Quick Answer: Whole-home tankless water heaters are roughly the size of a large wall-mounted suitcase — gas models run about 17–19 inches wide, 23–27 inches tall, and 10–13 inches deep, while whole-home electric models are thinner still at roughly 3.5–4.6 inches deep, per manufacturer spec sheets from Rheem, Navien, Rinnai, EcoSmart, and Stiebel Eltron. Point-of-use electric units are smaller yet, often under a foot in any dimension. The unit’s footprint is only half the space question, though — most manufacturer installation manuals also call for about 24 inches of clear working space in front of the heater for service access, which is the number that actually decides whether a tight closet works.
Every other guide on this site talks about flow rate and BTU — how much hot water a unit can deliver. This one answers a different, more literal question: how much physical space does the box on the wall actually take up, and how much room do you need to leave around it. That distinction matters most when you’re trying to fit a replacement into an existing closet, a shallow utility nook, or a cabinet under a sink, where an inch of depth is the difference between “fits” and “doesn’t.”
Real dimensions, by category
| Model | Category | Height | Width | Depth | Mount |
|---|---|---|---|---|---|
| EcoSmart ECO 11 | Point-of-use electric | 11.5" | 8" | 3.75" | Wall |
| Stiebel Eltron Tempra 24 Plus | Whole-home electric | 14.5" | 16.6" | 4.6" | Wall |
| EcoSmart ECO 27 | Whole-home electric | 17" | 17" | 3.5" | Wall |
| Rheem RTGH-95DVLN | Whole-home gas (condensing) | 23.7" | 17.7" | 9.8" | Wall |
| Rinnai RU199iN | Whole-home gas (condensing) | 26.4" | 18.5" | 11.5" | Wall |
| Navien NPE-240A2 | Whole-home gas (condensing) | 27" | 17" | 13" | Wall |
Dimensions per each manufacturer’s own published specification sheet or retailer listing (EcoSmart, Stiebel Eltron, Rheem, Rinnai, Navien). Depth is the figure that varies most and the one most worth double-checking against your specific model before ordering, since even units in the same output class can differ by several inches.
The pattern that jumps out: electric units are dramatically thinner than gas units, often by a factor of two to three on depth alone. That’s not a manufacturing choice — it’s physics. An electric tankless unit is essentially a resistance-heating element and a flow sensor in a box; a gas unit needs a combustion chamber, a heat exchanger, and (on condensing models) a secondary heat exchanger to pull extra efficiency out of the exhaust, all of which need internal depth. If wall depth is your binding constraint — a shallow interior wall, a recessed alcove — electric is the category to shortlist first, independent of whether gas would otherwise be the better fit for your climate and usage. Our electric vs. gas tankless water heater comparison covers the rest of that tradeoff (installed cost, GPM, operating cost) beyond just size.
The clearance space nobody measures until it’s a problem
The unit’s outer dimensions only tell part of the story. Per installation manuals published by major manufacturers including Rinnai and Navien, a typical whole-home tankless install also needs:
- About 24 inches of clear space in front of the unit — this is service and access clearance for a technician to remove the front cover and reach the heat exchanger for descaling or repair, not just decorative breathing room.
- Roughly 12 inches of clearance above the unit for the vent connection on gas models, more if the vent run needs to angle before it exits the wall.
- A few inches on each side, though this varies more by model than the front and top figures do — check your specific unit’s manual rather than assuming a universal number.
- A clear path from the unit to an exterior wall or roof for gas venting — our venting guide covers the PVC-vs-stainless-steel distinction and the maximum run-length math that can rule out an otherwise perfect-looking closet.
This is the step that trips up people converting a coat closet or a shallow utility nook: the heater itself might be under 12 inches deep, but if the door can’t fully open with 24 inches of clearance behind it, or there’s no route to an exterior wall for the vent, the space doesn’t actually work — regardless of how good the unit’s spec sheet numbers look on paper. Point-of-use electric units sidestep most of this, since they don’t vent and their access needs are minimal, which is why they’re the realistic pick for genuinely tight spots like an under-sink cabinet.
EcoSmart ECO 11 Electric Tankless Water Heater
- The smallest realistic option on this page — no combustion chamber, no vent run, and a footprint closer to a large book than an appliance.
- Built for a single fixture or a low-flow point-of-use application, not a whole house; see our best point-of-use water heater guide if this is your use case.
- Confirm your existing circuit can handle the amperage before buying — even a compact electric unit needs a dedicated circuit sized to its kW rating.
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Weight matters too, especially for wall mounting
Whole-home gas units aren’t just deeper than electric ones — they’re heavier. The Rinnai RU199iN weighs about 74 pounds and the Navien NPE-240A2 about 77 pounds, per each manufacturer’s spec sheet, which is enough that the wall bracket needs to land on real framing, not just drywall anchors. Electric units are lighter across the board, generally in the 15–30 pound range depending on kW rating, which is part of why they’re the more forgiving option for a wall that doesn’t have convenient stud spacing. Either way, use the manufacturer’s specified wall bracket and mounting hardware rather than improvising — a tankless unit that’s even slightly out of level can affect condensate drainage on condensing models. Our installation guide covers the full mounting and hookup sequence.
How to choose based on your actual space
- Sink cabinet or a genuinely tiny nook: a point-of-use electric unit under 12 inches in any dimension, no venting needed.
- Shallow closet or a wall with limited depth behind it: a whole-home electric unit — 3.5 to 4.6 inches deep is thin enough to work where a gas unit’s 10+ inches won’t.
- Standard utility closet or garage wall with normal depth: any whole-home gas condensing unit fits, but confirm the 24-inch front clearance and a viable vent path to an exterior wall before you commit to the spot.
- Converting a space that previously held a 50-gallon tank: you’ll have dramatically more room than you’re used to — the harder part is usually relocating venting and gas/electrical service to the new mounting point, not fitting the smaller unit itself. See our tankless vs. tank water heater guide for the full space comparison.
The bottom line
Tankless water heater dimensions cluster tightly by category: whole-home gas units run roughly 17–19” wide, 23–27” tall, and 10–13” deep; whole-home electric units are similar in height and width but only 3.5–4.6” deep; point-of-use electric units are smaller than a shoebox. All figures are per each manufacturer’s own published specifications (EcoSmart, Stiebel Eltron, Rheem, Rinnai, Navien) and can shift slightly between model revisions, so confirm the exact spec sheet for your chosen unit before finalizing a tight installation. The number that actually decides whether a space works, though, isn’t the unit’s footprint — it’s the roughly 24 inches of front service clearance most manufacturer manuals require, plus a viable vent path for gas models. Measure that before you order, not after.