Choosing a Rinnai Tankless Water Heater: Sizing, Cost, Maintenance & Most-Asked Questions

Choosing a Rinnai Tankless Water Heater: Your Most-Asked Questions, Answered

Rinnai is a category leader in gas tankless water heaters—known for efficient, on-demand hot water and long service life. Below you’ll find quick, practical answers to the questions people ask most when deciding which Rinnai to buy, how to size it, and how to maintain it.

Quick Sizing Basics (GPM + Temperature Rise)

1. List simultaneous fixtures (e.g., shower + dishwasher).
2. Add flow rates — shower ~2.0–2.5 GPM; bath faucet ~1.0–1.5 GPM; dishwasher ~1–2 GPM (varies).
3. Estimate temperature rise = 120°F desired − your winter groundwater temp (often 40–55°F).
4. Match the curve: Choose a Rinnai that delivers your required GPM at that temperature rise.

Example: Two showers (2.0 + 2.0) + a 1.0 GPM sink = 5.0 GPM. With 50°F groundwater, rise to 120°F = 70°F rise. Pick a model that produces ~5 GPM at ~70°F rise.

Top Questions About Rinnai 

How Does a Tankless Works?

Is a Rinnai tankless water heater any good?

Yes—Rinnai is a top brand with high efficiency (especially on condensing models), compact wall-mount form factors, and strong reliability when installed to spec. Many models offer built-in recirculation options for faster hot water at far fixtures.

How long does a Rinnai tankless last?

With proper installation and annual maintenance, many homeowners see 15–20 years of service. Register your warranty and keep records of professional service where required.

How often should I flush a Rinnai?

Annually in most homes; more often with hard water. Flushing (descaling) keeps efficiency high and extends the life of heat exchangers and valves.

What’s the most common problem with tankless heaters?

The most common issues are installation-related: undersized gas lines, improper venting/condensate handling on condensing units, or mineral scale due to skipped maintenance. Size the fuel and venting correctly and flush routinely to avoid headaches.

What size Rinnai do I need?

For many 1–2 bath homes, target ~5–7 GPM at 70°F rise. Larger homes or multi-shower use will need higher GPM models. We’re happy to sanity-check your numbers.

Condensing vs. Non-Condensing?

Condensing units are the most efficient and use cooler exhaust (often PVC/PP venting) but require a condensate drain. Non-condensing units cost less up front and use hotter exhaust (category III stainless vent) with no condensate line.

Indoor vs. Outdoor?

Indoor models vent through the wall or roof and are popular where freezing is a concern. Outdoor models save indoor space and don’t need venting but require a location safe from extreme weather per the manual. The RX Series can be installed in either location with the use of the Versa Vent. Outdoor Vent Cap easily adapts to any SENSEI® RX Series model for outdoor installations providing the ultimate flexibility (sold separately).

Natural Gas vs. Propane?

Rinnai offers both NG and LP versions. Choose the model built (or field-convertible) for your fuel and size the gas line to the input BTU—undersized piping causes error codes and poor performance. The units equipped with Smart Sense^™ offer proprietary Adaptive Gas Valve Technology adjusts to either Natural Gas or Liquid Propane Gas for optimal combustion - no conversion kit required.

Do I need recirculation?

If you have long pipe runs and don’t like waiting for hot water, yes. The RXP models offer built in pumps. You can use a dedicated return line or a crossover valve where allowed.

Cost & Installation (What to Expect)

• Equipment: Varies by GPM/output and condensing vs. non-condensing.
• Install: Venting route/material, gas line size/runs, recirculation hardware, and condensate management affect labor.
• Operating cost: High efficiency and no standby losses compared to tanks; maintenance (flush kits/filters) keeps costs predictable.

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Bottom line: whether you choose a high-efficiency SENSEI® condensing model (RX/RXP/RUCS/RUS) or the value-forward non-condensing line (RE/REP), a properly sized Rinnai tankless delivers endless hot water, compact installs, and lower standby losses than a tank. Match your GPM and temperature-rise needs, pick the venting/recirculation setup that fits your home, and you’re set for years of reliable performance.