The error code 20 means the High Limit Switch has been triggered. This error code generally requires a hard reset (unplugging or disconnecting power). The high limit switch is usually found on the copper pipe leaving the heat exchanger in a Pre-Mix style unit.
The purpose of the high limit switch is to trip if water temperature in or leaving the heat exchanger approaches a dangerous range.
Residential units will generally have lower trip temperatures while commercial units will have higher trip temperatures. Once the high limit switch is triggered combustion stops immediately and the unit will close the water control servo to prevent super heated water from leaving the unit and entering the plumbing.
The most common cause of an error code 20 (and most other overheat related codes) is scale build up in the heat exchanger. Scale build up reduces the heat transfer into the water passing through the heat exchanger. Reduced heat transfer into the water means other parts retain the heat and eventually overheat. If you repeatedly get error codes 20 and just reset the unit without investigating and resolving the issue, you’re likely to end up with an error code 14 which is not resettable and requires a much more involved repair process.
Additional causes could be related to poor combustion, a loose wiring connection and of course a failed high limit switch. If there is a bad wiring connection of the high limit switch to the main wire harness or a failed high limit switch, usually the error code will not reset even if the unit is cold. If the high limit switch and wiring is good, you can usually unplug the unit and let it cool down for awhile and when you plug it back in the code will be reset and the unit will attempt to operate normally. If the root cause of the overheat still exists, the code will eventually return.
In general, inspect the overall condition and cleanliness of the unit. If the unit is installed in an area with hard water and there is no softener, water treatment or records of descale flushes being performed it would be a good place to start by conducting a full service. Start by performing a descale flush and if the unit has signs of dust, dirt or lint build up also perform a combustion cleaning.
Descale procedures can be found in the installation/owners manual, on our Support Site and on our YouTube Page along with a general maintenance video.
For further technical assistance call 866-766-7489 or use our Contractor Locator to find a local service representative.