The first thing to do is write the logic of what (fan, light, anything else) should be on under different conditions.

For example, if the fan is to be on ONLY when the shower is running, then put a flow sensor (simple paddle device in a plumbing fitting) into the water path between shower controls and shower head. Have this wired in series with the fan. (This plumbing change may not be trivial depending on construction.)

However, if a more complex requirement exists, such as ventilating the bathroom of odor or exhausting residual humidity while drying oneself, then more complexity is needed.

Here's a picture of the shower controls. Would a flow sensor be easy to install?

What's the access like in the wall behind the control valve and where the feed to the shower head (gold strip) comes out? This is really a question for a plumber.

The wall where the control valve is mounted is next to the master bedroom closet. Because the original installer did a bum job (we could hear water leaking out in the wall), plumber #2 cut a 6" x 6" square hole in the drywall to access the shower from the closet side. We have not had that cut patched (just have the drywall square inserted in it) because it is in our closet and didn't care (can't see it because of clothing.). It was an expensive repair as I remember.

Would the plumber do the wiring or would you have to have a plumber for the sensor and an electrican for the wiring?

Just replace the motion sensor with a wall timer (https://www.homedepot.com/p/326493813).

As I wrote: Decide on the logic first, choose the components to accomplish it second. Note that @wdccruise's solution is widely used in hotels, except as a rotary control in my experience.

In case the flow switch is relevant to whatever the logic is, it seems that easy access to the plumbing exists.

The answer to plumber + electrician vs. some other mix of agents is not one I can predict, even if the residence location were provided, as it relates to local code enforcement. I would do it myself (I think I have a flow switch remaining from another project and do all my electrical with permits as required). Or, I would have the plumber insert the switch and modify the wiring myself (assuming it is reasonably accessible). There often exist contractors who have grandfathered plumbing and electrician status who could do both even in a rigid permit environment.

I had a similar problem and just replaced the fan switch with a timer switch, mine goes up to 60 mnutes. problem solved.

Timers that need to be set will not work with a son that will not turn on a fan. Big sigh.

" With the motion-sensing turned off in the shower, the fan and light in the shower now go off after a certain time..."

"Timers that need to be set will not work with a son that will not turn on a fan."

Then he'll be doing all his bathroom business in the dark because the light and fan will be on the same timer.

Electrician is coming tommorow morning. Thank you all for suggestions and advice.

Thanks for sharing your bathroom exhaust fan saga! Dealing with moisture and mold issues is no joke, especially in a small space like your bathroom. It sounds like you've had quite the adventure finding the right solution to keep things dry and mold-free.

Radiant Spy, you are very kind to say what you have said. Thank you for being so thoughtful!

I figured you can't be the first / only person with this problem -

Try this - a motion sensing exhaust fan, variable speed with a timer.

https://www.deltabreez.com/SMT130M.php

Thank you just_Janni for the suggestion, but I already paid to have a Broan motion sensing one with a timer installed. The Delta Breeze would fit the hole, but I can't return the Broan at this point.

I think part of the problem is that the electrical company that I hired is sending out some green kids that don't really know what they are doing.

I'm having the one here right now just pull the switch detector out because he did not think he could wire the switch motion detector and the fan motion detector to both operate the fan. He's struggling just reversing what the first kid did and on the phone with others for advice. Whatever.

I imagine that the water sensor idea was the right way to go, but I'm already a roughly $400 fan and 4+ hours the first day and an 1.5 hours so far today with electricians working on replacing an exhaust fan. Should have hired the industrial electricians that my husband uses for his office. The residential outfit is struggling. They were recommended by the HVAC people that we use. Hmmm. Live and learn.

https://www.broan-nutone.com/en-us/product/ventilationfans/zb80ml1

First, the ZB80ML1 should NOT be wired to a wall motion sensor switch. This model has a motion sensor built in.

If there is only one circuit for both the fan and light, the switch (referred to as the "Master Switch" in the installation instructions) for the circuit should be left ON. Control of the fan and light are then managed exclusively by the fan's motion sensor, its fan's user-adjustable controls, and the fan's light-timed-off control all of which are built into the fan.

Installation Guide: https://www.broan-nutone.com/getmedia/eda5f70a-85ff-4f8d-851e-f3e38303868c/Installation_Guide_ZB80ML1_ZB110ML1.pdf

You are way over-complicating this. Ditch the motion sensor and install a condensensation-sensing switch (Dewstop). It runs for the amount of time that you pre-select after sensing condensation. You can also turn it on manually whevever you like, and have it run for that same amount of pre-selected time. No need to keep a light switch on for no reason. No need to run the fan for no reason.

The motion sensor is built into the fan. You assume that the electrical box that would house a condensation-sensing switch is in the proper location inside the bathroom; it may not be. You also assume that there are separate circuits for the light and fan which we don't know. The OP doesn't need to buy more stuff; he has what he just needs to wire it properly...and simply.

Agreed- it appears the OP has a simple solution that needs a simple bit of wiring, as the fan controls all the timing, auto shutoff, motion sensing etc

The fan that I am replacing was a moisture sensing unit, and it was the moisture sensor that went bad after only 5 years .

Anyway, young electrician that was here today spent a lot of time on the phone with the young electrican who was initially here and then called his boss, who actually drove over to help. I had them pull out the light swtch motion sensor and just use the fan motion sensor. The light switch motion sensor was their idea, and they are supposedly not going to charge me for it. He was here for around 2.5 to 3 hours today.

The one here today went to school with my son, and both of them were very sweet and tried. I'll see how this gets billed to me. It shouldn't take 7 hours to replace an exhaust fan in a bathroom.

Thank you all for your advice and suggestions.

"I had them pull out the light switch motion sensor and just use the fan motion sensor."

If by this you mean that they removed the wall motion sensor -- substituting a simple on/off wall switch -- thereby causing the ZB80ML1's fan and light to be controlled exclusively by the motion sensor and timers built into the device, this was the correct solution.

Thank you wdccruise, that is exactly what the second electrician did. Perhaps the first electrician didn't know how to wire the fan properly, and that is why he recommended using the wall switch motion sensor.

Anyway, life is good now. Thank you!

He did read the spec sheet--big sigh. I gave both of them the instruction booklet.

Alls well that ends well.