Simpson Duet dryer, maybe 25 years old. The dryer works, but only on low heat. On the high heat setting, there is no heat. There is only one element, so what do you think might be the problem?

If the dryer has dual or triple heating elements, one element, or the relay controlling one element, could be open circuit, ( failed). mostly likely is a failed heater element. The elements work together or separately to combine the heat settings. there could be a high and low element, that do the different ranges. it could be that when it is on medium or high, it's really a lower combination of heat, and it may not be easy to observe, but on the low settings, the chosen element is completely open and failed and provides not heat at all. if medium and high are really low and medium, and low and medium-low is really nothing, then 1 element is probably open circuit. ( failed).

Don't have your model so I couldn't bring up the schematic some will run with an open thermal fuse I suspect thermal fuse which is non resettable I've only run across a bad thermistor once based on the age check the exhaust exiting the house and at the dryer this a safety fuse designed to open due to excessive heat if the dryer cannot breath restricted exhaust.

Take a look at some of these videos to help you diagnosis the no heat problem.
http://www.youtube.com/results?search_query=duet+dryer+no+heat&oq=duet+dryer&gs_l=youtube.1.0.0l4j0i5l6.2271.5049.0.7601.10.10.0.0.0.0.100.624.9j1.10.0...0.0...1ac.1.11.youtube.EGor8jRswXo

I would suspect the heating element. I bet you are asking why this clown is saying that when you already told him that on high heat it worked fine. SIMPLE! Your heating element is made up of five coils. For low heat your dryer uses two heating elements and for high all five. So if one or two of the heating elements are open and they happen to be the low temp dry setting ones, that would explain it. That model doesn't have the dryness sensor on it (according to my documentation) so the heating element is the most likely cause.
Hope this helps!
ImJustAvg aka Hal

Hi,
If you have a gas dryer.
The most common problem with a gas and no heat is the ignitor going bad...

Here is a tip that I wrote about that problem...
Gas Dryer not Working Gas Dryer Not Heating
http://www.fixya.com/support/r4398592-gas_dryer_not_working_gas_dryer_not

If your dryer is electric..
There could be a number of different problems...
Check out this tip that I wrote about this problem...

Dryer Troubleshooting Electric Dryer won t Heat
http://www.fixya.com/support/r3866345-dryer_troubleshooting_electric_dryer_won

heatman101

If its only 4 years old you probably have a bad thermal protector, Take the bottom front cover off and check with a meter to see if any are open. They cost about 35$. Otherwise look for a local guy to do the job for 150$

try heat element

FIX IT!!!!

I believe it is your theremostat, maybe the heating element,
I would use a multimeter if you have one and set it on ohms touch it to the terminals of the thermostat, and the heating element (if you don't have a multimeter walmart sells a cheap one that will work fine for what you are doing) if you do not get a reading replace, http://www.appliancepartsworldwide.com/Store_Part.aspx?Id=493992
Here is the list for all your parts
http://www.appliancepartsworldwide.com/Store_SearchResults.aspx?S=392096&ST=PBM

it could be several things but the element could should be the last part i would check first if you are still using fuses change them both if that doesnt work call for service call a good service man stay away from major companies they are expensive i know i worked for most of them

check the vent outside it seems to be getting too hot and cycling the element off and on your safety therm.