The smallest electric tankless water heater in 2026 is the Bosch Tronic 3000T at just 10.5 x 7.0 x 3.2 inches — roughly the size of a hardcover book — delivering 3.4 kW for a single handwashing sink. The smallest gas model is the Rinnai V53DeN at 20.5 x 14.0 x 8.7 inches, packing 120,000 BTU/h into a unit barely larger than a carry-on suitcase.
If you need hot water in a tight space — under a sink, inside a closet, in an RV, or mounted on a cramped utility wall — these compact models deliver surprising performance from minimal footprints. For RV-specific and camping models with battery ignition, see the outdoor portable tankless heaters guide.
Compact Electric Tankless Water Heaters
Electric models dominate the "smallest" category because they have no combustion chamber, no heat exchanger fins, and no venting requirements. The heating elements are compact, and the housing is essentially a small junction box.
| Model | Dimensions (H x W x D) | Weight | Power | GPM at 35F Rise | Best For | Price |
|---|---|---|---|---|---|---|
| Bosch Tronic 3000T ES2.5 | 10.5 x 7.0 x 3.2 in | 3.5 lbs | 3.4 kW | 0.6 GPM | Single sink | $140–$190 |
| Stiebel Eltron DHC 3-2 | 7.9 x 6.5 x 2.9 in | 3.3 lbs | 3.0 kW | 0.5 GPM | Handwash station | $180–$240 |
| EcoSmart ECO 8 | 11.5 x 8.0 x 3.75 in | 6.5 lbs | 8 kW | 1.4 GPM | 1 sink or lavatory | $170–$230 |
| Rheem RTEX-04 | 10.0 x 7.5 x 3.5 in | 4.8 lbs | 3.5 kW | 0.6 GPM | Single sink | $150–$200 |
| EcoSmart ECO 11 | 11.5 x 8.0 x 3.75 in | 6.5 lbs | 13 kW | 2.2 GPM | 1 bathroom | $200–$300 |
| Stiebel Eltron Tempra 12 Plus | 14.5 x 11.5 x 4.5 in | 8.0 lbs | 12 kW | 2.0 GPM | 1 bathroom (premium) | $350–$450 |
| Rheem RTEX-13 | 10.0 x 7.5 x 3.5 in | 5.5 lbs | 13 kW | 2.2 GPM | 1 bathroom | $220–$290 |
| EcoSmart ECO 18 | 17.0 x 14.0 x 3.75 in | 7.5 lbs | 18 kW | 3.1 GPM | 1–2 fixtures | $280–$380 |
Bosch Tronic 3000T ES2.5 — Smallest Overall
At 10.5 x 7.0 x 3.2 inches and 3.5 pounds, the Bosch Tronic is smaller than most routers. It hides under a bathroom vanity or behind a utility sink panel with room to spare. The 3.4 kW draw requires a single 20A/240V circuit — one of the few tankless units that works with standard residential wiring without dedicated heavy circuits.
The limitation is obvious: 0.6 GPM at a 35F rise serves only a single handwashing faucet. This is a purpose-built point-of-use heater for commercial restrooms, office kitchens, or adding hot water to a remote sink where running a pipe from the main heater is impractical.
Installation: Mounts with two screws. Two 3/8-inch water connections (standard compression fittings). One 20A/240V circuit with #12 AWG wire. Total install time: 1–2 hours. Total cost: $250–$400 installed.
EcoSmart ECO 11 — Best Small All-Rounder
At 11.5 x 8.0 x 3.75 inches, the ECO 11 packs 13 kW into a unit the size of a laptop bag. It delivers 2.2 GPM at 35F rise — enough for a full shower in warm climates or a bathroom sink plus handwashing in cold climates.
This is the go-to model for guest bathrooms, tiny homes, garage apartments, and under-sink installations. A single 60A/240V circuit with #6 AWG wire handles the load. Lifetime warranty on the heat exchanger.
Stiebel Eltron DHC 3-2 — Smallest Footprint
The DHC 3-2 measures just 7.9 x 6.5 x 2.9 inches — the most compact tankless unit available from any manufacturer. At 3.0 kW, it is designed exclusively for single-sink handwashing. The German build quality and brass fittings justify the premium over the Bosch Tronic.
Best for: High-end commercial restrooms, dental offices, medical exam rooms, and any application where a tiny footprint and reliability are paramount.
Compact Gas Tankless Water Heaters
Gas units are inherently larger because they contain a combustion chamber, heat exchanger, exhaust fan, and gas valve. The smallest gas models are still larger than the largest electric point-of-use units. However, they deliver far more heating power.
| Model | Dimensions (H x W x D) | Weight | BTU/h | GPM at 35F Rise | Best For | Price |
|---|---|---|---|---|---|---|
| Rinnai V53DeN | 20.5 x 14.0 x 8.7 in | 34 lbs | 120,000 | 5.3 GPM | Small whole-home, warm climate | $700–$900 |
| Noritz GQ-C2457WX-FF | 21.0 x 14.0 x 8.0 in | 38 lbs | 120,000 | 5.3 GPM | Small whole-home, warm climate | $750–$950 |
| Rinnai RL75iN | 23.0 x 14.0 x 9.3 in | 44 lbs | 150,000 | 6.6 GPM | Medium whole-home | $800–$1,050 |
| Rheem RTG-70XLN-3 | 22.5 x 14.0 x 9.0 in | 40 lbs | 150,000 | 6.5 GPM | Medium whole-home | $750–$950 |
| Takagi T-H3J | 20.3 x 13.8 x 7.3 in | 32 lbs | 100,000 | 4.0 GPM | Small home or apartment | $600–$800 |
Rinnai V53DeN — Smallest Gas With Real Power
The V53DeN squeezes 120,000 BTU into a 20.5 x 14.0 x 8.7-inch enclosure, making it the smallest gas tankless that can serve a whole home in warm climates. At 5.3 GPM (35F rise) and 34 pounds, it mounts on any load-bearing wall with standard bracket hardware.
This is a non-condensing unit (UEF 0.81), requiring Category III stainless steel venting. However, the compact size and 120,000 BTU output make it ideal for condos, small homes, and retrofit situations where cabinet space is tight. Available in both natural gas (V53DeN) and propane (V53DeP).
Takagi T-H3J — Most Compact Gas Frame
At 20.3 x 13.8 x 7.3 inches, the Takagi T-H3J has the shallowest depth of any gas tankless — only 7.3 inches from wall to front. This matters in tight utility closets and between-stud installations. At 100,000 BTU and 32 pounds, it is the lightest gas model available.
The trade-off is capacity: 4.0 GPM at 35F rise limits it to single-fixture use in moderate climates and light multi-fixture use in warm climates. Best for apartments, studios, and small homes under 1,200 sq ft.
Size Comparison: Tankless vs Tank
To appreciate the space savings, here is a direct dimensional comparison:
| Unit Type | Footprint (Floor Space) | Height | Volume | Weight (Full) |
|---|---|---|---|---|
| 40-gallon tank | 2.6 sq ft (18 in diameter) | 48 in | 5.4 cu ft | 340 lbs |
| 50-gallon tank | 2.9 sq ft (20 in diameter) | 54 in | 8.2 cu ft | 440 lbs |
| 75-gallon tank | 3.8 sq ft (22 in diameter) | 60 in | 11.5 cu ft | 650 lbs |
| Gas tankless (full-size) | 0 sq ft (wall-mounted) | 24 x 18 x 10 in | 2.5 cu ft | 55–85 lbs |
| Gas tankless (compact) | 0 sq ft (wall-mounted) | 20 x 14 x 9 in | 1.5 cu ft | 32–44 lbs |
| Electric tankless (whole-home) | 0 sq ft (wall-mounted) | 17 x 17 x 4 in | 0.67 cu ft | 8–20 lbs |
| Electric tankless (POU) | 0 sq ft (wall-mounted) | 12 x 8 x 4 in | 0.22 cu ft | 3–7 lbs |
A compact electric point-of-use unit is 97% smaller by volume than a 50-gallon tank. Even a full-size gas tankless is 70% smaller and frees 3 square feet of floor space.
Under-sink installation: Electric point-of-use units (8–13 kW) fit inside standard 24-inch vanity cabinets. Mount the unit to the back wall of the cabinet, connect to existing hot/cold lines, and run the electrical from the nearest junction box. No visible hardware, no floor space lost.
Sizing Guide for Compact Units
Small does not mean inadequate if you match the unit to your demand:
| Application | Fixtures Served | GPM Needed | Recommended Unit | Power/BTU |
|---|---|---|---|---|
| Single handwash sink | 1 lavatory faucet | 0.5 GPM | Bosch 3000T or Stiebel DHC 3-2 | 3.0–3.4 kW |
| Kitchen sink only | 1 kitchen faucet | 1.0–1.5 GPM | EcoSmart ECO 8 or Rheem RTEX-08 | 8 kW |
| One bathroom (sink + shower) | 2 fixtures (not simultaneous) | 2.0 GPM | EcoSmart ECO 11 | 13 kW |
| One bathroom (simultaneous) | 2 fixtures (simultaneous) | 3.0 GPM | EcoSmart ECO 18 | 18 kW |
| Studio/small apartment, warm climate | Whole unit | 3.5 GPM | Rinnai V53DeN or Takagi T-H3J | 100–120K BTU |
| 1-bed apartment, warm climate | 1 bath + kitchen | 4.5 GPM | Rinnai V53DeN | 120K BTU |
| Small home, any climate | 2 bath + kitchen | 5.0+ GPM | Rinnai RL75iN or Rheem RTG-70XLN | 150K BTU |
Installation Considerations for Tight Spaces
Clearance Requirements
Even compact units need minimum clearances for service access and (for gas) combustion air:
Electric POU: 1 inch clearance on all sides. No combustion air needed. Can be installed in enclosed cabinets.
Gas units: Minimum 12 inches of clearance in front for service access. 1-inch clearance on sides and top. Combustion air supply per local code (usually 1 sq in per 1,000 BTU for confined spaces). Venting must maintain minimum clearances from combustible materials.
Wall Strength
Gas tankless units (32–85 lbs) need mounting on a load-bearing wall or with toggle bolts rated for 100+ lbs. Electric POU units (3–20 lbs) can mount on drywall with standard anchors.
Vibration and Noise
Gas units vibrate slightly when the burner fires (50–65 dB). Avoid mounting on shared bedroom walls. Use rubber isolation pads between the bracket and wall to reduce transmitted vibration. Electric units are nearly silent (under 40 dB).
Key Takeaways
- Smallest electric: Stiebel Eltron DHC 3-2 (7.9 x 6.5 x 2.9 inches) for handwash; EcoSmart ECO 11 (11.5 x 8 x 3.75 inches) for a full bathroom
- Smallest gas: Takagi T-H3J (20.3 x 13.8 x 7.3 inches) for apartments; Rinnai V53DeN (20.5 x 14 x 8.7 inches) for small whole-home
- Electric POU units fit under any standard vanity cabinet — true invisible installation
- Space savings vs tank: 70–97% volume reduction depending on unit type
- Match unit to demand — a 3.4 kW unit for handwashing, 13 kW for a bathroom, 120K BTU for an apartment
- Compact gas units still need venting — factor vent routing into your space planning
Frequently Asked Questions
Electric tankless units can be installed in any closet with proper electrical access. Gas tankless units can be installed in closets only if combustion air requirements are met — typically requiring louvered doors or air ducts providing adequate ventilation. Check local building codes for confined-space installation rules for gas appliances.
Sources & References
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