What is inside a floor sensor

A typical floor sensor consists of a small NTC thermistor bead — the most common stock values are 10 kΩ, 12 kΩ, 15 kΩ or 100 kΩ at 25 °C, with B-values from 3380 K to 3960 K depending on the thermostat ecosystem — soldered to a two-conductor cable. The bead and the solder joints are encapsulated: in the cheapest builds with a single layer of adhesive heat-shrink tubing, in better builds with a moulded plastic body, and in the best builds with a fully over-moulded thermoset body that fuses to the cable jacket.

The cable itself runs 3-5 m to the thermostat. The bead end is installed inside a flexible conduit set into the screed, so the sensor can in theory be pulled out for replacement if it fails — in practice the conduit fills with screed dust during the pour and the sensor is permanent.

The heat-shrink failure mode

A heat-shrink tube relies on a hot-melt adhesive layer between the tube and the cable jacket. When the tube is shrunk down with a heat gun the adhesive flows and bonds to the cable. The bond is mechanically strong on day one but has two long-term weaknesses:

  1. Thermal cycling — in a Class B underfloor system the sensor is hit with 50 daily cycles between 18 °C and 35 °C. The differential expansion between the heat-shrink polyolefin and the PVC or silicone cable jacket creates a small reciprocating shear stress at the boundary. Over months to years this opens a microscopic gap.
  2. Moisture diffusion — cured screed releases moisture into the conduit for years after installation. The adhesive in a typical heat-shrink is hygroscopic; once water gets to it, the bond degrades from the inside out.

The end result, observed in field returns over the years following installation, is that resistance readings start drifting. The thermostat compensates as long as it can, then shows an error code or starts overshooting the setpoint by several degrees. The homeowner’s call to the installer is for replacement of the whole heating zone, because the sensor cannot be replaced without lifting the floor.

What over-moulding actually does

An over-moulded sensor uses a single thermoset polymer cure that bonds the encapsulation to the cable jacket without an intermediate adhesive layer. The most common materials are polyurethane (PU) and PVC for low-cost builds, and silicone or thermoset epoxy for premium builds.

Two properties matter for in-slab reliability:

  • No joint — the encapsulation and the cable jacket are the same polymer family (or have been chemically bonded during cure), so there is no boundary for moisture to find.
  • IP68 by construction — a properly over-moulded sensor is rated IP68 not as a test result but as a structural property. The same part can be submerged in water continuously for the service life.

The cost increment over heat-shrink is real — mostly driven by the moulding tool and cycle time rather than the resin cost. The pay-back is that the failure mode that dominates field returns goes away. The industry trajectory has been a steady shift from heat-shrink to over-moulded construction for new premium-grade thermostat SKUs, with heat-shrink retained mainly in cost-driven builds.

Why the over-mould pays back: the failure mode that dominates in-slab field returns — moisture ingress at the heat-shrink-to-cable joint — is structurally eliminated by a monolithic over-moulded body. A correctly over-moulded sensor can be expected to outlive the thermostat it is connected to.

Other failure modes (much less common)

  • Crush damage during installation — the contractor stepping on the sensor while pouring screed. Modern oval-profile over-moulded sensors are designed to resist this.
  • Lightning / mains surge — uncommon but real for sensors run in the same conduit as the heating cable. A small TVS diode at the thermostat end fixes it for pennies.
  • NTC drift — long-term resistance drift in the thermistor itself is below 1 % per year for modern NTC ceramics. Translated to temperature error this is < 0.3 °C drift over the life of the installation. Not the dominant failure mode by a long way.
  • Cable rodent damage — mice eat PVC cable jackets if the conduit is open at the thermostat end. Specify silicone jacket or fully sealed conduit to avoid it.

What thermostat compatibility means

Each thermostat brand fixes the NTC parameters its firmware expects. The wrong R25 or B-value will not make the sensor “not work” — it will read a temperature, just the wrong one. The major R25 values seen in the field are 10 kΩ, 12 kΩ, 15 kΩ and 100 kΩ, with B-values from 3380 K to 3960 K. PT1000 (linear, no B-value) is also widely used in European commercial systems.

When supplying a replacement sensor — or designing a new thermostat — always confirm the exact R25 and B-value documented for the specific thermostat model, not just the “NTC 10K” label. A 10 kΩ / 3 380 K NTC reads several degrees different from a 10 kΩ / 3 950 K NTC at typical floor-warmth temperatures — enough to make a previously comfortable floor feel too hot or too cold. Send us the thermostat model number and we will confirm the matching part from our stock.

What Jianlu offers

Our IM-series over-moulded floor sensors are built specifically for the heat-shrink failure mode described above. Available in 10 kΩ, 12 kΩ, 15 kΩ, 100 kΩ NTC and PT1000, B-values selectable to match any thermostat ecosystem, with PVC, silicone or Teflon cable jackets. Private-label branding and custom cable length supported for thermostat OEMs. Drop-in replacements available for any heat-shrink sensor we receive a sample of.