Sounds like you’re talking about a tub with single mixing valve handle. The type I was referring to (and Emmanuel too I suspect) are the tubs with separate hot and cold. Those don’t have any adjustments.
Thank you and I agree, Matt and Manny… 
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Hey Larry,
They are typically used for stand alone tubs that do not have single handle sets but two handles mounted typically on the tub. Other handle sets that combine hot and cold will have the adjustment for mixing hot and cold. Sometimes even on new construction I’ll run across a wall mounted handle set that was actually intended as a tub/shower handle set but obviously has no shower head above.
Almost all my work now is new construction and I see quite a bit of interesting stuff being used. As for that set-up in Stuart’s post they could have placed that valve in a master closet wall with an access panel or under the tub if it had an access panel for it. Of course that might require extra plumbing in the wall and we know how that goes.
I see approximately 50% under sinks and of those many that have crappy installs blocking access for storage and exposing the piping and connections for potential damage. Even if they are back against a wall I report that the client needs to use caution when storing items underneath.
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Thanks Manny…I was a little slow on my uptake and response to your other post but get what you are referring to now.
Single mixing valve handles for tubs/showers has been the only type used in my area on new construction for several years. No separate hot and cold handle valves. That eliminates the temperature regulating valve, and the mess the OP has at the sink base cabinet.
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That’s why I think it may be a commode, Christopher. 
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Another thing that needs to be watched for are these pedestal style, single handle faucet sets. They are getting popular here with the free standing tubs. Many don’t have mixing valves and some are real high dollar. Encountered one over $2K and no temp valve for it anywhere.
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Gotcha, thanks…
Looks like a “Watts Automatic Valve Control” was installed in the bathroom piping system. If it was, then these automatic valves greatly improve supply and distribution system efficiency and offer precise control of pressure, flow, liquid level, temperature, etc. Inspecting them for proper functionality is beyond the scope of the inspection.
Not two hot pipes the three pipes are a hot and a cold in & the mixed (lower temperature) water out.
To keep the drain pipe from getting knocked around by stuff. 
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I’ve occasionally seen the same valves installed at water heaters for an entire house. Years ago at a conference I heard about a potential code change requiring this but apparently it never got any traction.
The problem is the temperature that water heaters hold water at (+/- 120F) is prime breeding ground for bacteria. From my food service days I always remember the “critical zone” of food storage was 40-140F. Mainly, they were talking about Legionnaire’s Disease and there are a few documented cases of it being traced back to water heaters. So, the idea with those valves was to hold water in the heater at 140 and blend in some cold to bring it back to 120 as it goes out for use.
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I have one on my water heater, but for a different reason. We are on an off-peak electrical program, part of which is only running the water heater at night. The water is heated to well above the desired tap temperature and then mixed down at the water heater. This gives us more daytime capacity out of our 105 gallon Marathon water heater. Even when we had five of us in the house, we never ran out of hot water.
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In my area, I see quite a number of Apollo Hydroheat water heaters(now state water heaters) and Aquatherm air handlers on older small single family homes and smaller condos. This knock off hydronic heating system uses a water heater for the hot water at plumbing fixtures and for heating. The hot water from the water heater gets delivered to the air/water heat exchanger in the air handler and a blower fan blows over the copper tubing of the heat exchanger and voila you get “hot” air in the winter.
Only problem is that if you set the water temperature to 120F at the water heater, it looses heat quickly on its way to the heat exchanger and the warm air that comes off of the heat exchanger cools significantly before it actually reaches the rooms. To make this heating system work, the temperature at the water heater is often set at 140F and a thermostatic mixing valve is installed right above the water heater. This system delivers mildly hot air from 80 to 90 degrees F out of the supply registers and is a lot cheaper than a full blown boiler system. I think I’ve seen maybe 1 or 2 gas fired boiler systems total in my area. It just doesn’t get that cold here midway up the east coast.
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There’s a big set of condos in my area built around 2000 that have systems like that. Heated water from the water heater is routed to an air-handler in the attic. I’ve often called it a “poor man’s boiler”. I suppose it works but I have to think the water heater gets a lot of use and will probably fail more quickly. But, of course, water heaters are cheaper than full boilers. Aside from these condos (Hillsboro - Orenco Station, NE Cornell Road for anyone from Oregon stumbling across this) I very rarely see systems like this.