Soaking in a traditional Japanese ofuro is a transformative experience, but it does take some planning. These tubs hold significantly more water than a standard American bathtub — roughly 80 to 140 gallons compared to the usual 25 to 45 gallons.
The good news: most US homes can handle this without a massive utility overhaul. You just need to understand your hardware.
Typical Water Heaters
In the US and Canada, your setup usually falls into one of two categories.
What Homes Typically Have
50-Gallon Water Tanks: This is the standard "giant cylinder" in the garage or basement.
The Reality Check: A 50-gallon tank doesn't actually give you 50 gallons of hot water because cold water mixes in as you use it. You get about 35 to 40 gallons of "true" hot water.
The Strategy: For a smaller ofuro, you can fill it in one shot. For larger tubs, you'll fill it until the tank runs low, wait 30 to 60 minutes for "recovery," and then top it off.
Tankless Whole-House Heaters: A compact wall-mounted unit. These are in about 15 to 17 percent of US homes and growing. They heat on demand, meaning you have an unlimited supply of hot water at a steady flow.
Propane vs. Natural Gas
Natural Gas: Piped in via city infrastructure. If you have it, it's the gold standard for cost and speed.
Propane: Delivered by truck to an on-site tank. While common for off-grid or remote locations, you can absolutely use propane in a suburban home if natural gas isn't available. The equipment is virtually the same; it just uses a different orifice to burn the fuel.
The Comparison: Both are significantly cheaper to operate than electric. A gas tank costs about $330 per year to run, while an electric tank averages $590 per year.
How They Do It in Japan
In Japan, they use a system called oidaki (reheating). A gas boiler provides household water and connects to the tub via supply and return lines. You press a button, and the system auto-fills the tub to a specific level and temperature, then recirculates and reheats the water to keep it at that exact degree. Since Japanese bathers wash before entering the tub, the water stays clean for the whole family to use in sequence.
Nothing Beats a Gas Tankless Heater
For filling a soaking tub, high-flow gas tankless units are the clear winner. Brands like Rinnai, Navien, and Noritz put out 150,000 to 199,000 BTUs. You can fill a 100-gallon tub in about 15 to 20 minutes and top off the heat indefinitely, just like a standard small bathtub.
How to Find Out What Water Heater You Have
- The Eyeball Test: A cylinder is a tank; a small wall-box is tankless.
- The Label: Check the yellow EnergyGuide label for capacity.
- The Rating Plate: Look for a metal sticker listing BTU input and model number.
- Fuel Check: If there is a pipe leading to the bottom, it's gas or propane. If there are only heavy wires, it's electric.
Efficiency
What are BTUs? A British Thermal Unit is the energy needed to raise one pound of water by 1°F.
Gas vs. Electric: Natural gas and propane heaters heat water faster and cost significantly less to operate. A gas tank runs about $330 per year versus $590 for electric. Electric heaters have a higher point-of-use efficiency rating (95 percent), but that number is misleading — electricity costs roughly three times more per BTU than gas, and electric units deliver heat much more slowly.
The Math: Heating a medium ofuro (110 gallons) from 55°F to 104°F requires about 44,900 BTUs. A gas tankless unit handles this with ease. An electric tank will struggle to keep up with that volume of demand, requiring long recovery periods between fills.
Keeping Water Warm
Natural Insulation: Cedar has roughly half the thermal conductivity of acrylic. At 1.5 inches thick, our tub walls hold heat noticeably longer than a conventional acrylic or fiberglass bathtub.
The Lid Strategy: 70 percent of heat loss happens at the surface through evaporation. A lid changes everything. We can provide 6-inch-wide cedar lid planks; these prevent heat loss and double as a tray for drinks or books.
Topping Off: For indoor ofuros, you can always drain a bit of cool water and top off with fresh hot water. If you have a tankless heater, this is an infinite loop of comfort.
A Note About Temps
The 104°F Limit: In the US and Canada, "hot tubs" (with integrated heaters and pumps) are legally capped at 104°F. However, bathtubs filled from a faucet are governed by plumbing codes that allow delivery up to 120°F. You control the final soak temp by mixing hot and cold.
What Different Temperatures Feel Like
| Temperature | What to Expect |
|---|---|
| 90–95°F | Warm and gentle. Good for long, leisurely hangs. |
| 100–102°F | The "sweet spot." Deeply relaxing for 20 to 30 minutes. |
| 104°F | Hot. You will sweat. This is the standard US spa maximum. |
| 108–110°F | The traditional Japanese onsen range. Very hot — experienced soakers only. |
Warnings on Temps
While soaking is restorative, heat is a serious element.
Listen to your body. If you feel dizzy or nauseous, get out. Your core temp rises about 1 degree every 15 minutes at 104°F.
Safety first. Pregnant women should stay below 100°F. Those with heart conditions or high blood pressure should consult a doctor.
If you're planning a tub installation and have questions about your water heater setup, get in touch. We're happy to talk through the specifics.