Choosing the best tankless hot water heater propane depends on more than the advertised gallons per minute. Your incoming water temperature, household peak demand, fuel supply, venting options, and installation location all affect how well a unit works. These five options range from high-capacity whole-house models to compact indoor heaters, so compare their stated output and installation needs before you buy.
Table of Contents
Quick Comparison
| Product | Listed capacity | Installation | Notable detail |
|---|---|---|---|
| Rheem RTG Series 7.0 GPM | 7.0 GPM; 160,000 BTU | Outdoor, according to title | High stated flow; listing details conflict on fuel and location |
| Rinnai RUCS65IP | Up to 6.5 GPM | Venting options described | 12-year heat exchanger warranty |
| APUS Indoor Propane Heater | 4.42 GPM; 95,000 BTU | Indoor | Temperature setting from 95°F to 140°F |
| GEARPASS 6.3 GPM Heater | 6.3 GPM; 160,000 BTU | Indoor, with exhaust venting | Stainless steel housing and digital controls |
| ThermoMate CZ422LP | Up to 5 GPM; 102,000 BTU | Indoor, with 3-inch vent connection | CSA certification stated in product details |
Our Picks: Best Tankless Hot Water Heater Propane reviews
Rheem RTG Series: High Listed Output for Outdoor Installation
The Rheem RTG Series stands out on paper for its listed 7.0 GPM output and 160,000 BTU rating. The listing says it can provide hot water for two to three simultaneous showers at a 35°F temperature rise, making it a candidate for a larger household with several bathrooms.
Its title identifies the unit as a liquid propane model for outdoor installation. Outdoor placement can avoid the need for indoor venting, but it still requires a suitable location, correct gas supply, and installation by a qualified professional. The listing also describes a digital temperature control, freeze protection, flame-failure protection, and a metal cabinet.
There is an important detail to resolve before ordering: the product information also refers to natural gas and indoor installation, which conflicts with the title. Confirm the exact fuel type, installation location, and model documentation with the seller or manufacturer before purchase. Do not assume the unit can be converted between fuels.
Pros:
- High listed flow rate and 160,000 BTU output
- Outdoor design may simplify venting needs
- Digital temperature adjustment and listed safety features
- Listed 15-year heat exchanger and 5-year parts warranties
Cons:
- Product details conflict about fuel type and installation location
- Professional installation is required
Verdict: Consider the Rheem for a high-demand home only after confirming that the exact unit is an outdoor propane model and checking its output against your local winter water temperature.
Rinnai RUCS65IP: A Flexible-Venting Option for Household Use
The Rinnai RUCS65IP is a compact propane tankless heater with a stated maximum flow of 6.5 GPM. Its product details describe service for as many as five appliances at once, though actual performance depends on how much hot water those fixtures use and the temperature rise your home needs.
Rinnai lists multiple venting options, including PVC or CPVC when used with the specified adaptor. That flexibility may help when planning an indoor installation, but vent materials and layout must match the heater’s instructions and local requirements. The brand recommends professional installation and provides a locator for qualified technicians.
The listed residential warranty is 12 years for the heat exchanger, five years for parts, and one year for labor. Compared with the Rheem, the Rinnai has a lower stated maximum flow but avoids the same listing conflict about fuel and installation type. Its product details do not provide a BTU rating or UEF, so compare those figures with your installer’s sizing advice.
Pros:
- Up to 6.5 GPM listed flow
- Multiple venting options are described
- Compact design intended to save space
- Listed 12-year heat exchanger warranty
Cons:
- BTU capacity and UEF are not provided in the supplied product details
- Professional installation is recommended
Verdict: The RUCS65IP is worth considering if you want a compact propane unit with flexible venting and a clearly stated heat exchanger warranty, but confirm that its capacity fits your peak demand.
APUS Indoor Propane Heater: Compact Choice for Moderate Demand
The APUS is an indoor propane model rated at 95,000 BTU and up to 4.42 GPM. Its product details say it can serve two showers and a sink, and its low-flow start is listed at 0.66 GPM. Those figures make it a more moderate-capacity choice than the Rheem or GEARPASS, rather than an automatic fit for every large household.
The unit has an LED touchscreen and a stated temperature range of 95°F to 140°F. APUS lists a compact body, a 120V power connection, freeze protection, and several safety shutdown features. The product details also call for draining the unit when it will be unused for a long time in freezing conditions; built-in freeze protection should not be treated as a substitute for following the manual’s winterizing instructions.
APUS lists a 0.81 UEF and up to 98% thermal efficiency. These are manufacturer-provided figures and should not be compared as if they were the same measurement. This model’s stated two-year coverage is shorter than the heat exchanger warranty listed for the Rheem and Rinnai.
Pros:
- Compact indoor design with a touchscreen control
- Listed 4.42 GPM flow and 95,000 BTU output
- Low-flow start specification may suit smaller draws
- Freeze and other safety features are described
Cons:
- Lower listed capacity than the highest-output options here
- Long periods of cold or disuse require careful winterizing
Verdict: Choose the APUS if its stated flow matches your household needs and you want a compact indoor model, but plan for proper venting and cold-weather care.
GEARPASS 6.3 GPM: High-Capacity Indoor Propane Option
GEARPASS lists this indoor propane heater at 6.3 GPM and 160,000 BTU. That puts its stated flow above the APUS and ThermoMate, while its BTU rating matches the Rheem. Actual hot-water output will still vary with incoming water temperature and simultaneous use.
The unit has a stainless steel housing, an oxygen-free copper heat exchanger, and an LED control panel with a stated setting range of 95°F to 149°F. The product information includes a horizontal vent pipe and says proper exhaust installation is required. It also lists standard electrical power and G 1/2-inch water and gas connections, so have an installer check compatibility with your home before ordering.
GEARPASS describes the heater as suitable for homes, RVs, cabins, and rental units. That broad use claim does not replace checking local code, the installation manual, and the exact ventilation and gas requirements for your intended location. A two-year warranty is listed, but the supplied details do not state an efficiency rating or a certification.
Pros:
- High listed capacity of 6.3 GPM and 160,000 BTU
- Digital temperature control with a broad setting range
- Stainless steel housing and copper heat exchanger are specified
- Two-year warranty is listed
Cons:
- Indoor installation requires correct exhaust venting
- Certification and efficiency details are not stated in the supplied information
Verdict: The GEARPASS is a plausible high-output indoor option if its installation requirements and documentation check out, but verify certification and venting details before committing.
ThermoMate CZ422LP: Feature-Rich Indoor Model with Listed CSA Certification
The ThermoMate CZ422LP is listed at up to 5 GPM and 102,000 BTU. The manufacturer says it can supply two showers at once under specified conditions, including a 65°F incoming water temperature. That qualification matters: colder inlet water can reduce the flow available at your chosen output temperature.
This indoor model has a 3-inch vent connection, a 120V plug, a 3.5-inch LED display, and temperature settings from 90°F to 140°F. Its product details list eight safety protections and say it is CSA-certified. The unit is also described as having automatic frost protection, but proper draining and winterizing instructions still matter when it will be unused in freezing conditions.
ThermoMate lists a 0.81 UEF and up to 98% thermal efficiency, along with a two-year warranty. Its stated output is lower than the GEARPASS, Rheem, and Rinnai, but it offers more installation and monitoring detail than some of the other options. Confirm the unit’s venting and gas requirements with a qualified installer.
Pros:
- CSA certification is stated in the product details
- Digital display shows operating information and error codes
- Listed safety protections and frost protection
- Provides a stated flow and BTU rating for easier comparison
Cons:
- Listed maximum flow is below the higher-capacity models
- Indoor installation requires compatible venting and electrical power
Verdict: The ThermoMate is a good candidate if you want an indoor heater with a detailed control display and listed CSA certification, provided its 5 GPM maximum suits your household’s peak demand.
How to Choose a Propane Tankless Water Heater
Size for Flow and Temperature Rise
Gallons per minute tells you how much water a heater may deliver under stated conditions, but the number alone is not enough. List the fixtures you might use at the same time, such as showers, a sink, or a dishwasher, and estimate their combined flow. Then consider temperature rise: the difference between the cold water entering the heater and the hot water you want at the tap. Colder incoming water means the heater must work harder, so its available flow may be lower than the headline maximum.
Confirm Propane Compatibility
Check that the exact model is designed for liquid propane, not natural gas. Do not rely on a broad product title if other listing details disagree. The Rheem listing, for example, contains conflicting references to propane, natural gas, indoor installation, and outdoor installation. Ask the manufacturer or seller to confirm the model and fuel type, and have a qualified installer verify that your propane supply is suitable.
Choose Indoor or Outdoor Installation
Indoor models need an approved exhaust setup and enough clearance for safe installation. Outdoor models avoid indoor venting but still need an appropriate location and protection that follows the manufacturer’s instructions. Check the manual for vent type, vent length, clearances, and any location limits before buying. Do not assume a vent pipe included with a heater is suitable for every home.
Check Power, Connections, and Local Requirements
Some tankless heaters need household electricity for controls, fans, or freeze protection. Compare the listed voltage and plug with the installation site, along with gas and water connection sizes. Ask a licensed professional to check local code, permits, gas-line capacity, and required safety measures. Product claims about possible uses do not mean every model is approved for every location.
Plan for Freezing Weather
Freeze protection can help, but it is not a guarantee against damage during a power outage, extreme cold, or long periods without use. Read the manufacturer’s winterizing instructions before installation. Some product details specifically call for draining the unit when it will be unused for an extended time in freezing conditions. Outdoor placement makes it especially important to understand the model’s cold-weather limits.
Compare Warranty and Efficiency Carefully
Warranty coverage can differ for the heat exchanger, parts, and labor, so check each term rather than comparing a single warranty number. Efficiency figures also need context: UEF and thermal efficiency are different measures and should not be treated as interchangeable. Look for complete documentation, including certification details, maintenance steps, and warranty conditions, before making your final choice.
Frequently Asked Questions
How many gallons per minute do I need?
Start by adding the flow needs of the fixtures you may use at the same time. Then ask an installer to account for your incoming water temperature and desired hot-water setting. A heater’s maximum GPM may not apply at the temperature rise your home needs.
Can a propane tankless heater run on natural gas?
Do not assume it can. Propane and natural gas models are not automatically interchangeable. Use only the fuel specified for the exact model, and ask the manufacturer and a qualified installer about any approved conversion options.
Does a tankless heater provide unlimited hot water?
It can heat water on demand rather than storing a fixed tankful, but it still has a maximum heating capacity. If several fixtures draw more hot water than the unit can heat at once, flow or temperature may fall.
Do indoor propane tankless heaters need venting?
Indoor gas heaters need an exhaust system installed according to the manufacturer’s instructions and local requirements. Check the approved vent material and layout for the exact model, and have a qualified professional handle the installation.
Is built-in freeze protection enough for an outdoor heater?
Not in every situation. Freeze protection may depend on power and operating conditions, and it may not cover extended outages or severe cold. Follow the unit’s manual for drainage and winterizing, especially if the heater will sit unused.
Are the UEF and thermal-efficiency numbers directly comparable?
No. They are different measures, so a thermal-efficiency percentage should not be compared directly with a UEF figure. Use the same type of rating when comparing models, and check the manufacturer’s full product documentation.
Final Recommendation
For a high-demand household, the Rheem and GEARPASS have the highest listed output, but each requires careful verification of installation details—and the Rheem listing has a notable fuel and location conflict. The Rinnai offers a strong stated flow and a longer listed heat exchanger warranty, while the APUS and ThermoMate may better suit households with more moderate demand. Before ordering any propane tankless water heater, confirm the exact fuel, size the unit for your coldest incoming water, and have a qualified installer review the venting and gas supply.
