Testing electric underfloor heating during installation: resistance and insulation checks

An electric heating mat is cheap to replace before it is tiled over and very expensive afterwards. Testing as you go tells you straight away if the cable was damaged in the box, by a trowel, a dropped tile or a screw, while you can still lift it. It also gives you the readings the warranty and the electrician will ask for.

This guide explains the two tests, when to do them, what readings to expect, how to work out the right resistance for your mat, and what to record. It uses heating mat manufacturers' installation guides and BS 7671 guidance for floor heating. If a mat already in the floor has stopped working, see electric underfloor heating fault finding instead.

When should I test a heating mat?

After unpacking, after fixing it down and after the floor is finished, and keep checking during the job. Manufacturers' guides list exactly these stages. The point is to catch damage at the stage it happens.

1. Out of the box: before you lay it, to prove the mat is sound. If it fails now, return it unused.
2. After fixing down: once it is stuck or taped in place and the floor sensor is in, before any adhesive or levelling goes on.
3. During covering: while the levelling compound or tile adhesive goes down, so a damaged cable is found while it can still be reached.
4. After tiling or floor finish: before grouting is complete and before the electrician connects it.
5. At connection: the electrician tests again as part of the BS 7671 inspection and testing.

Never switch the mat on to "see if it works" before it is fully covered and the adhesive or compound has cured. Follow the curing time in the adhesive maker's instructions.

How do I test the resistance of a heating mat?

Set a multimeter to ohms and measure between the live and neutral conductors of the cold lead, with the mat disconnected from everything. Compare the reading with the resistance printed on the mat's label or in its manual. One manufacturer's installation guide says the ohmic resistance must be between -5% and +10% of the labelled value, and that a mat outside the range should be replaced. Use the tolerance given for your own mat.

If the label is missing, you can work out the expected value from the mat's wattage:

Resistance (Ω) = 230 x 230 ÷ watts

This matches manufacturers' tables. For example, one manufacturer lists its 50W timber floor mat at 1,058Ω, and 230 x 230 ÷ 50 is 1,058.

Mat size at 150W/m²	Mat wattage	Expected resistance	-5% to +10% band
1m²	150W	353Ω	335 to 388Ω
2m²	300W	176Ω	168 to 194Ω
4m²	600W	88Ω	84 to 97Ω
6m²	900W	59Ω	56 to 65Ω
10m²	1,500W	35Ω	34 to 39Ω

Larger mats have lower resistance, so use a meter that reads small values accurately, and zero it first by touching the probes together. Readings also rise a little with temperature, so test at room temperature.

What do the resistance readings mean?

A reading close to the label means the heating cable is whole. A very high reading, or OL on the display, means the cable is broken somewhere. A reading well below the label suggests a short between the conductors. Both mean the mat must not be connected.

Reading	Likely meaning	What to do
Within tolerance of label	Cable whole	Carry on and record it
OL or infinite	Open circuit: cable broken	Stop; find the damage before covering
Far below label	Short circuit	Stop; do not connect
Slightly outside tolerance	Meter or temperature, or damage	Retest with a known good meter

What is an insulation resistance test?

It checks that the heating conductors are fully insulated from the earth screen, using a high test voltage from an insulation resistance tester. A normal multimeter cannot do this test, so it is usually done by the electrician or an installer with the right tester. The reading is in megohms (MΩ). Manufacturers set a minimum that the mat must exceed:

• One manufacturer's guide for heating cables and mats: above 20MΩ after one minute at a minimum of 500V DC.
• One manufacturer's thin mat guide: at least 50MΩ, tested at 1000V DC or more for one minute.

Use the voltage and minimum in your own mat's instructions. Test between each heating conductor and the earth screen, with the mat disconnected from the thermostat, because the high test voltage can damage electronics. A low insulation reading means moisture or damage has reached the conductors, and the mat should be replaced before it is covered. In service, an insulation fault is what trips the 30mA RCD that BS 7671 requires on floor heating.

What should I record after testing?

Every reading at every stage, plus a photograph and sketch of where the mat and sensor lie. BS 7671 guidance says concealed heating needs full labelling and records, covering the equipment fitted, the maker, where it is laid, the power and the cold-state resistance of the elements. Manufacturers also ask for readings on the warranty card at installation and connection.

Stage	Resistance (Ω)	Insulation (MΩ)	Date and initials
Label value		n/a
Out of the box
After fixing down
After tiling
At connection

Keep the record with the house documents and give a copy to the electrician. If there is ever a fault, the photographs and readings save a lot of time.

Should I test the floor sensor too?

Yes, check it is in place and undamaged before the floor goes down, and lay it in a conduit so it can be swapped later. Manufacturers' guides put the sensor in an insulating conduit, sealed at the floor end, between two runs of heating cable. If the sensor fails later, a thermostat such as the Heatmiser neoStat-e shows an E1 error, and a sensor in conduit can be pulled out and replaced without lifting tiles. See our guide to floor probe installation.

When the thermostat is first powered up, check it reads a sensible floor temperature. Our neoStat troubleshooting guide explains what to do if it does not.

Who should do the final tests and connection?

A qualified electrician. You or your tiler can do resistance checks while the floor goes down, but the connection to the supply, the RCD protection and the final inspection and testing must be done by a qualified electrician to BS 7671. In England, a new circuit, or adding to a circuit around a bath or shower, is notifiable under Part P, and a registered electrician can self-certify the work and issue a certificate. Our guide to whether you can install underfloor heating yourself covers what is and is not DIY.

For the full fitting process, see how to install an electric underfloor heating mat.

Products for this job

• MaxHeatUFH Electric Underfloor Heating Mat 150W/m²
• Heatmiser Temperature Sensor Probe
• All electric underfloor heating

Frequently asked questions

Should I test an electric underfloor heating mat before tiling?

Yes. Test it out of the box, after fixing it down and again after tiling. A fault found before tiling costs a mat; a fault found after costs a floor.

What ohm reading should a heating mat have?

The value on its label, within the maker's tolerance. If there is no label, 230 x 230 ÷ the mat's wattage gives the expected value, so a 600W mat should read about 88Ω.

Can I test a heating mat with a multimeter?

You can test resistance with a multimeter. The insulation resistance test needs an insulation resistance tester, usually used by an electrician.

What insulation resistance should a heating mat have?

It depends on the maker. Manufacturer guides ask for above 20MΩ at 500V DC, or at least 50MΩ at 1000V DC for thin mats. Check your mat's manual.

What does OL mean on my meter?

Open circuit. The heating cable is broken somewhere, and the mat must not be covered or connected until the cause is found.

Can a damaged heating mat be repaired?

Sometimes, but a repair under a tiled floor means finding the break and lifting tiles, which is why finding damage before tiling matters so much. Our fault finding guide covers repairs.

Do I need to keep the test readings?

Yes. BS 7671 guidance asks for records of concealed heating, including cold resistance, and makers often need readings on the warranty card.

Sources: Building: BS 7671 and underfloor heating, ELECSA: BS 7671 Section 753 floor heating, Heatmiser: neoStat E0, E1 or E2 error codes, Planning Portal: electrics and Part P