How Do I Know If I Have a Tankless Water Heater?

Graphic: summary of this guide, made by our team from the cited sources.

Tank or tankless: the quick comparison

Clue	Tank (storage) water heater	Tankless water heater
Size and shape	Tall cylinder, typically 20 to 80 gallons	Flat box about the size of a carry-on suitcase
Where it sits	On the floor in a basement, garage, closet or utility room	Mounted on a wall, indoors or, in warm climates, outdoors
EnergyGuide label	Shows tank size and a first-hour rating in gallons	Grouped by a gallons per minute rating
Hot water	Can run out after a long shower, then needs time to reheat	Heats water only while it flows, so it does not run out the same way
Gas venting	Often a vent pipe from the top into a flue or chimney	Usually vented straight out through a wall

Check 1: Size, shape and where it hangs

Go to where your water heater should be: basement, garage, utility closet, attic or an outside wall. A tank model is a large cylinder standing on the floor. A tankless model hangs on the wall, with water pipes, a gas line or electrical connection underneath and a vent at the top or side.

Some tankless units are outside. The Building America Solution Center says outdoor installation is typically only suitable in warm climates (climate zones 1 and 2) where the water lines will not freeze, and that manufacturer guidelines should be followed. If you live somewhere warm and find no water heater inside, walk around the outside walls.

Check 2: Read the yellow EnergyGuide label

If the original yellow EnergyGuide label is still on the unit, it answers the question. Under the Federal Trade Commission's labeling rule, a storage water heater's label must show its capacity as both tank size (storage capacity) and first hour rating. The label for an instantaneous (tankless) model instead compares it with other models by its gallons per minute rating (16 CFR 305.17).

So if you see "Tank Size" and "First Hour Rating" in gallons, you have a tank. If you see a gallons per minute rating, you have a tankless unit.

The rule sorts tankless models into four flow groups for cost comparison, and sorts tank models into four first-hour groups:

Graphic: made by our team from 16 CFR 305.17 and U.S. Department of Energy material.

EnergyGuide group	Tankless: gallons per minute	Tank: first hour rating
Very small	0 to 1.6	Fewer than 18 gallons
Low	1.7 to 2.7	18 to 50.9 gallons
Medium	2.8 to 3.9	51 to 74.9 gallons
High	4.0 or more	75 gallons or more

No label? Look for the manufacturer's rating plate and search the model number on the brand's website.

Check 3: Look at the vent

Gas water heaters have to vent exhaust. The Building America Solution Center says most fuel-fired tankless water heaters are direct vent and blow exhaust out through a vent in the wall, not up a flue or chimney. Condensing gas tankless models are typically vented through a sidewall, and the Solution Center shows one with PVC vent pipes. Rinnai notes that non-condensing tankless models need a metal-based exhaust vent system.

A tank-type gas heater in an older home often vents up into a chimney; the Solution Center mentions this as something that has to be dealt with when switching to tankless. One caution: white PVC pipes on their own do not prove you have a tankless unit, because high-efficiency furnaces use them too. Follow the pipe to the appliance it comes from.

Check 4: Watch what happens when you turn on the hot water

A tankless heater only works while water is flowing. U.S. Department of Energy material explains that a sensor detects the water flow and turns on the burner or heating element, which shuts off when the flow stops (Building Science Education, DOE). If you turn on a hot tap and hear a fan start or see the unit's display change, it is probably tankless.

A tank heater behaves differently. After a large draw such as a long shower, the burner fires to heat the whole tank back up, and hot water may not be available until it recovers. If your hot water runs out and takes a while to come back, that points to a tank. (If yours turns cold after a few minutes for other reasons, see why your shower turns cold after 5 minutes.)

Check 5: No water heater at all? Look at the boiler

Some homes heated by a boiler have no separate water heater. Vermont Energy Saver explains that tankless systems can be "coil" or "no coil" types, and that a tankless coil heats water inside the boiler. It also calls tankless coils one of the least efficient ways to heat domestic hot water (Vermont Energy Saver). If you have a boiler and cannot find a tank or a wall unit anywhere, ask a plumber or heating technician whether a coil supplies your hot water.

Why it matters which one you have

• Lifespan. The Building America Solution Center says tankless water heaters are generally considered to last up to 20 years, compared with 10 to 15 years for tank-type models.
• Flow limits. Whole-house tankless units often deliver 2 to 5 gallons per minute, according to DOE material, and Vermont Energy Saver notes that a shower and a dishwasher at the same time can stretch one to its limit. The Solution Center adds that not every tankless unit can deliver very low flows below 0.5 gallons per minute, which matters for hand washing.
• Service and replacement. Troubleshooting, parts and maintenance differ between the two types, so knowing which one you have makes any call to a plumber faster.