An F20 fault code on a Vaillant Ecotec means the boiler has to be reset as the water temperature inside the boiler has gone too high.
What causes a Vaillant F20 fault
F20 faults are produced when (in Vaillants words):
“Maximum temperature at the flow/return temperature sensor is too high when the safety cut-out function is operating using the NTC”
In layman’s terms:
One or more temperature sensors have told the control board that the water inside the boiler is too hot. To remain safe the boiler has switched itself off.
F20 fault caused by failing temperature sensor
The simplest version of this fault is a faulty temperature sensor. The sensors are designed to send a reading to the control board to report on the temperature in the boiler.
This is just a resistance reading that changes depending on the temperature. A low resistance means a high temperature and a high resistance means a low temperature. The value of resistance changes as the temperature goes up and down.
If the sensor becomes inaccurate it can trick the boiler into overheating or not getting hot enough.
The sensors can’t be repaired and will have to be changed by a gas safe engineer.
The other (much more rare) sensor fault is that they aren’t connected to the pipe properly. It’s very unusual and normally happens only after work has been done. They can be refitted at a slight angle so they don’t detect the temperature properly.
Sticking pump caused F20 fault code on a Vaillant Ecotec
The pump on the boiler transfers the heat away from the boiler as the gas is burnt.
The boiler has no idea if the pump is running. If the pump sticks the gas continues to burn causing the boiler to overheat.
This is normally accompanied by a banging boiling sound as the water boils inside the heat exchanger.
The solution is to either replace the pump or get the pump turning again.
F20 fault requires replacing the pump
The new high efficiency pump can be bought online. Check the part number against the type of boiler you have before ordering anything.
The older style pumps aren’t available from Vaillant but are still available online.
Both require a qualified engineer as they are inside the case which should not be removed.
Releasing a stuck pump due to a Vaillant F20 fault
The older style silver pumps can be released by taking the silver cap off the center and turning it with a small screwdriver.
If the pressure is still on the boiler, a lot of water will spray. Be sure to drain it before taking the cap off.
The new style square pumps have a small hole in the front that a thin pozi 1 screwdriver can fit in. Push in to engage the impeller then spin it around without any water leaking out.
F20 fault due to air in the boiler
Too much air trapped inside the boiler can cause an F20 fault code on a Vaillant Ecotec
Water is used to transfer the heat from the burning gas to the rest of the house and to other parts of the boiler. If there are too many big bubbles of air in the boiler this heat transfer can’t happen. This makes the boiler overheat.
F20 fault caused by an auto air vent.
The boiler normally vents air from the boiler through an automatic air vent. This opens to allow air out but closes to stop any water from leaking out when the air is gone.
The auto air vent (called a de-aerator by Vaillant) is an air vent with a float attached.
When there is air present the float drops opening the valve and allowing the air to escape.
When the air is gone the valve is closed by the float floating on the water. This allows the air to escape but doesn’t allow the water to leak out.
Auto air vents can stick shut or become blocked (or be closed instead of replacing them if they leak). This stops any trapped excess air from escaping.
If the air can’t escape the boiler can’t transfer the heat fast enough which causes a F20 fault.
Auto air vents are easy to change once the boiler is drained. However, as you must open up the boiler case it should only be done by a qualified heating engineer.
F20 fault cured by clearing the air from the boiler
Another way of clearing air from the boiler is by sending it to the radiators. Turn on the heating then run it with the boiler thermostat set as low as possible.
The pump will run but the boiler will not burn much gas so it shouldn’t overheat.
The second (more complicated) but much more effective way to clear the air is the use of the bleeding test program. This is specifically designed to remove trapped air in the boiler.
Summary of why you have an F20 fault
The F20 fault is due to an overheating situation. Unless you can clear the air or blockage yourself you will most likely need a qualified heating engineer to replace or repair the faulty parts.