Zurn Wilkins ZW-1070 Aqua-Gard thermostatic mixing valves lot, $5.

Whenever I have the chance to buy a quantity of plumbing stuff that looks expensive, like these thermostatic mixing valves, I get interested. Now... what the heck is a thermostatic mixing valve?

The water heater in an average residence is usually set at something like 115º F, which normally works perfectly fine except for the unusual cases where the tank runs out of hot water from lots of people taking showers in quick succession. If this is a chronic problem, though, the water heater temperature can always be turned up a little.

Now, let's consider the hot water requirements in places like hotels, hospitals, restaurants and other institutions that go through lots of hot water. In places like these, it would obviously be impractical to maintain a huge tank full of 125º F water to ensure that it never ran out. So, the solution is to keep a lesser amount of water stored at a much higher temperature, like 180º F to 190º F.

Of course, as you can imagine, routing this nearly-boiling water right out of the tap is simply inviting injury, so the trick is to run the high-temp water to the sink, and then mix the scalding-hot water with cold water right before it comes out the faucet, bringing the temperature of the hottest water you can get out of the tap down to a more manageable 115º F.

As you look at the mixing valve in the picture, you'll notice the two inlets beside the blue handle - one for cold water from 40º F to 75º F, and the other for hot water from 120º F to 195º F. The outlet is located opposite the blue knob, and outputs out this mixed, warm water to the tap. When the hot water tap is opened at the sink, the mixing valve automatically mixes hot and cold water to get the desired final temperature, which is adjusted via the blue valve to anywhere between 95º F and 115º F. Is that cool, or what?

One lesson to be learned here is how important it is to pay careful attention whenever you see larger numbers of brand new, identical items that are related to a specific interest, craft, hobby or industry. Even if the item isn't necessarily that expensive as a single unit, it can often be worthwhile to unload a bunch of them at a time.

But there's an even more valuable lesson here that I'd like to go over, and it bears very careful consideration, because it has to do with liability. Not for me, obviously, but for the guy who ends up installing them. The more likely that a particular item's failure may result in personal injury, the larger a disparity you're going to see between its suggest price and what it brings on eBay. Think about it. A plumber installs one of these valves and it fails and someone gets burned. Who pays? Well, if he got the valve from a Zurn Wilkins supplier, Zurn would likely be at fault. But if the plumber says he got the valve off eBay, where it's literally impossible to track where that valve had been, he will likely be liable to a degree, even if the valve was brand new.

These valves sold as a single lot on eBay for $89.

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