Tankless vs Tank Water Heater: Real Costs and Trade-Offs

The installed cost is where most tankless dreams die. A tankless unit often costs two to three times a tank installed. The equipment itself is not the whole story. Most gas tankless units need a 3/4-inch or larger dedicated gas line. If your existing line is 1/2-inch, the gas line upgrade runs $300 to $1,200. Venting is the other gotcha. Tankless units use stainless steel or PVC venting depending on the model. An existing chimney flue may not work. A retrofit vent can add $1,000 to $2,500. Electric tankless units need 120 to 150 amps of dedicated capacity. If your panel is full, add $1,500 to $3,500 for a panel upgrade. Those numbers are why a “tankless swap” quote can double after the contractor opens the wall.

The endless-hot-water promise has a flow limit. Tankless units are rated in gallons per minute at a specific temperature rise. In St. Louis, winter groundwater comes in around 40°F to 45°F. If you want 120°F at the tap, that is a 75°F to 80°F rise. A 199,000 BTU gas tankless unit delivers roughly 4.5 to 5.5 GPM at that rise. A shower is 1.5 to 2.5 GPM. A dishwasher is 1 to 2. A laundry tap is 2 to 3. Two showers plus the dishwasher can exceed the unit’s capacity. Northern homes get fewer gallons per minute than the brochure’s summer numbers. It is endless hot water until you run three fixtures at once.

Descale maintenance is not optional in hard water. St. Louis water is hard enough that scale builds in the heat exchanger. Yearly descaling runs $150 to $300 if you hire it. A DIY kit with a pump and vinegar runs $50 to $100 and takes about an hour. Skip it for a few years and the flow drops, the unit works harder, and the warranty may be void. A tank does not need descaling, but it does need a yearly flush and an anode rod every few years. Neither system is maintenance-free.

Tank’s honest advantages: it is simple, cheap, and forgiving. A tank holds hot water during a short power outage if it is a gas atmospheric unit. It does not need a special gas line or vent retrofit in most replacements. It tolerates hard water better if you flush it. It is easier to repair. A tank is the default for a reason.

Verdict by household

• One or two people, small home, tight budget: tank. The payback on tankless is not there.
• Three or four people, gas available, space tight: tankless can work if you size it to winter flow and commit to descaling.
• Five or more people: one tankless unit may not keep up. Two tankless units or a tank with a recirculation loop is more realistic.
• Electric-only home: whole-house electric tankless usually is not worth it unless the panel already has capacity. A heat-pump tank is often the better efficiency play.
• Hard water and no maintenance budget: tank.

No brand picking here. Size by flow rate and temperature rise. Add up the fixtures you might run at the same time: shower 1.5–2.5 GPM, sink 0.5–1.5, dishwasher 1–2, laundry 2–3. Use the winter incoming water temperature, not summer. In St. Louis, that is 40°F to 45°F. Size the unit to deliver your peak flow at that rise. Do not size to the summer number on the spec sheet.

STOP: If you smell gas around a tankless unit or its gas line, do not touch the valve or any electrical switch. Leave the house and call your gas utility from outside.

STOP: Water from a tankless unit can be set to 140°F or higher. Scalding water can cause third-degree burns in seconds. Do not test temperature with your hand. Use a thermometer, and set the unit to 120°F unless you have a specific reason.

STOP: Electric tankless units pull high current. Do not open the unit or touch the wiring with the breaker on. Mains voltage can kill you.