The high limit switch is one part of a safety loop circuit that must work before a heater can fire. It's purpose is to ensure tha tthe heater will shut off in the event of a thermostat failure or if the internal core gets too hot due to a lack of water flow. If the switch is failing, it needs replaced. There are 2 so you can replace both or make an attempt to determine which one is bad. It is best to replace the pair.
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The first thing you need is a multimeter, get one at Walmart you don't need a expensive one and the an be used for a lot of things around the house, pull wires off heater turn meter to ohms check between frame of dryer and each terminal it should show nothing if you are showing anything on the meter get a new heater
High limit switches are very cheap 1 time switches. They do not go out without a reason. When is the last time the heat exchanger was flushed? Has the burner tray been cleaned recently?
Is there enough water flowing through the heater? Is the heater and pump set on a timer?
If you have a sand or DE filter, do you backwash frequently enough?
All these things will affect the temperature of the outgoing water and the temperature of the high limit sensor(switch). I would say it is most likely flow related, but could be that the gas valve isnt tuned up too.
I know this is not a solution, but hopefully it will help you to figure out what is going on.
I think you are on the right track with a faulty high limit switch. When i serviced these units we always replaced the operating thermostat and the high limit switch together they came in a kit. The operating thermostat in theory should never let it get hot enough to trip the high limit switch, which is really just a safety backup for the operating thermostat. Get the kit and replace both, and also make sure your exhaust vent going outside has good flow at the outside vent,a restricted vent system will cause similar issues.. Good Luck
There are two thermostats, a 'limit' or 'operating' thermostat which turns the heating element on and off, and a 'high limit' or 'safety' thermostat which oversees the 'operating' thermostat by opening and removing power to the heating element when the upper limit temperature is reached. Basically, the only difference between these two are the temperature ratings. The 'high limit' is higher than the 'operating' thermostat. The thermal fuse is the last resort protection.
First and foremost, you must know that every inch of your exhaust vent pipe is clean.
Usually, a bimetallic limit switch will degenerate in the direction of too low of a temperature range because as the parts wear out inside, the mechanical tolerance widens (the tiny rod that pushes the contacts together gets shorter).
The high limit thermostat is by the thermal fuse. The operating thermostat is in the blower housing.
Check the fan impeller for broken vanes and lint. The impeller is directly coupled to the motor and should not turn without the motor. The fan housing and ducting should be free of excess lint.
Either heater is burned up, or high limit is bad. If you take dryer apart and get to heater you will be able to visibly see the broken heater wire. To check high limit switch disconnect the only two wires on the switch and using Ohm meter put one lead on each terminal. Should read closed if switch is good. High limit switch is located on panel covering heater coils If heater is not broken, and high limit switch is closed then you are losing power on your temperature control.
To Test the limit switch put the wires together (if just 2 terminal) and see if it works. If so replace the switch assuming it is not a manual reset. REPLACE the switch, Don't just leave it by passed. If it has 3 or more terminals you will have to find the wire diagram and decide which terminals should be closed and if any should be open, or if the switch has some kind of built in heater on the part.
If the same thing happens you know its not this part but something else and you'll have to get a volt meter to start eliminating whats working from whats not and when its working to get to the bottom of the problem and finding what it is.
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It may also be the high temp switches under the 2" inlet/outlet header. The 150 degree high-limit switch sits horizontal and corrosion happens often there. But be careful removing as if the header is corroded and the switch is also, water will flow out , so be ready to bypass the heater. You will have to replace the header. Try to jumpe that switch out and I'll bet the heater fires up.