Answering your search for reliable indoor gas tankless water heaters, this guide highlights top options that deliver on-demand hot water, compact installation, and energy efficiency. Ideal buyers include homes needing continuous hot water, small apartment setups, or upgrades from traditional storage tanks. The featured models balance performance, efficiency, and indoor-suitable venting. Below, you’ll find a quick product snapshot table, followed by in-depth sections for each selected heater and a practical buying guide.
| Product | Brand | Approx. Output | Best For |
|---|---|---|---|
| Ranein Natural Gas Tankless Water Heater, 8.5 GPM | Ranein | 190,000 BTU | |
| Rheem RTG Series 7.0 GPM | Rheem | 160,000 BTU/hr | |
| Rheem RTGH Series 8.4 GPM | Rheem | 157,000 BTU/hr | |
| Westinghouse Tankless Water Heater Gas Indoor, 7.9 GPM | Westinghouse | 180,000 BTU | |
| APUS Natural Gas Tankless Water Heater, 140,000 BTU | APUS | 6.8 GPM |
Below are detailed overviews for the five most relevant options. Each section covers who it’s best for, why it’s a fit for indoor installation, and practical considerations to help you compare models side by side.
Ranein 8.5 GPM Indoor Gas Tankless
Best for: Homes needing robust on-demand hot water in a compact footprint. This model’s 190,000 BTU input supports consistent performance even with higher simultaneous demand. The design includes a safety-minded air intake and blower system to promote safe combustion and low noise operation.
Why selected: It offers strong heating power in a relatively small 24.25″ x 15.08″ footprint, making it suitable for indoor installation where space is limited. The setup uses standard 3/4″ NPT fittings for water and air intake, facilitating straightforward installation.
Important note: While powerful, ensure proper venting and clearance per local code. The unit’s performance depends on stable venting and adequate airflow for efficient combustion.
Rheem RTG Series 7.0 GPM Indoor
Best for: Larger homes or families with multiple simultaneous hot-water needs, such as two showers plus sinks. This non-condensing model emphasizes reliable on-demand performance with efficient gas use.
Why selected: It provides a well-matched balance of flow (7.0 GPM) and 160,000 BTU/hr output, designed for indoor installation with a direct vent system. It’s a practical upgrade from traditional tanks for energy savings with continuous hot water.
Caution: Indoor venting requirements are essential; improper venting can impact efficiency and safety.
Rheem RTGH Series 8.4 GPM
Best for: Large households with multiple baths where precise temperature control is valued. Condensing technology drives high efficiency and low NOx emissions.
Why selected: With 8.4 GPM at 35°F rise and 157,000 BTU/hr, it supports 3–4 simultaneous showers, making it a strong all-around indoor option for bigger homes.
Limitation: Condensing units can require additional venting considerations and may have higher initial installation complexity.
Westinghouse 7.9 GPM Indoor
Best for: Homeowners who want smart control and reliable temperature stability in a single-system solution.
Why selected: It offers up to 7.9 GPM and 180,000 BTU, with WiFi and voice control, plus precise temperature stability via self-modulating technology. Ideal for homes seeking modern smart-home integration.
Note: Connectivity features rely on compatible devices and stable network conditions.
ThermoMate CZ422NG 5 GPM
Best for: Smaller homes or apartments with modest hot-water needs. It emphasizes safety and cost savings with efficient operation.
Why selected: It features safety protections and a 0.81 UEF, delivering up to 98% thermal efficiency and improved energy use compared with older tank heaters.
Limitation: Maximum flow of 5 GPM may limit simultaneous usage in high-demand households.
APUS 6.8 GPM Indoor
Best for: Homes needing robust flow without extremely high BTU input, and simple 120V operation.
Why selected: It provides instant hot water with 140,000 BTU and built-in booster pump increasing flow up to 6.8 GPM, supporting two showers and two sinks with minimal temperature fluctuations.
Note: Operation relies on standard electrical supply; check compatibility with your electrical panel before installation.
Buying Guide
What should I look for in an indoor gas tankless water heater?
Key factors include input BTU rating, flow rate (GPM), and the Water Heating Efficiency (UEF). Indoor installations require proper venting, vent pipe sizing, and clearance from combustibles. Consider whether you need a condensing model for higher efficiency and lower NOx, or a non-condensing unit for simpler venting and potentially lower upfront cost.
How to choose by household size and hot-water demand?
Small homes or apartments with 1–2 fixtures typically do well with 5–7 GPM units. Larger homes with multiple showers simultaneously will benefit from 7.5–8.5 GPM models or higher BTU inputs. Always match the unit’s GPM rating at the expected temperature rise to your climate and usage pattern.
What installation considerations matter indoors?
Indoors, you’ll need appropriate venting (direct-vent or ventless where permitted), gas line capacity, and adequate space for maintenance. Some models offer compact footprints and standard 3/4″ water connections, which simplify installation. Ensure clearance for combustion air and follow local building codes.
What about energy efficiency and operating costs?
Condensing models typically achieve higher efficiency (UEFs around 0.93 or higher) and lower ongoing energy costs. Tankless units heat water on demand, reducing standby losses compared to tank-type heaters. Look for UEF and NOx emission ratings to estimate long-term savings and environmental impact.
Maintenance and safety considerations
Regular venting checks, gas line inspections, and periodic descaling (especially in hard water areas) help maintain performance. Look for models with protections such as boil-dry, flame-out, overheat, and pipe blockage protections to improve reliability in daily use.
FAQ
- Is indoor venting required for all gas tankless water heaters? Yes, most require direct-venting to exhaust combustion byproducts and bring in combustion air. Check the manufacturer’s venting instructions.
- Can I install these myself? Installation is possible for some units with basic plumbing and venting knowledge, but professional installation is recommended to ensure safe venting and gas connections.
- What size GPM should I choose for a family of four? For a family of four with multiple hot-water needs, look for 7.5–8.5 GPM models to support 2–4 points of use simultaneously.
- Are there any quiet options? Yes, many models incorporate low-noise blowers and optimized combustion air paths to reduce noise during operation.