Key takeaways
- Choosing a Rinnai tankless water heater? Learn fuel and venting types, how to size by flow rate and temperature rise, and what installation involves.
Short answer: when you shop for a Rinnai tankless water heater, the brand name is only the starting point. The model that suits your home depends on four things: the fuel you have available, how much hot water you draw at once, how cold your incoming water gets, and whether the unit can be vented and installed where you plan to put it. Get those right and most of the choice follows. This guide explains the types, shows how to estimate sizing with a worked example, and lists what to confirm on any listing. It does not rely on specifics about any particular model, so check the manufacturer’s documentation for exact figures.
How a tankless water heater works
A conventional storage heater keeps a tank of water hot around the clock. A tankless, or on-demand, heater heats water only when a hot tap opens. Cold water flows through a heat exchanger, a burner or electric element warms it, and the heated water continues to the fixture. When the tap closes, the unit stops.
That design avoids keeping a full tank warm, which is why tankless units are often promoted for efficiency and for a continuous supply of hot water. The trade-off is that output is limited by the unit’s heating capacity. A tank holds a reserve, while a tankless unit has to heat everything as it flows. If the demand is higher than the capacity, the temperature falls or the flow drops.
Some people also like the smaller wall-mounted footprint. Others are surprised that installation can be more involved than replacing a tank, because gas lines, venting and electrical connections are often different from what the old heater used.
The main types you will see
| Choice | Options | What it affects |
|---|---|---|
| Fuel | Natural gas, propane, electric | Output, running cost, needed utilities |
| Location | Indoor or outdoor rated | Venting, freeze protection, weather exposure |
| Combustion design | Condensing or non-condensing (gas) | Efficiency, vent material, condensate drain |
| Venting | Direct vent or other arrangements | Air supply and exhaust path |
| Features | Recirculation, Wi-Fi, temperature control | Convenience and wait time for hot water |
The same maker can offer several of these combinations. A listing for a Rinnai tankless water heater may cover indoor or outdoor units, natural gas or propane versions, or condensing and non-condensing models, which is why the exact model name and number matter. Confirm them on the manufacturer’s spec sheet, not just in the headline.
Gas or electric: choosing a fuel
Gas units usually have higher heating capacity, so they are common for whole-home use where several fixtures may run together. They require a gas supply of adequate size, a safe venting path and, for many designs, a condensate drain. A gas line sized for an old tank heater may be too small for a high-output tankless unit.
Electric units avoid combustion and venting, which can simplify placement. However, higher-output electric models may demand substantial electrical service, and an older panel may need an upgrade. Smaller electric units can be a fit for a single sink or a small bathroom rather than a full house.
Propane units are a variation on gas that depends on the tank or supply at your property. Natural gas and propane models are generally not interchangeable without the correct conversion components, so check which version the listing is for.
Sizing by flow rate and temperature rise
Sizing is the step people most often skip. Two numbers matter: flow rate in gallons per minute, and temperature rise, which is the difference between your incoming water temperature and the temperature you want at the tap.
A common rule of thumb says that heating one gallon per minute by one degree Fahrenheit takes about 500 BTU per hour. Multiply flow by temperature rise by 500 to estimate the heat required, then account for the unit’s efficiency. This is a rough estimate, not a substitute for the manufacturer’s performance data.
Worked example with stated assumptions
Assume a household that might run a shower at 2.0 gallons per minute and a dishwasher or kitchen tap at 1.5 gallons per minute at the same time, for 3.5 gallons per minute total. Assume incoming water at 50 degrees Fahrenheit in winter and a target of 120 degrees, so the rise is 70 degrees.
| Step | Calculation | Result |
|---|---|---|
| Peak simultaneous flow | 2.0 + 1.5 gpm | 3.5 gpm |
| Temperature rise | 120 – 50 F | 70 F |
| Heat needed (ideal) | 3.5 x 70 x 500 | about 122,500 BTU/h |
| Adjusted for an assumed 90% efficiency | 122,500 / 0.90 | about 136,000 BTU/h input |
In this example you would look for a model whose published performance at a 70-degree rise meets at least 3.5 gallons per minute, and you would still add a margin. If your coldest incoming water is colder than the assumption, or if you run a third fixture, the requirement rises. Conversely, a warmer climate lowers the needed output. An installer can check your local water temperatures and codes.
Venting, gas supply and electrical needs
A gas tankless heater needs a safe way to bring in combustion air and send exhaust outside. Venting materials and clearances depend on whether the unit is condensing and on local rules. Poor venting can cause carbon monoxide hazards, so do not improvise. Most gas models also need a small electrical connection for controls and fans, and some need a condensate drain.
Check the gas line diameter and length, the meter capacity and the regulator, because a high-input unit can demand more gas than a tank heater did. Also consider water pressure, since a unit may require a minimum flow to start and may include an inlet filter that needs periodic cleaning.
Because these tasks involve gas, electricity and plumbing, hire a qualified professional, pull permits if your area requires them, and follow the manufacturer’s instructions and local codes.
Water quality and maintenance
Hard water leaves mineral scale, which can build up on the heat exchanger and reduce flow and efficiency over time. Many manufacturers recommend periodic flushing with a descaling solution, and some suggest more frequent service in hard-water areas. If your supply is hard, consider treatment upstream. Our page on whole-house water softener systems explains what softening does and how to think about sizing.
Other routine tasks include cleaning inlet filters, checking vents and condensate drains, and watching for error codes. Keep the owner’s manual, record the install date and serial number, and note when you last flushed the unit. In cold climates, find out what freeze protection the model offers and what happens during a power outage, since many units need electricity to operate.
Recirculation, wait times and comfort
Tankless heaters deliver hot water on demand, but the pipes between the heater and the fixture still hold cooled water, so you may wait for hot water at a distant tap. Recirculation systems keep warm water moving through the lines and cut that wait, though they can add energy use and require extra plumbing. Some models have built-in recirculation features, while others need an external pump.
You may also notice brief temperature swings when you shut a tap and reopen it quickly. That behavior is common in on-demand systems and can often be reduced with settings or accessories. Ask about minimum flow requirements too, because very low flows can prevent a unit from firing.
What to confirm on a listing
- The exact model name and number, and whether it is indoor or outdoor rated
- Fuel type: natural gas, propane or electric, and any conversion requirements
- Maximum flow rate at a stated temperature rise
- Whether venting parts, valves or isolation kits are included or sold separately
- Warranty terms, including conditions such as professional installation
- Whether the seller is an authorized dealer or installer
- Return policy for heavy or special-order items, since freight returns can be costly
- Requirements for gas line size, electrical supply and drainage
Who it suits and who might skip it
A tankless unit tends to suit households that want endless hot water in long showers, have limited floor space, or are replacing a tank that is near the end of its life and can accommodate the venting and gas needs. It can also suit second bathrooms or additions where running a long pipe from a central tank would waste water and time.
It may be a poorer fit if your home has a small gas line that would be costly to upgrade, if the water is very hard and you will not maintain the unit, or if you mostly use small amounts of hot water in short bursts, where the savings are small. Households that run many fixtures at once may need a large unit or two units, which changes the cost picture. When in doubt, ask for a sizing calculation from the installer in writing.
Common mistakes
Typical errors include sizing by the number of bathrooms instead of by peak flow and temperature rise, assuming the existing gas line is adequate, buying the wrong fuel version, forgetting the cost of venting, and skipping maintenance. Another is trusting a listing title that mentions a brand but not the exact model. Ask for the model number before you commit, and check it against the maker’s documentation.
Costs to plan for
The price of the unit is only part of the picture. Installation can include gas line upgrades, new venting, electrical work, a condensate drain, isolation valves, permits and disposal of the old heater. These costs vary widely by home and region, so get written quotes from licensed professionals. Compare quotes line by line, and ask what is included. Then consider the running cost, which depends on fuel prices, your usage and the model’s efficiency. Savings compared with a storage heater are possible but are not guaranteed, particularly for small households.
Related guides
- Indoor Electric Heaters For Large Rooms
- Electric Tankless Water Heaters
- Reverse Osmosis Water Filters
Frequently asked questions
How do I size a tankless water heater for my home?
Add up the hot water fixtures you expect to run at the same time, note their flow rates in gallons per minute, and estimate the temperature rise between your cold incoming water and your target. A heater must deliver that flow at that rise. Manufacturers publish performance charts, and an installer can confirm sizing for your climate.
Should I choose gas or electric?
Gas models, fueled by natural gas or propane, generally have higher output and suit whole-home demand, but they need venting and a suitable gas supply. Electric models avoid venting yet may need heavy electrical service. Your existing utilities, local codes and the model’s rated output decide which is practical, so ask a qualified professional.
Can I install a tankless water heater myself?
Gas, venting, electrical and plumbing work carries real risks, including gas leaks, carbon monoxide and scalding, and many areas require permits and licensed trades. Follow the manufacturer’s instructions and local codes, and hire a qualified professional for the installation. Warranty terms may also depend on professional installation, so read them before you buy and keep the installer’s paperwork.
Do tankless heaters need maintenance?
Yes. Mineral scale from hard water can build up inside the heat exchanger and reduce performance, so manufacturers typically recommend periodic flushing or descaling, along with checks of inlet filters and venting. Follow the schedule in your model’s manual, and consider water treatment if your supply is hard. Skipping service can shorten the life of the unit and may affect warranty coverage.
Why does my hot water sometimes turn cold briefly?
Tankless systems can produce a short burst of cold water when hot water is turned off and on quickly, because water left in the pipes cools and the unit needs a moment to restart. Low flow rates can also keep a unit from firing. Ask about recirculation options and minimum flow requirements for your model.
Bottom line
Pick the fuel and location first, size the unit by flow rate and temperature rise, and plan for venting, gas and electrical needs before you compare prices. Confirm every detail on the manufacturer’s documentation, hire a qualified professional for installation, and plan for maintenance if your water is hard. Follow the manufacturer’s instructions and local codes throughout.
