You pressurise the loops and leave them pressurised while the concrete is poured. If someone puts a trowel through a pipe, the gauge tells you immediately and the pipe is right there in front of you. Skip it, and the same damage surfaces six months later as a damp patch, by which point you are lifting a finished floor to find it. Every installer knows this and it still gets left out when a job runs behind schedule.”
— Maggie Shen, Director of Legom
Components That Reduce Leak Risk
Leak prevention begins with what goes into the floor, because those components cannot be changed afterwards.
Legom manufactures floor heating pipe with oxygen barrier protection to DIN 4726, manifolds with flow meters on each branch, thermal actuators, room thermostats, and the HVAC valves that control the circuit, all at our facility in Jiaxing, Zhejiang Province.
Because the components are produced together, connection standards and pressure ratings are matched by design rather than assumed, which removes one category of joint failure from the installation. We supply partners in more than 90 countries, with OEM and ODM services available across the range.
For leak detection in refrigerant and air systems rather than hydronic circuits, see our guide to HVAC leak detection methods.
Frequently Asked Questions
How do I know if my underfloor heating is leaking?
Falling system pressure is the primary sign, and a sealed system should hold its pressure without regular topping up. Other indicators include a persistent damp patch on the floor that is warm to the touch, one zone behaving oddly while others are fine, recurring air requiring frequent venting, unexplained water consumption where the system tops up automatically, and a damp or musty smell from a floor void. Falling pressure with no visible leak at the manifold points to buried pipework.
Can I find the leak myself before calling someone?
You can narrow it down considerably. Start by inspecting the manifold and plant area, since most hydronic leaks are at accessible connections rather than in buried pipe. Dry everything and check again after the system has run. Rule out a failed expansion vessel, which produces pressure loss that looks exactly like a leak. Then close every loop at the manifold and pressurise: if pressure holds, the leak is in a loop, and opening one at a time identifies which.
How is a leak in buried pipework located?
Thermal imaging detects the heat signature of warm water escaping under the floor, and it works well on underfloor heating specifically because the water is warm. Tracer gas is the most sensitive method: the system is drained and charged with a safe hydrogen and nitrogen mixture that escapes through even a small defect and is detected at the surface. Acoustic listening and moisture meters support both. Combining thermal imaging to find the area with tracer gas to pinpoint it avoids exploratory excavation.
Should I keep topping up the pressure?
Not as an ongoing practice. A system needing pressure every few weeks has a defect, and repeatedly refilling makes matters worse. Every top-up introduces fresh water carrying dissolved oxygen, which accelerates internal corrosion of pumps, valves, and heat exchangers. So an unaddressed slow leak causes damage in two places: at the leak itself, and throughout the system from the water used to compensate. Find the cause rather than managing the symptom.
Can a leak in underfloor heating be repaired without replacing the floor?
Usually yes, once it has been located precisely. The floor is opened at that point only, the damaged section cut out, and a repair coupling fitted. The consideration is that a joint now exists beneath the floor, and joints are the most likely site of future leaks, so use a coupling rated for burial and record its position. An alternative is abandoning the damaged loop and running a new circuit from the manifold, which is more disruptive initially but restores an unjointed run.
Why do underfloor heating pipes leak?
Most buried leaks are caused during construction rather than by pipe failure: a nail or screw through a loop, or an impact before the screed was poured. The second cause is joints beneath the floor, which is why each loop should run continuously from the manifold with no join in between. A third is internal corrosion, which occurs where pipe without an oxygen barrier allows oxygen into the water. That last one affects the whole circuit rather than one point, so repairing a single pinhole buys time rather than solving it.
What is an oxygen barrier and why does it matter?
A co-extruded layer, typically EVOH, that prevents oxygen diffusing through the pipe wall into the circulating water. Without it, oxygen enters continuously and corrodes every metal component it reaches: pump bodies, valve internals, and heat exchangers. The insidious part is that the pipe itself is unaffected, so the damage appears years later in expensive components far from its cause and the connection is rarely made. Specify pipe with an oxygen barrier to DIN 4726, since it cannot be corrected once the screed is poured.
How can leaks be prevented during installation?
Four measures cover most of it. Run each loop continuously from the manifold with no joint beneath the floor. Pressure test before the screed is poured and hold pressure while it goes down, so any damage during pouring shows immediately while the pipe is still accessible. Record the pipe layout with photographs and a marked drawing so later trades do not drill into a circuit. And specify oxygen barrier pipe, since internal corrosion is the failure mode that cannot be fixed afterwards.
Reviewed by Maggie Shen, Director at Legom, on September 13, 2026. This guide to underfloor heating and hydronic system leaks was reviewed for technical accuracy, including the loop isolation diagnostic, oxygen barrier requirements, and pre-screed pressure testing procedure.