Radiant Floor Heating Not Working? What to Check

The basics the Radiant Professionals Alliance checks first

Its list, before you call. Is there fuel for the system, gas or oil, and are all the fuel shutoff valves open at the tank and the boiler? Check the emergency switch and the service switch, as there may be more than one, and the breaker or fuse for the heating.

The thermostat. It should be set above the room temperature with heating, not cooling, selected. A night setback or “unoccupied” function should not be active, and the alliance notes it may affect your hot water too.

What changed. It asks whether the system was serviced recently or its control settings adjusted, and whether another heat source, such as an appliance, is warming the thermostat so it turns the heating off.

The boiler. If the boiler itself will not run, that is in a boiler not working.

The mixing control and warm weather

Cooler water, on purpose. Caleffi describes a radiant panel system supplied with lower temperature water through a mixing valve, whose controller may also set the supply temperature by outdoor reset. Weil-McLain says its WM-ODR reset control raises or lowers the boiler supply water temperature as the outdoor temperature drops or rises.

Off on a mild day. With its outdoor sensor connected, Taco’s Radiant Mixing Block turns off the boiler and its injection pump when it is warmer outdoors than its Warm Weather Shut Down setting. If its mixing target reading shows three dashes, Taco says it is either in that shutdown or there is no demand for heat.

Warmup’s sheet. For no heat in any zone, it checks that the controls are on, the heat source is supplying heat, the main isolation valves are open, the mixing unit’s bypass is closed and the circulator’s isolation valve is open.

One zone cold: its thermostat, actuator or valve

The thermostat and the actuator. Warmup’s sheet lists a faulty actuator: a technician checks that the zone’s thermostat is calling for heat and that signal voltage reaches the actuator, and if it does, replaces the actuator. It also lists crossed controls, where a thermostat controls the wrong circuit.

How an actuator works. Caleffi says a valve actuator on a manifold valve holds it closed until 24 volts warms a wax element inside, which lifts its shaft. The valve typically reaches fully open two to three minutes after the actuator is powered.

Valves. For a zone that does not warm, Warmup checks that the circuit’s isolator cap is open, and for one slow to warm, that the cap is fully open and the actuator fully opening. Where a whole manifold is one zone, Caleffi says it can have its own zone valve or zone circulator instead: a zone valve not working and a boiler circulator pump not working.

One loop cold: air, or the balancing

Air. Warmup’s sheet gives an air lock in the pipes as a cause of zones that do not become warm, cleared by filling and purging. Caleffi says air has to be removed as fully as possible, and loops on a manifold with valves are purged one at a time until the water runs out free of air.

Balancing. Warmup lists a manifold incorrectly balanced, both for zones that stay cold and for ones slow to warm. Caleffi says balancing can be done with the integral circuit valves built into some manifolds, and circuit flow meters show each loop’s flow, greatly speeding the job and helping with troubleshooting.

Who does it. Caleffi’s purge uses hoses on the manifold’s purging valves and a pump or a pressurized water source, so it is for the technician, along with the balancing.

Warm, but it never keeps up

Warmup’s causes. For a zone taking a long time to warm up, it lists a manifold incorrectly balanced, a flow temperature set too low, and high heat losses. It says some rooms and floor finishes lose more heat and take longer, which can be offset by setting those zones to run longer.

New rugs and carpet. The Radiant Professionals Alliance asks whether anything changed in the house, such as new floor coverings like rugs or carpeting, new windows or doors, or a room addition.

The water temperature. Weil-McLain’s typical design figure for radiant floors is 100 to 130 degrees, and its reset control lowers the water as it gets milder outside. The reset settings are for the installer, and the Maximum Supply setting on Taco’s Radiant Mixing Block can limit the water temperature where certain system components need protecting from high temperatures.

Slow to warm, or it overshoots

The slab. Caleffi says rooms with high thermal mass, such as a heated floor slab, warm significantly slower. Rooms with very different mass on one thermostat can respond very differently, which it says is likely to lead to complaints.

Setbacks. Caleffi says a high mass space that is well insulated and tightly sealed sees minimal savings from daily thermostat setbacks. In its example, a room with a 4-inch slab set back from 70 to 60 on a 0 degree night does not reach 60 even after 10 hours.

The thermostat’s timing. Google says radiant systems generally take much longer to heat up and can overshoot, because the pipes stay hot after the heating stops. Its True Radiant feature on some Nest thermostats learns this, turning the heat on early and off early.