“Gremlins,” imaginary mischievous, furtive creatures, were jokingly, maybe half-jokingly, blamed for tricky aircraft malfunctions among mechanics in World War II.

Gremlins pop up in other contexts, more mundane and with no life-or-death impact. A reviewer of a very highly rated old-school electronics repair shop explained that his vintage Technics turntable had an odd intermittent problem which couldn’t be fixed or even diagnosed, even by this very skillful old hand. My dad’s work on his car’s starter motor, turned out to have nothing, apparently, to do with the starter motor after all.

These things remind me of ghost stories. There’s a reason “gremlins” as a trope exists. It’s not just the impulse to blame something on an airplane crash that kills servicemembers. You can blame a faulty part, a bad mechanic, or weather, after all. It’s also the feeling that sometimes inanimate objects appear to behave in ways that cannot be explained by their physical properties alone. It’s why the idea of the haunted house exists. There’s a reason creaky old houses and hotels, and almost never new ones, are the focus of these stories. And the answer is detective work and canny knowledge. Working on a house is a lot like being a detective, or a doctor—specifically, a diagnostician. The more you look, the more you see.

All of this is to introduce the fact that, recently, I had a gremlin in my water pipes.

One of the bath/shower faucets in our house—an older revision of the classic Moen single-handle pull-out-the-knob fixture—never quite seemed to work. The shower always worked fine, though this faulty faucet is in the side bathroom, so we rarely use it for showers; only my parents really shower there, when they visit. I do, however, use that tub/shower to take a bath, because our main tub has those uncomfortable no-slip stickers on the bottom, and this one doesn’t.

So, the problem. Run the hot water to fill the tub, and it’s fine—too hot, too cool, just right. But I often undershoot on heat, and need to top the bathwater off with a little extra super-hot water. Maybe two thirds of the time, this faucet would only give me lukewarm-to-warm water, not truly hot water, no matter where I turned the knob. It went perfectly from freezing to cold to cool to lukewarm to warm, and then stopped, like when an old potentiometer-driven volume control drops out for part of the range.

Sometimes wiggling and jiggling the faucet would cause the hot water to suddenly start flowing again. It was usable enough, and I take baths rarely enough, that I just left it alone. Perhaps there was a certain, exact way to shimmy the faucet to make the hot water come on reliably—what I like to call getting good at doing things wrong—but I never found one.

Back in December, however, I had taken a bath and this problem occurred again, and this time no amount of playing with the faucet fixed it. (I wrote about that adventure last week—this is a sort of update.) The hot water was so stubbornly absent that it occurred to me that possibly there was an issue with the hot water heater this time—the kind of thought a homeowner has when expecting a cold winter. I asked my wife to try a couple of faucets in the house, expecting and hoping the hot water itself was fine. But—same problem. Lukewarm water and nothing warmer, from every faucet in the house.

This necessitated an adventure in the basement—shutting off the hot water heater’s circuit breaker, opening up the panels, turning the breaker back on, carefully testing the heating elements with a multimeter, playing with the thermostats—and finding nothing wrong. Except that one of the heating elements was 1500 watts over the maximum wattage the heater is designed for.

The more you look.

We concluded that one of the thermostats had likely gotten “stuck,” a thing that…I think can happen with old thermostats. And these are old—the heater is about 20 years old, past its expected life, and at the point where a lot of people would chuck it just to be on the safe side. I’m not like that. I think more along the lines of, this one is built different and might make it to 25 or 30! Is there a Guinness World Record for longest-lived hot water heater? I’m gunning for it.

A couple of weeks ago, I rethought that. I was taking a bath, went to top off the water, and got a frustrating mix of warm and cool water. Warm and cool at the same time, almost a weird feeling. Another house faucet—warm. Hot water was gone again.

Two hot water droughts in about a month, during winter, with a 20-year-old water heater. Time to give it the old heave ho, right? Some homeowners might go ahead and do that, or maybe call in an expensive plumber visit. But as I thought about it, I realized that the hot water never disappeared, except a few minutes after having filled a bath with that particular faucet. What were the chances that the hot water heater malfunctioned in exactly the same way, only after using one particular faucet? Pretty much zero.

“Can a faucet mess up hot water in a whole house reddit” I googled, along with a couple of other differently worded searches. And there you go. Several people in Reddit threads and home improvement forums reported that their whole house’s hot water supply was impaired, and that the problem ultimately traced to just one single-handle faucet somewhere in the house.

Not the faucet, per se, but the cartridge. Luckily, modern faucets almost all use cartridges: little removable and replaceable devices that basically “do” everything. What you think of as the “faucet” is just a shell and a lever that actuates the cartridge. And somehow, the cartridge in a single-handle faucet can fail in a way that allows cold water to mix into the whole house’s water supply. Even if the faucet is turned off. Even if it isn’t dripping. Even if it sort of works most of the time.

I had never heard of this, and neither had my dad, who does a lot of his own work on my parents’ house. But it was obviously a real phenomenon, and a new Moen single-handle faucet cartridge is a hell of a lot cheaper than a new hot water heater. $23 to be exact, at either of the big-box home improvement stores. (The generic part is about $18, but I always go for the real thing.)

The actual cartridge replacement is very simple. The only thing you have to remember to do is to turn off the water supply valve to the whole house. Luckily, it works, and luckily, we have one. In our old condo, the units didn’t have individual water shut-off valves; only the whole building did. So the building had certain days and times where the water was shut off so you could schedule your plumbing calls on those days. Yes, most plumbing-related devices in the condo had local shut-off valves, but those often freeze up, and if you bust one without turning off the building’s supply, well…

Because the cartridges can become stuck, Moen provides a little plastic piece that goes over the cartridge and gives you some leverage to loosen it up before pulling it out. It worked like a charm. New cartridge in, much smoother movement than the old one, wonderful to use, and perfect and precise temperature control from freezing cold to burning hot. (I did initially reverse the hot and cold, by putting the cartridge in upside-down. But that was an easy fix.)

So that was the gremlin—a faulty old faucet cartridge intermittently mixing cold water into the whole house. There may be some detail I missed along the way, in terms of what exactly caused it and whether there was a precise thing to do with the broken faucet to resolve it. That doesn’t really matter. It’s always a really cool feeling to resolve some mystery like this, to locate a ghostly problem in a little bit of plastic and metal.

As to the plumbing phenomenon at issue here, it’s called “plumbing crossover,” the “crossover” referring to the crossing over of cold water into hot water. It’s interesting how hard it is to distinguish “there’s no information about this thing” and “I’m not calling it by the right name.” Another one of those principles of home-maintenance detective work.

And it doesn’t have to be a bath/shower faucet, or even a faucet at all. Somehow—I don’t quite understand it technically—a dual-handle faucet is mechanically different enough that this isn’t possible. Here’s a plumbing company with some helpful information crossover:

And here’s a way to test it. I never did any definitive tests, I just concluded that the faucet was the culprit because of its behavior. But this is useful:

You just never know, do you?

Related Reading:

A Hard Day’s Wash, Part 2

Rural New Mexico Blues

A Repair Journey Through Low-Cost Manufacturing

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