Quick answer
Radiant floor heating that is not working has usually lost flow or water temperature in one of five places: a manifold actuator or zone valve stuck closed, air in a loop, a failed circulator, a mixing valve failed or set too low, or a floor-sensor thermostat fault. First rule out slow response: a concrete slab takes several hours, sometimes a full day, to warm from cold.
Before deciding your radiant floor heating is not working, give it time and then check the pipes instead of the floor. A hydronic radiant floor warms by pumping lukewarm water through plastic tubing, and the floor surface is the last thing to respond. The manifold, the panel where the tubing loops start and end, tells you in minutes what the floor takes hours to show.
Slow or broken? How to tell on a slab
A concrete slab stores a great deal of heat and takes a long time to change temperature. From a cold start in the fall, or after the system has been off, expect several hours before the surface feels warm and as much as a full day before the room settles. Thin systems, such as tubing in plates under a wood subfloor, answer faster. Turning the thermostat up 5 °C does not speed any of them up.
The test is at the manifold. With the thermostat calling for heat, wait 15 to 20 minutes and feel the supply and return lines for the zone. Supply warm and return a little cooler means water is flowing and heat is being left in the floor: the system is working and you are waiting on the slab. Supply warm and return stone cold after half an hour means no flow in that loop, and both cold means the heat source or the mixing valve is not delivering.
No flow: stuck actuator or zone valve, air in a loop, failed circulator
- Manifold actuator stuck. Each zone on a manifold has a small thermal actuator that opens its valve when the thermostat calls. It takes a few minutes to open, and most show an indicator on top that rises or changes colour. An actuator that stays down with the thermostat calling has failed or is not getting 24 volts.
- Zone valve failed. Systems without manifold actuators use a motorized zone valve in the mechanical room for each floor or area. A failed valve head leaves the valve shut.
- Air in a loop. A bubble in a long loop of tubing stops flow in that loop alone, so one room or one strip of floor stays cold while the rest is warm, and you may hear gurgling at the manifold. The cure is a purge: each loop is isolated and flushed with fill water, one at a time, until it runs free of air.
- Failed circulator. No hum and no flow in any loop, or a hum and a very hot pump body, points to the pump or its relay. How to tell a boiler circulator pump is bad lists the checks.
Flow but no warmth: mixing valve and floor-sensor thermostat
Radiant floors run on much cooler water than radiators. A slab is typically supplied at roughly 30 to 45 °C, well below what a conventional boiler makes, so a mixing valve blends return water into the supply to bring it down. When the thermostatic element in that valve fails, or someone has turned the dial down, the floor gets water barely warmer than the room and never catches up in cold weather.
Many radiant thermostats read a sensor buried in the floor as well as, or instead of, the room air. A broken or disconnected sensor wire puts the thermostat into a fault and it stops calling; most display a sensor error when this happens. A floor limit set too low does the same thing quietly. Thermostats over hardwood are usually given a maximum floor temperature to protect the wood, and if that limit is below what the room needs on a cold day, the heat stops at the limit and the room stays cool with nothing broken.
Check the boiler or water heater feeding the floor
The floor is only as good as its heat source. A dedicated boiler or combi boiler shows a fault code or a low-pressure reading when it has locked out, so confirm the gauge reads about 12 psi or more. Some basement slabs and heated garages are fed from the home's tank or tankless water heater through a heat exchanger and a small pump. On those, a water heater set low, a tripped unit or a scaled heat exchanger shows up as a cold floor, and if the taps are also running cool, start with the water heater.
Electric floor warming, the thin mats under bathroom tile, is a different system and a different trade. If its thermostat shows a ground-fault trip or the floor is dead, that is electrical work under the Electrical Safety Authority's rules, not a hydronic repair.
Check this yourself first, and when a radiant floor needs a technician
- The thermostat is in heat mode, set above room temperature, with fresh batteries if it uses them and no sensor error on the screen.
- The heat source is on and shows no fault code, and its pressure gauge is not at zero.
- Each actuator's indicator has risen after a few minutes of calling.
- No thick rug or underlay has been added. A heavy rug insulates the floor from the room and can make a working zone feel dead.
Book a radiant floor service visit when:
- One loop stays cold after a day and the others are warm: it needs purging, or an actuator or valve replaced.
- The supply to the manifold is cold with the boiler running, or far hotter than the mixing valve setting.
- The system pressure keeps dropping. Tubing leaks inside a slab are rare, and manifold fittings and the boiler's own parts are checked first.
- The house is unheated in freezing weather. Tubing at a garage edge or along an outside wall can freeze.
ZK Mechanical diagnoses radiant systems at the manifold with temperature and flow readings before recommending parts, and quotes the repair in writing; the diagnostic visit is $90–$150 and comes off the repair bill if you proceed. Start with radiant floor heating repair, or see radiant floor heating for new zones and replacements. Call (647) 801-1252 or tell us what the manifold is doing.