Warm Water from Cold Taps: Fugitive Household Goes Plumb(ing) Crazy

Earlier this month, I noticed something odd around the Fiction Fugitive homestead. The cold water was starting to run warmer.

No wonder they call him "super".

No wonder they call him “super”.

One day, after using the shower, we noticed the cold taps were running lukewarm water from every faucet–a problem I would come to learn is called “hot-cold crossover”. This wasn’t some simple, obvious leak, either–this one would take some detective work.

Time to call a plumber, some would say. Or time to buy a new water heater, for the hell of it, then call a plumber. I bet some of you reading this are already getting a knot in your stomachs, recalling past traumatic experiences:

“Oh great, a mysterious plumbing issue. There goes a thousand bucks!”

One of the pillars of my strategy for financial independence is lowering your living expenses. And one of the main tactics for lowering living expenses is to become a Renaissance Man–that is, to cultivate a great variety of skills that allow you to meet your wants and needs with little or no money.

If you own a home, plumbing is just one of the many skill-sets you’ll need to tackle. Thus, although my past DIY experience is limited to unscrewing light fixtures and changing air filters, I endeavored to conquer this lukewarm water on my own, rather than forking out the hundreds of dollars to a plumber.

In this post I’ll share with you my harrowing quest and pass on some DIY wisdom.

Spoiler alert: I fixed it for $70, a few days of research, and a couple hours of labor.

Hot-Cold Crossover


Diagram of a simple (non-recirculating) plumbing layout. You may have more fixtures but the principle is the same.

First step: research. Google is your friend. Turns out the technical plumbing term for my problem is “hot-cold water crossover”. It is what it sounds like–hot water crossing over into cold lines (or vice versa). I knew the symptoms, but what was the cause?

The handy diagram to the left, courtesy of yours truly, shows what a typical home plumbing installation looks like.

The hot and cold pipes are, for the most part, closed systems. The cold water line branches off to deliver cold water to the fixtures before it even reaches the water heater. How, then, can hot water leech into the cold lines?

In most plumbing layouts, there are only three places where the separate lines meet and could potentially mix hot water into cold water lines (or vice versa):

1. At the recirculating manifold, if you have a hot water recirculating system. From what I understand, these are fancy plumbing connections that mix a little hot water into cold water lines at certain places in order to bring warm water to a fixture faster. You can tell you have one if you have more pipes than just an intake/outtake pipe coming out of your heater, or if you have any bizarre looking hookups (manifolds) under any sinks. I don’t have one, so I didn’t investigate this further.


In a non-recirculating system, the only possible points where the lines meet are at the heater and at each fixture.

2. At the hot water heater. Without proper heat traps, hot water can rise back into the cold water line through the process of convection.

3. At any single-handle or mixer faucet fixtures. Think about it: when you move a single-handle lever to the middle to get lukewarm water, you are essentially creating a local cross-over–mixing the hot and cold at the faucet. This action is conducted by a special mixer cartridge inside the faucet, which can allow water to cross over continuously if it fails.

Since I don’t have a recirculating system, that narrows down the source of the warm water in cold lines to either convection at the water heater or crossover at a failed faucet cartridge.

The Heat Trap

Convective heat transfer, often called simply convection, is the physical process by which heat transfers from one place to another by the movement of fluids. When an increase in temperature produces a reduction in density, this causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity.


Hot water can rise by natural convection up the intake pipe on the heater, mixing hot and cold water in the cold water line.

What this means for us, in layman’s terms, is that heated water has a tendency to rise upward and displace colder water. For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid which falls.

Most hot water heaters are equipped with “heat traps” between the intake/outtake pipes and the heater itself. These are essentially dielectric nipple or ball valve inserts that prevent convective heat from rising while allowing cold water to pass.

These can fail, or may never have been installed correctly, allowing hot water to “climb up” the cold water line via convection, resulting in lukewarm water in the cold line. This process can be especially bad if the cold water line has a long vertical rise by which the hot water can climb.

The simple test? Put your hand on the cold water intake pipe! Is it cold, or is it hot? It’s probably normal for it to be a little warm nearest the heater, but if the pipe is warm even a foot or so up, you might be a victim of natural convection.

Convection can force hot water UP, but it can’t push it past a vertical drop DOWN.

Convection can force hot water UP, but it can’t push it past a vertical drop DOWN.

You can replace the dielectric heat traps. These are cheap, about $10. If, however, the first couple feet of the intake and outtake pipes on your water heater are made of sweated copper–like mine–this would require desoldering the copper pipe, replacing or installing the heat traps, and resoldering the pipes.

If the cold water intake line is comprised of PVC or similar plastic pipe, either directly from the heater or above the initial copper pipe, you can also make an “improvised” heat trap by incorporating a 6-12″ vertical drop in the line. Recall that the problem is convection–hot water’s tendency to rise upward. It doesn’t like to go downward because it lacks density. Therefore, if we introduce a loop or drop in the piping, cold water will continue to flow as intended due to water pressure, but the hot water rising by convection will be trapped because there’s no path to rise.

This is the solution I implemented. After purchasing a tube cutter, PVC pipe, elbow joints, solvent, and primer (the most expensive part was the tube cutter, a useful tool; the rest was about $10), I shut off the water at the main intake valve and opened all the faucets to allow them to drain.

The Fugitive Heat Trap

The Fugitive Heat Trap (Click for larger view)

Then, I cut away the piece of pipe I would be replacing and allowed the remaining water in the pipes to drain and dry (have a bucket and towels ready). Then I built my own improvised heat trap “loop”. Working with PVC is fun and easy; even a plumbing newbie like me figured it out with a little help from Google.

When the new plumbing dried and I opened the main valve again, I immediately noticed a difference. The hot water was hotter, the cold water was cooler. I could tell the loop was doing its job–the top part of the loop, where the heat is trapped, was warm to the touch, while the lower section of the loop, after the vertical drop, was noticeably cooler. Success!

…Or so I thought.

After running for long enough, the cold water turned lukewarm again!

The Mixer Cartridges

This bad boy.

This bad boy.

With backflow of hot water from the heater ruled out, I knew the only place left where water could cross-over was at the fixtures themselves. By systematically closing the main valves at each fixture–under the sink on faucets, behind the faceplate on showers–I determined the most likely culprit was the Moen single-handle faucet in my kitchen.

Moen single-handle faucets are also notorious for causing the hot-cold crossover issue. If you have a Moen, learn from my mistake–make this Suspect Number One right from the start!


The Moen 1225 Cartridge

Fortunately, this is an incredibly easy fix (way easier than improvising your own heat trap). Simply identify the brand of sink you have and find out via the manufacturer’s website or elsewhere online what type of cartridge it takes. (Note: The correct cartridge type for my sink, the Moen Renzo, is 1225. This was misidentified on the Moen website when I looked, so double check!)

There are countless video tutorials for replacing these cartridges online. If you can’t find the right one, I’d call the manufacturer or just see if you can puzzle it out from similar models. All I needed was the correctly sized Allen wrench/hex key to open the faucet, a socket wrench, and a screwdriver.

Replacing the cartridge had the added benefit of making the faucet handle move and operate much more smoothly.

Most importantly, it fixed the problem!

No More Crossover!

And I’m that much closer to the ideal of the Renaissance Man.

The heat loop cost about $35 to build, the cartridge fix about the same. In both cases, a majority of the expense was tools (tube cutter and set of Allen wrenches) that will be reusable for future projects. On the other hand, plumbers routinely charge $80-100 an hour, and by all accounts on the internet this is a rare problem that even a skilled plumber might have had difficulty solving if he hadn’t seen it before. Who knows how much money I saved?

As I write this, I’m sipping delicious ice cold water fresh from my cold water tap–but what tastes even better is the victory. Not only did I save money, I increased my skill set and sense of agency. I’m now extremely familiar with the plumbing system in my house, know how to maintain and repair every fixture, know how to repair and replace any section of plumbing, and will be totally confident handling whatever plumbing issue comes at me next. A simple leak will seem like a piece of cake after this!

And now that this plumbing shit is done, I can get back to flying my dinosaur! Fuck yeah!

And now that this plumbing shit is done, I can get back to flying my dinosaur! Fuck yeah!

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2 Responses to Warm Water from Cold Taps: Fugitive Household Goes Plumb(ing) Crazy

  1. chris says:

    I have single shower handles with no cartridge only a steel ball that rotates over the hot and cold inlets. A couple of questions.
    First of all These are new faucets. On this bathroom line there is a double sink with cold and hot valves and a tub with cold and hot valves. Two shower single handled faucets. The back flow, or mixing, is only in this area. The other plumed parts of the house works as they should. How does this leaking take place, and why, with the single shower faucets in off position, with no dripping.? Can you also have crossover with individual hot and cold valves? I can see an old cartridge and rubber seals. This is head scratching at it’s best.

    • AEHodge says:

      Hi Chris. I can only really speak to the specifics of my problem and my solution for that problem, and even then I hasten to add (again) that I’m just a cheapskate DIYer, not an actual plumber, so YMMV. That said–since this is new construction, is it possible the intake valves were switched? That is, the hot water line was connected to the cold water valve, or vice versa? The hot water pipe should usually be on the left side in North America. I understand that switching these can cause crossover issues. I think the only way for crossover to occur separately (meaning, without mixing at hot and cold valves) would be for hot water from the heater to climb up the cold water intake pipe as discussed in the post, but if it’s a localized issue, I doubt that could be it.

      Not sure what you mean by “I can see an old cartridge and rubber seals,” but if you can see these components are old, go ahead and replace them and see what happens. Even if it doesn’t fix your current problem, it’s cheap maintenance and prevents problems down the line. I have some of the non-cartridge, rotating ball-type mixer valves in my bathroom sinks. These also wear down, particularly the springs and rubber seals. When mine failed, it caused the faucet to drip out the back when operated, but I imagine, like any problem at the mixer valve, it could also cause crossover issues. If you know the make/model, you can find a replacement pack of the springs/seals for under $10 at a big box store.

      Hope this helps. I know how frustrating this crap can be! Hang in there, and if you have to call a plumber, shadow him and learn how to deal with it next time… cuz there’s always a next time. 😉