9 Best Propane Tankless Water Heater For Radiant Heat of 2026 To Keep Floors Warm

1

APUS Tankless Water Heater Propane Gas, Up to 4.42 GPM,

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  9.8
• Brand
  APUS

Key specs2

Heating Capacity & Temp Range

95,000 BTU/hr with 95°F to 140°F range

Continuous Hydronic Suitability

The 95,000 BTU rating fits small radiant zones, but buyers must verify closed-loop space heating approval before connecting to radiant manifolds.

Pros & cons42

Pros

• Delivers 95,000 BTU output with precise digital temperature control from 95°F to 140°F
• Low-flow ignition activates at 0.66 GPM to support lower circulation rates
• Listed operating noise of 40 dB allows quiet indoor utility room placement
• Oxygen-free copper heat exchanger provides over 85 percent thermal efficiency

Cons

• Hydronic space-heating certification is not specified by the manufacturer
• Maximum temperature caps at 140°F which limits high-temp baseboard installations

2

Tankless Water Heater Propane,2.64GPM 10L Outdoor Portable Water Heater,with 2

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  9.6
• Brand
  forimo

Key specs2

Off-Grid Portability

D-cell battery ignition with included 12V pump

Venting & Space Heating Fit

Built specifically for outdoor camping and washing, this unit lacks an indoor flue adapter and cannot be operated safely inside living areas.

Pros & cons32

Pros

• Bundled with a 12V 2 GPM pump and CSA-approved propane regulator for off-grid setups
• Lightweight portable frame with folding handles weighs very little for easy transport
• Capable of delivering a maximum temperature rise of 167°F for demanding thermal needs

Cons

• Outdoor-only design lacks dedicated indoor venting required for occupied living spaces
• D-cell battery ignition and 68,000 BTU output are not rated for continuous radiant duty

3

NEW Rinnai RX199iN Condensing Smart Sense Natural Gas or Propane

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  9.0
• Brand
  Rinnai

Key specs2

Thermal Output & Efficiency

199,000 BTU with 0.98 UEF condensing rating

Whole-Home Radiant Capability

With 199,000 BTU and a 0.98 UEF rating, this condensing model can sustain substantial radiant heating loads across multiple domestic zones.

Pros & cons42

Pros

• Massive 199,000 BTU capacity provides ample heat energy for demanding radiant floor loops
• Exceptional 0.98 UEF condensing efficiency minimizes long-term liquid propane consumption
• Versa Vent technology allows flexible indoor or outdoor installation configurations
• Smart Sense system automatically configures for propane or natural gas fuel sources

Cons

• Condensing setup requires dedicated drainage routing for internal acidic condensate
• Substantial 55-pound cabinet weight and complex plumbing require professional installation

4

Rinnai RE140iP Non-Condensing Propane Tankless Water Heater, Up to 5.3

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  9.0
• Brand
  Rinnai

Key specs2

Warranty Coverage

15-yr heat exchanger, 5-yr parts, 1-yr labor

Mid-Sized Loop Heating

The 140,000 BTU burner provides dedicated indoor heating power for workshops or home additions, supported by comprehensive factory warranty terms.

Pros & cons42

Pros

• Solid 140,000 BTU output supplies steady continuous heat for mid-sized floor loops
• Backed by a 15-year residential heat exchanger warranty and 5-year parts protection
• Dedicated indoor non-condensing design avoids complex condensate drain routing
• Digital LED display provides diagnostic service reminders and error codes

Cons

• Non-condensing design yields lower thermal efficiency than condensing alternatives
• Requires professional technician installation to validate residential warranty coverage

5

Ranein Propane Gas Tankless Water Heater, Indoor Max 3.6 GPM,

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  8.6
• Brand
  Ranein

Key specs2

Dual Chamber Design

Dual internal heating chambers help stabilize water temperature between 95°F and 149°F, reducing rapid cycling in modest radiant runs.

Burner Output

80,000 BTU with 3.6 GPM maximum flow

Pros & cons42

Pros

• Dual heating chamber design improves temperature uniformity and heat transfer
• Digital dynamic linkage maintains steady water output between 95°F and 149°F
• Compact 18-pound indoor footprint mounts easily in cramped mechanical spaces
• Oxygen-free copper heat exchanger resists corrosion under continuous water circulation

Cons

• Output of 80,000 BTU limits heating coverage to small rooms or well-insulated zones
• Hydronic loop compatibility and duty cycle ratings are not documented in the manual

6

Propane Tankless Water Heater, 4.74GPM 18L Indoor,Constant propane water heater,Instant

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  8.6
• Brand
  VELPAX

Key specs2

Included Venting Hardware

Includes 2.5-inch horizontal vent pipe and fittings

Low-Pressure Operation

Ignites at just 3.6 PSI of water pressure, allowing circulation in low-head loops, though the 68,000 BTU rating restricts total square footage.

Pros & cons42

Pros

• Includes a 2.5-inch horizontal vent pipe in the box to facilitate proper indoor exhaust
• Low 3.6 PSI activation threshold accommodates lower-pressure closed loops
• Durable stainless steel exterior shell resists corrosion in humid utility environments
• Wide temperature range from 95°F to 149°F matches typical radiant slab parameters

Cons

• Output rating of 68,000 BTU will struggle to satisfy radiant loads in cold climates
• Requires manual drainage in freezing conditions if loop circulation ever stops

7

Outdoor Propane Tankless Water Heater - 6L 1.58 GPM Portable

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  8.4
• Brand
  Amzgachfktch

Key specs2

Minimum Starting Pressure

2.5 PSI minimum water activation threshold

Cabin & Camp Utility

Designed for outdoor off-grid washing, this 40,000 BTU heater lacks the heating output and indoor venting required for permanent floor loops.

Pros & cons32

Pros

• Ultra-low 2.5 PSI starting pressure activates under minimal hydronic pump head
• Lightweight 12-pound build with folding handle provides easy mounting in utility sheds
• Complete setup kit includes CSA-approved regulator and garden hose adapters

Cons

• Modest 40,000 BTU output cannot sustain residential radiant floor heat loops
• Strictly outdoor application with no safe exhaust venting for indoor living spaces

8

Tankless Water Heater Propane with Digital Display, 3.66 GPM 14L

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  8.2
• Brand
  Nessxa

Key specs2

Maximum Water Temperature

113°F maximum temperature setting

Low-Temperature Loop Fit

The 113°F maximum output matches low-temperature concrete slab parameters, but verify indoor flue requirements before installation.

Pros & cons32

Pros

• Produces 85,300 BTU output at a light total weight of 10.67 pounds
• Operates within a low-pressure window starting at 3.6 PSI up to 116 PSI
• Digital display provides straightforward monitoring of active water temperature

Cons

• Maximum temperature ceiling of 113°F prevents higher water temperature requirements
• Venting hardware and closed-loop hydronic certifications are not specified in the data

9

Tankless Water Heater Propane 4.21GPM 16L Outdoor Portable GasHot Water

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  7.4
• Brand
  forimo

Key specs2

Plumbing Thread Standard

BSP pipe fittings require US plumbing adapters

Hydronic Limitation

Intended for outdoor recreational use with 68,000 BTU output, this model lacks indoor venting provisions for permanent radiant heating.

Pros & cons32

Pros

• Delivers up to 4.21 GPM water flow with a CSA-approved gas regulator included
• Operates across a wide water pressure spectrum between 3.6 PSI and 110 PSI
• Compact casing with advanced burner combustion helps suppress operational noise

Cons

• Equipped with British Standard Pipe threads requiring adapters for standard US plumbing
• Outdoor-only design lacks an exhaust flue connector for safe indoor installation

Last update on 2026-09-25 / Affiliate links / Images, Product Titles, and Product Highlights from Amazon Product Advertising API

Can a standard propane tankless water heater run an in-floor radiant heat loop?

It depends on the specific heater design and whether the manufacturer certifies the unit for closed-loop hydronic space heating. Domestic on-demand units are engineered for intermittent hot water draws, while radiant floor loops circulate water continuously for hours. High-output models like the Rinnai RX199iN supply up to 199,000 BTU, and the Rinnai RE140iP provides 140,000 BTU, but you must check the manual to confirm space-heating approval before connecting to a radiant manifold. Connecting an unapproved domestic heater can void your warranty and lead to premature heat exchanger stress.

How many BTUs are needed to keep radiant floors warm?

Your required heating capacity depends on the heat loss of your structure and total loop footage rather than domestic fixture counts. Propane tankless options range from compact residential units like the Ranein at 80,000 BTU and the APUS at 95,000 BTU up to larger units like the Rinnai RX199iN delivering 199,000 BTU. Radiant loops require sufficient continuous thermal output to overcome slab or subfloor heat loss during freezing weather. If an isolated room or workspace falls outside your radiant manifold coverage, using a space heater for indoor use can warm that zone without overloading your central hydronic setup.

What venting setup does an indoor propane radiant heater require?

Indoor propane units require dedicated venting engineered to exhaust combustion gases safely out of the building. Condensing models like the Rinnai RX199iN feature versatile venting designs like Versa Vent for indoor or outdoor configurations, venting exhaust safely through dedicated pipe runs. Non-condensing units such as the Rinnai RE140iP, the Ranein 80,000 BTU model, and the APUS AP-AWH-Q13LPG require direct indoor exhaust venting. You must check manufacturer clearance requirements and vent pipe specifications to prevent combustion gases from entering living or work areas.

Can you use a portable outdoor propane water heater for radiant floor heating?

No, portable outdoor propane water heaters are not built for permanent radiant floor systems. Compact units rated around 40,000 BTU or models designed as a tankless water heater for outdoor shower use are intended strictly for intermittent, open-discharge outdoor tasks. These units, including portable 2.64 GPM models with integrated 2 GPM water pumps, lack the pressure containment, indoor venting channels, and continuous duty heat exchangers needed for closed hydronic circulation. Running a portable outdoor heater indoors also creates a severe carbon monoxide hazard.

What flow rate is needed to activate a propane tankless heater on a radiant loop?

Your hydronic circulation pump must push enough water volume to meet or exceed the heater's minimum flow activation threshold. If loop resistance restricts water movement below this starting rate, the burner will not fire. The APUS AP-AWH-Q13LPG lists a low-flow activation requirement of 0.66 GPM with a maximum flow rate of 4.42 GPM. Other models support higher capacities, such as the Ranein at 3.6 GPM and the Rinnai RX199iN at up to 11.1 GPM. Verify that your circulator pump and manifold design maintain flow above the unit's activation rate across all active heating zones.

How does temperature regulation affect radiant floor heating?

Radiant floor loops require steady temperature control to avoid overheating finished flooring materials or cycling the burner erratically. Certain propane models feature precision digital regulation, such as the APUS unit maintaining water temperature within 1°F of the setpoint, or the Ranein providing an adjustable range from 95°F to 149°F. Other units, like the 85,300 BTU 14L heater, list operating temperatures reaching up to 113°F with a minimum of 17°F. Finished flooring materials such as hardwood, tile, and laminate have strict surface temperature limits, so verify your heater's minimum output temperature aligns with your hydronic system design.