BoilerFaultCodesYour boiler has received a signal from your underfloor heating system to stop running, likely because a safety thermostat has triggered to prevent the floor from getting too hot.
This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.
Technical description: Underfloorheatingcontactopen
What causes this fault?
This fault occurs when the safety sensor designed to protect your floors detects that the water temperature has become too high. To prevent damage to your flooring, the sensor sends a signal to the boiler to shut down the heating immediately. The most common reasons are a faulty mixing valve, a failed underfloor pump, or a sensor that has become too sensitive over time.
This code is most frequently seen during the first cold snap of autumn when underfloor heating systems are turned on for the first time after being idle all summer.
DIY fix possible
Check if your underfloor heating thermostat is set too high.Ensure the underfloor heating pump is running correctly.Wait for the floor temperature to cool down, as the boiler may restart automatically once the limit thermostat resets.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like S.39 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.