Underfloor Heating Fault Finding


In the event that your underfloor heating (UFH) system is not working correctly, here are some points to check to help find the cause of the problem:

  • Check that the air is completely out from the system. Do not fill from the boiler; fill the UFH system as described in the instructions. Many times the plumber might still inject air in the system afterwards when connecting to the boiler, so even if the UFH installer has flushed the system, you might need to do it again.
  • The most common problem with UFH systems is air in the loops!
  • Check that the pump is running on speed 3 (max). Also installed right way.
  • Check that the mixing valve is not set at minimum. Normal setting is 45 degrees C for concrete floors and 55 degrees C for timber suspended floors. The indication mark is below the mixer cap. Turn anticlockwise for higher water temperature and clockwise for lower temperature. If rooms are a bit cold, then increase the water temperature to the UFH system.
  • Check that flow and return to the mixing valve is correctly fitted from the boiler. Flow is closer to the wall on the mixing valve, i.e. the one marked ‘H’.
  • Check that correct room thermostat is doing the correct loops, i.e. the correct actuators!!! This is quite a common problem. One room will always be cold as the wrong room thermostat is doing the wrong actuator. A simple electrical test will clarify the problem.
  • Many times the plumber that fits the boiler/radiators does not flush through the system and rubbish will get into the boiler pump, mixing valve and the underfloor heating pump. Take out the mixing valve and clean it with hot water, flush the system before fitting the mixing valve back again. This is quite a common problem. The flow gauges on the manifold will show very low flow rate, which means that not enough flow is getting into the UFH system as the mixing valve is full of rubbish.

  • If one room is a bit cold, then you can adjust some of the other loops by reducing the flow to them. Less flow to the loops means less heat to them. This could happen if the longest loop is struggling to heat up, then by reducing the flow for the shorter loops, you will have more water going into the longer loop. See manual for adjusting the loops. Also you can increase the water temperature on the mixing valve. Note, if you have radiators in your house it is very important that you adjust them, else they might take all the water from the UFH system.

  • Remove the red clips on the actuators. Sometimes the electricians forget to remove them. This means the actuators are always open. Also note the actuators take 4 minutes to open when a room thermostat is calling. It is important that the actuators are screwed tightly to the return manifold. When the room thermostats call for heat, the UFH pump will start and the actuators will start to open.
  • All room thermostats should be set at 20 – 21 degree C. This is normal UK design temperature for inside climate.
  • The system should be on 24 hours a day during the heating season, use the set-back to lower temperatures at night, which normally means that the heating system does not come on at night, but is very quick to respond in the morning.
  • Max heat output for any UFH system is 100 W/sqm for concrete floor and 70 W/sqm for timber suspended floor. The property needs to be built to current building regulations for any UFH system to work correctly.
  • In a Conservatory you might loose more heat than 100 W/sqm, so the UFH system will extend the time you can use the Conservatory for, not keep it at 20 degrees C all year.
  • If you have a combination boiler check that it will work with a secondary pump before fitting the UFH system.
  • The correct way to fit an UFH system is with an S-Plan system, i.e. a two port valve for each system so you can have independent control if you are mixing UFH and radiators.
  • It will take 24 h for the UFH system to operate correctly when started for the first time. Same will apply if you do any changes on the controls.
  • If you have a large one zone system without actuators, always check that the blue caps are fully open.
  • When the system is running check that water is running through the flow meters on the flow manifold. If there is no water coming through the flow meters, then you have air in the system.

  • If the pump is not running, check that the electrician has wired it in correctly (see our manual). This is a common problem. L and N does not go into the relay, see our manual.
  • The boiler relay normally gives power to a two port valve and the volt free contacts on the two port valve will start the boiler, i.e. like and S-Plan system. If no two port valves are fitted then use the boiler relay as a Volt free contact.

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