Quick Answer: For the thinnest wall-mounted footprint, the EcoSmart ECO 27 is the best compact whole-home tankless water heater — 17 x 17 x 3.75 inches per EcoSmart’s published spec, nearly the profile of a large flat-screen TV, while still delivering close to 3 GPM at a 37°F temperature rise. The Stiebel Eltron Tempra 24 Plus (16.63 x 14.5 x 4.63 inches) is the best alternative if you want self-modulating flow control. For a single fixture, the EcoSmart ECO 11 (11.5 x 8 x 3.75 inches) is the smallest unit here. Gas buyers should expect roughly double the depth: the Rheem RTGH-95DVLN condensing unit, one of the slimmer gas options, still runs 9.75 inches deep per Rheem’s own submittal sheet, because it has to house a burner and heat exchanger that electric units skip entirely.
“Compact” means something different depending on fuel type, and most buyers don’t find that out until they’ve already measured their closet for the wrong category. This guide ranks the genuinely thinnest units in each fuel type, by manufacturer-published depth, not marketing copy.
The depth gap, by the numbers
Electric tankless units run 3.75 to 4.63 inches deep. A relatively compact condensing gas unit still needs 9.75 inches. That gap isn’t a manufacturing difference between brands — it’s a difference in what has to physically fit inside the housing. An electric unit is a shell around heating elements, a flow sensor, and a control board. A gas unit needs room for a burner assembly, a heat exchanger, and a port for intake/exhaust venting, none of which an electric unit requires at all.
That’s also why the EcoSmart ECO 27 and ECO 11 share the exact same 3.75-inch depth despite one being a whole-home unit and the other a single-fixture unit — depth is set by what’s inside the box, not by how much hot water it’s rated to deliver.
Our top picks at a glance
| Model | Best for | Dimensions (H x W x D) | Power | Flow | Price |
|---|---|---|---|---|---|
| EcoSmart ECO 27 | Thinnest whole-home electric | 17" x 17" x 3.75" | 27 kW / 240V | ~3 GPM @ 37°F rise | ~$460–580 |
| Stiebel Eltron Tempra 24 Plus | Self-modulating alternative | 14.5" x 16.63" x 4.63" | 24 kW / 240V | 4.68 GPM rated | ~$700–720 |
| EcoSmart ECO 11 | Smallest, single fixture | 11.5" x 8" x 3.75" | 13 kW / 240V | 1.3–3.1 GPM | ~$210–300 |
| Rheem RTGH-95DVLN | Best compact gas | 27.5" x 18.5" x 9.75" | 199,000 BTU / NG | 9.5 GPM rated | ~$1,200 |
1. EcoSmart ECO 27 — Thinnest Whole-Home Electric
EcoSmart ECO 27 Electric Tankless Water Heater
- At 3.75 inches deep, per EcoSmart's published spec, it's close to the profile of a large flat-screen TV mounted on the wall — the thinnest whole-home unit on this list.
- Rated by EcoSmart for nearly 3 GPM at a 37°F rise, enough for one shower plus a fixture running simultaneously in a warmer climate.
- Energy Star certified and draws up to 113 amps, so it needs a correctly sized double-pole breaker — confirm your panel capacity before ordering.
- Colder-climate performance drops below that 3 GPM figure, since a bigger temperature rise always costs flow at a fixed kW rating.
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This is the pick if a shallow closet or utility nook is the hard constraint and you’re in a warm-to-moderate climate. Its 3.75-inch depth means it adds almost nothing to the wall it’s mounted on, but the amperage draw is real — at 113 amps it’s one of the heavier electrical loads in this entire category, so check your service panel first. See our best electric tankless water heater guide if you want the full ECO lineup sized by household bathroom count.
2. Stiebel Eltron Tempra 24 Plus — Self-Modulating Alternative
Stiebel Eltron Tempra 24 Plus (24 kW)
- Advanced Flow Control, a feature Stiebel Eltron patents and keeps exclusive to its own tankless lineup, automatically trims flow slightly to hold a steady output temperature rather than letting it swing.
- At 4.63 inches deep it's still well under half the depth of a gas condensing unit, though almost an inch thicker than the EcoSmart ECO 27.
- Rated to cover up to 2 bathrooms in a warm climate, or roughly 1 to 1.5 bathrooms in a cold climate, per Stiebel Eltron's own coverage guidance.
- Priced noticeably above the ECO 27 for a smaller rated GPM ceiling — the premium buys flow-modulation smoothness, not more hot water.
Choose this over the ECO 27 if you’ve had trouble with temperature swings on a self-modulating-free electric unit before, or you specifically want Stiebel’s flow-control patent rather than EcoSmart’s simpler on/off element staging. It’s still a genuinely compact unit — just not the single thinnest one here.
3. EcoSmart ECO 11 — Smallest Unit, Single Fixture
EcoSmart ECO 11 Electric Tankless Water Heater
- At 11.5 x 8 inches of wall footprint, this is small enough to mount inside a sink cabinet or a shallow utility closet most other units wouldn't fit into at all.
- EcoSmart rates it for a single bathroom, small sink, or office breakroom — it is not sized for whole-house use the way the ECO 27 is.
- Self-modulating technology adjusts power to actual flow and inlet temperature rather than running at a fixed output.
- Needs its own dedicated 60-amp double-pole breaker despite the small housing — the electrical requirement doesn't shrink with the case size.
This is the right pick if you need compact for a single fixture — a detached bathroom, a workshop sink, a breakroom — not a whole house. Our best point-of-use water heater guide covers the even-smaller 120-volt plug-in category if this is still larger than you need.
4. Rheem RTGH-95DVLN — Best Compact Gas
Rheem RTGH-95DVLN Condensing Tankless Water Heater
- At 9.75 inches deep, per Rheem's own submittal sheet, this is on the thinner end of whole-home condensing gas units, though still more than double the EcoSmart ECO 27's depth.
- Rated for 9.5 GPM, well above any electric unit on this list, because gas combustion delivers far more BTU/hr than a household electrical panel can supply.
- Still requires a PVC intake/exhaust vent run to an exterior wall — a space and permitting requirement none of the electric units on this list have.
- At roughly $1,200, it costs about 2 to 4 times as much as the electric options here before installation and venting labor.
Pick this only if you already have a gas line and need the higher GPM a whole house with multiple simultaneous showers actually requires — no electric compact on this list matches 9.5 GPM. If you don’t already have gas service, the venting and permitting cost usually outweighs any “compact” advantage over an electric unit. Our best gas tankless water heater guide covers the full gas lineup if 9.5 GPM is more than you need.
How to choose
- Thinnest wall-mounted whole-home unit, warm-to-moderate climate: the EcoSmart ECO 27.
- Want self-modulating flow control in a similarly thin housing: the Stiebel Eltron Tempra 24 Plus.
- Single sink, breakroom, or detached fixture — not a whole house: the EcoSmart ECO 11.
- Need whole-house GPM and already have a gas line: the Rheem RTGH-95DVLN, accepting the larger depth.
- Renting and can’t install anything hardwired at all: see our best tankless water heater for apartment guide for plug-in options that need zero landlord sign-off.
The bottom line
If physical depth is the actual constraint, electric wins by a wide margin: the EcoSmart ECO 27 runs just 3.75 inches deep and still covers a shower plus a fixture in a warm climate, while even a relatively compact gas unit like the Rheem RTGH-95DVLN needs 9.75 inches for its burner and heat exchanger. Go gas only if you already have a line in place and need more GPM than any electric compact can deliver. Whichever you pick, remember that depth isn’t the whole space question — manufacturer manuals still call for roughly 24 inches of clearance in front of the unit regardless of how thin the housing is, a detail our tankless water heater dimensions guide covers model by model.