Deconstructing the RV Tankless Heater: The "Cold Water Sandwich" and the Physics of On-Demand

For any experienced RVer, the 6-gallon tank water heater is a familiar, if frustrating, companion. It’s a technology of limits. It demands you plan your day around its 20-minute recovery time, and it forces you to master the “military shower”—a hurried, 3-minute race against the inevitable cold.

The engineering problem is simple: a traditional tank is a “storage” system. It wastes energy (via standby heat loss) to keep a small, finite amount of water hot, whether you need it or not.

The tankless (or “on-demand”) water heater is presented as the perfect solution. A machine like the forimo RV Tankless Water Heater is a “heating” system, not a storage one. It promises “unlimited” hot water by only activating when you need it. This is not a review, but a deconstruction of that promise and the complex, real-world trade-offs it demands.

Deconstructing On-Demand Physics (The Theory)

The sales pitch is compelling. A tankless unit like the forimo promises 65,000 BTU of propane-fired power, heating 3.9 Gallons Per Minute (GPM) of water, instantly.

The process is a clean, on-demand event:
1. You turn on the tap. A flow sensor detects the demand (requiring a minimum 3.6 PSI).
2. Ignition. The 12V DC power (essential for off-grid RVs) powers the stable DC motor and igniter.
3. Combustion. The 65,000 BTU burner instantly heats the water as it passes through an oxygen-free copper coil (a highly efficient heat-transfer material).
4. Hot Water. The result is an endless supply of hot water, all while using “roughly 40% less energy” because there is zero standby loss.

On paper, this is a flawless upgrade. In practice, it introduces a new problem that is unique to the RV lifestyle: The “Cold Water Sandwich.”

The Great Trade-Off: How Water Conservation Breaks the Tankless Model

In a house, you turn on the shower and leave it on. In an RV, you never do this. You are trained to conserve water to protect your limited fresh supply and, more importantly, your limited grey tank capacity.

The “military shower” is the standard:
1. Turn on water to get wet.
2. Turn off water to lather up.
3. Turn on water to rinse.

It’s this water-saving step—turning the water off—that conflicts directly with the tankless heater’s logic.

A 3-star review from user “idahomars” for the forimo heater provides a perfect, and painful, deconstruction of this phenomenon. He describes taking a shower in a “cold and snowing” climate:

”…I started my shower and lathered up with soap. I was hoping to enjoy the shower for a few minutes, but then the water went ice cold. I had to finish my shower in a snowstorm, rinsing and cold water, which made me very unhappy.”

Deconstructing the “Cold Water Sandwich” (The Reality)

What “idahomars” experienced is a predictable, three-part sequence caused by the “military shower” method:

1. The First Slice (Hot): You turn the water on. The flow sensor engages, the burner fires, and you get hot water. You get wet.
2. The “Off” Cycle: You turn the water off to lather. The flow sensor reads 0 GPM. The 65,000 BTU burner immediately and correctly shuts down. The machine is now “cold.”
3. The “Sandwich” (Hot-Cold-Hot): You turn the water back on to rinse.
   • Stale Hot: For 1-2 seconds, you get the hot water that was already in the pipe between the heater and the showerhead.
   • The “Ice” (The Cold Meat): For 3-5 seconds, you get a blast of pure, unheated water. This is the fresh water from your tank that has just entered the now-cold heater, and it’s being pushed to your showerhead. The burner is only just now re-igniting because the flow sensor has detected the new demand.
   • The Lag: The “Smart Segmented Combustion” must sense the flow, ignite, and heat the copper coils back up to temperature.
   • Hot Water Returns: The hot water finally arrives, but only after you’ve been hit with an “ice cold” blast “in a snowstorm.”

This is the “cold water sandwich.” It’s not, as “idahomars” notes, a “flaw” in the heater, but a fundamental incompatibility between on-demand technology and the RVer’s water-conservation habit.

Specs vs. Reality: The 3.3-Star Case Study

The Forimo RV heater, with its 3.3-star rating, is a case study in this conflict. The technology already has a high-friction user experience (the cold sandwich). This is then compounded by what appears to be significant quality control issues, which turn a “quirk” into a “failure.” * User “Michele Delgado” (1-star): “Horrible. This is now my 2nd unit and it doesn’t work . I’ve paid to have both units installed just to have this one not work either.” * User “Eizaria” (2-star): “Doesn’t work at all… came with damage burner and thermostat doesn’t function at all.” * User “Stephen Shelton” (3-star): “It works. Water is not as hot as old tank type heater & takes longer to get.”

This last comment—“takes longer to get”—is the key. He is not describing a failure; he is describing the normal ignition lag (the “cold water sandwich”) that is inherent to the design.

Conclusion: The Two-Compromise System

The forimo heater, and the tankless RV category in general, forces the user to make a conscious choice. There is no “perfect” solution, only a choice of which compromise you prefer.

• Option 1: The Tank Heater (Old Tech): You are limited to 6-10 gallons of hot water. You must plan your showers. But that hot water is consistent.
• Option 2: The Tankless Heater (New Tech): You have a theoretically unlimited supply of hot water and higher efficiency. But you must give up the “military shower.” To avoid the “cold water sandwich,” you must leave the hot water running the entire time you are in the shower, which, in an RV, is often a cardinal sin.

The technology works—but it demands you change your habits. For a user stuck in a snowstorm, that change is a cold, hard lesson in the physics of on-demand.