Plumbing · Compare
Tank vs tankless water heater: which to spec
Tank wins on first cost and forgiving spiky peaks; tankless wins on space, standby loss, and life if the gas line supports it.
Short answer
Pick a tank when first cost and a spiky demand curve rule the job; pick tankless when you need to free floor space, kill standby loss, and get longer service life. The single biggest deciding factor is the building's utility feed: a tankless fires far higher input (often 150,000 to 199,000 BTU/hr vs a tank's ~40,000) and commonly needs a 3/4 in or 1 in gas line and a meter to match. If that gas supply is not there, the tankless never reaches its rated output and a tank is the honest choice.
Tank water heater vs Tankless water heater: side by side
| Factor | Tank water heater | Tankless water heater |
|---|---|---|
| How it delivers | Stores 40 to 120 gal hot and ready; buffers spikes, then drops to recovery rate | Heats on demand as water flows; flat output capped at rated GPM, never fades |
| Upfront cost | Lowest first cost of common types | Higher: unit plus upsized gas line and venting |
| Typical efficiency (UEF) | ~0.60 to 0.70 gas; ~0.90 to 0.95 electric resistance | ~0.80 to 0.96 gas (condensing at top of range) |
| Standby loss | Sheds heat from stored mass around the clock | None; no stored mass to lose |
| Performance limit | Volume/recovery ceiling: drain faster than it reheats and it goes cold | Flow ceiling: exceed rated GPM at cold-inlet rise and every fixture goes lukewarm |
| Install demand | Needs floor area and headroom; standard gas or electric feed | Wall-hung, frees floor; needs 3/4 in or 1 in gas line and matching meter |
| Maintenance | Anode rod checks/replacement and periodic sediment flush | Descale flush on a schedule, especially on hard water |
| Life expectancy | ~8 to 12 years, set by anode and water chemistry | ~15 to 20 years with descaling; no large tank to rust through |
| Quirks | Sediment buries burner/element and cuts recovery | Cold-water sandwich on short stop-start draws; needs recirc-rated unit for a loop |
Which should you pick?
Choose Tank water heater when
- Budget is tight and first cost drives the decision.
- Demand is spiky and short (house morning rush, hotel 7 a.m., dorm) where stored gallons ride out the peak.
- The gas line and meter cannot feed a high-input tankless without a utility upgrade.
- You want a forgiving unit that tolerates a demand it was not perfectly sized for.
Choose Tankless water heater when
- Floor space is tight and a wall-hung unit frees the mechanical room.
- Demand is long and steady and you want endless hot water with no standby loss.
- You are optimizing life-cycle cost and can absorb the higher first cost over a longer service life.
- The building already has (or can get) the larger gas line and meter the unit needs.
Bottom line
It depends on the demand curve and the utility feed. A tank is the cheaper, more forgiving pick for spiky short peaks and tight budgets, and it fails by running out of volume. A tankless eliminates standby loss, frees the floor, and lasts longer, but it fails by hitting a flow ceiling and only performs if the gas line and meter can feed its much higher input. Both need thermal expansion control on a closed system and a compliant T&P discharge regardless of type. Size the unit to the peak-hour demand at the coldest-month inlet, then pick the type the building can actually run.
FAQ
What is the difference between a tank and tankless water heater?
A tank stores 40 to 120 gallons hot and ready, which makes it forgiving of a spiky draw but adds standby loss and a recovery ceiling. A tankless heats water on demand with no stored mass, so no standby loss and no volume limit, but it has a flow ceiling: exceed its rated GPM at the cold inlet and it goes lukewarm.
Is a tankless water heater worth it over a tank?
Tankless is worth it when you value endless hot water, a freed-up floor, lower energy use, and longer life (often 15 to 20 years vs a tank's 8 to 12), and the building can feed the bigger gas line it needs. It costs more up front and demands descaling on hard water. On a tight budget with an undersized gas line, a tank often wins on total cost.
Do both tank and tankless heaters need an expansion tank?
Yes, on a closed system, which is most of them once a backflow preventer, check valve, or pressure-reducing valve sits on the cold supply. Water expands about 2 percent from cold to hot and the pressure has nowhere to go, so an expansion tank absorbs it. This holds for any type, tank or tankless, per IPC Section 607.3 and the corresponding UPC provisions.