For more than four decades, This Old House plumbing and heating expert Richard Trethewey has been splitting plumbing components in two to reveal their hidden inner workings. Armed with band saws, rotary tools, and hacksaws, he has dissected at least 60 different items, from garbage disposers and water heaters to dozens of valves and pipes. His self-proclaimed role as “the great bisector” has helped millions of homeowners understand how their plumbing systems truly function. These cutaways are not just props for television; they are powerful teaching tools that transform abstract concepts into visible, tangible mechanisms anyone can grasp. Each slice reveals the engineering inside everyday fixtures that most people never think about, making complex home systems accessible to everyone.
The value of these cutaways extends far beyond mere curiosity. When you understand how a pipe fitting or a plumbing valve works internally, you can better diagnose problems, communicate with service professionals, and make informed decisions about repairs and replacements. Richard’s collection of bisected parts has even made appearances on The Tonight Show, proving that there is genuine public appetite for understanding what happens behind the walls and under the sinks of our homes.
The Art of the Cutaway: Making the Invisible Visible
Richard Trethewey’s first cutaway was a humble garbage disposer, created for an episode of Ask This Old House. That initial bisection taught him something profound: a cutaway reveals not just how something works, but also how to repair it. The disposer cutaway showed the blades, the motor, and the reset button on the bottom, giving homeowners a complete mental map of a device they use daily. As Richard explains, the inside of a disposer actually contains no blades at all. Instead, two stainless-steel lugs whirl on a turntable at 1,725 rpm, using centrifugal force to smash food debris against a stationary ring that shreds waste into particles small enough to flush down the drain.
One practical tip that emerges from understanding the disposer’s inner mechanics is the importance of running cold water during operation. Cold water congeals grease, allowing it to be ground up and flushed away rather than coating the inside of drain pipes. This simple insight, made obvious by seeing the cutaway, can prevent costly clogs and extend the life of your unit. Kitchen plumbing maintenance becomes far more intuitive when you can picture the internal components at work.
Valves That Control Your Water Supply
One of the most remarkable innovations in residential plumbing is the pressure-balanced valve, also known as the thermostatic shower valve. Richard calls it “perhaps no one thing more amazing in residential plumbing” for its ability to take hot and cold water and deliver a consistent temperature regardless of pressure fluctuations elsewhere in the house. When someone flushes a toilet while you are in the shower, a pressure-balanced valve prevents that sudden, shocking temperature spike. The cutaway reveals the internal mechanism that constantly monitors and adjusts the mix, providing both safety and comfort that homeowners often take for granted. This kind of engineering precision explains why high-end homes invest in premium plumbing fixtures that offer reliable performance and lasting durability.
The pressure-reducing valve is another essential component that Richard has bisected. Installed on the water main where municipal water enters a building, this device regulates incoming pressure when it exceeds 75 psi. Inside, a spring-loaded core acts as a shock absorber, delivering water at the ideal household pressure of 45 to 50 psi. Without this valve, high municipal pressure could damage fixtures, cause leaks, and shorten the lifespan of appliances throughout the home. It is a small device with a critical job, and seeing its internal spring mechanism makes its function immediately clear.
Toilets and Water Heaters: Everyday Miracles of Engineering
The siphon jet toilet is perhaps the most universally used piece of plumbing equipment in the world. Billions of toilets function flawlessly every day based on a remarkably simple principle. The design incorporates a short leg and a long leg inside the trapway; water is pushed through a small hole at the base of the bowl to start a siphon, and the combined hydraulic effect flushes everything away. Richard’s toilet cutaway has appeared on The Tonight Show and on Ask This Old House at least ten times, making it one of his most-viewed teaching tools. The cutaway makes it obvious why toilets rarely clog when used properly and why basic toilet repairs are often simpler than homeowners assume.
No cutaway in Richard’s collection has seen more use than “Old Rusty Water Heater,” a gas-fired tank-type model that reveals the critical components inside every storage water heater. The cutaway shows the dip tube that delivers cold water to the bottom of the tank, the anode rod that sacrifices itself to prevent corrosion, and the steel tank construction that gives every water heater a limited lifespan. Seeing the rust and sediment buildup inside an aged tank explains why water heaters eventually fail. Understanding these internals helps homeowners recognize when water heater replacement is necessary and why regular maintenance like flushing the tank can extend service life.
Valve Technology: From Rubber Washers to Stainless Steel Balls
Plumbing valves have evolved dramatically over the years, and Richard’s collection of bisected valves tells the story of that progression. The standard stop-and-waste valve, still used for branch lines in many houses, relies on a rubber washer that compresses to stop water flow. While functional, this design has a clear weakness: the washer wears out over time and eventually needs replacement. The gate valve represented a significant improvement, using a guillotine bronze wedge driven down into a gate to close off water. Gate valves have long been used on water mains for their robust construction, but they have their own limitation. Over time, electrolysis can rot the threads, potentially leaving the valve stuck in a closed position.
The invention of the ball valve was truly revolutionary. It uses a stainless-steel ball with a hole through its center, inserted next to a specially engineered seat material that will never wear out. A quarter-turn of the handle rotates the ball from fully open to fully closed, providing reliable shutoff with minimal effort. Many professional plumbers now use ball valves everywhere in new installations because of their long service life and dependable operation. When homeowners ask about upgrading their home plumbing system, switching to ball valves is often one of the first recommendations professionals make.
| Valve Type | Internal Mechanism | Typical Use | Primary Limitation |
|---|---|---|---|
| Stop-and-Waste | Rubber washer compression | Branch lines in houses | Washer wears out over time |
| Gate Valve | Bronze wedge on threaded stem | Water mains | Electrolysis can rot threads |
| Ball Valve | Stainless-steel rotating ball | Modern residential systems | Higher initial cost |
| Pressure-Reducing | Spring-loaded diaphragm | Water main entry points | Requires periodic adjustment |
| Pressure-Balanced | Internal spool with hot/cold ports | Shower mixing valves | Can scale up in hard water |
Safety Devices and Noise Solutions Every Homeowner Needs
The temperature and pressure relief valve, or T&P valve, is what Richard calls “absolutely one of the most important safety devices in a house.” Installed on every water heater, this valve is designed to relieve pressure if the tank ever reaches an unsafe level. Without it, a water heater could become a dangerous projectile. Richard warns against a critical mistake homeowners sometimes make: when the valve drips, people place buckets underneath to catch the water. This seemingly harmless response can lead to someone later deciding to plug the valve entirely, which would fundamentally turn the water heater into a bomb. This safety device must always be allowed to function freely and should never be blocked or removed.
Water hammer is one of the most common plumbing complaints. Richard explains that many calls begin with a homeowner saying, “It makes a noise like this,” imitating the banging sound that occurs when water moving through a pipe stops suddenly. The solution is a water-hammer arrester, a chamber that sits inside the plumbing system with a neoprene gasket inside and air pressure on one side and water on the other. When oncoming water hits this device, the gasket acts like a trampoline or shock absorber, cushioning the sudden stop and eliminating the noise. Understanding how this simple device works can save homeowners from hours of frustration trying to stop pipe banging.
Richard’s collection of cutaways represents more than forty years of teaching experience. Each bisected pipe, valve, and fixture tells a story about the invisible systems that make modern life comfortable and safe. Whether it is the centrifugal force inside a disposer, the siphon action of a toilet, or the spring-loaded precision of a pressure-reducing valve, these cutaways transform mystery into understanding. For any homeowner interested in understanding your home’s plumbing system, these visual demonstrations offer insights that no diagram or written description can match. The next time you turn on a faucet or flush a toilet, remember the engineering inside those walls, and appreciate the work of masters like Richard Trethewey who have spent decades helping us all understand it better.