Rheem Tankless Water Heater: Types, Sizing and What to Check

A Rheem tankless water heater heats water only when a tap or appliance calls for it, so it does not keep a stored tank warm around the clock. Whether that is a better fit than a tank depends on how much hot water you run at once, which fuel you have, how far the exhaust must travel, and how hard your water is. This guide explains the choices in plain terms so you can walk into a conversation with an installer or a product page and ask the right questions.

It is a general explainer built from published manufacturer documentation and common industry practice. It does not recommend a specific model, and it makes no claims about particular units. Always confirm specifications, ratings and installation requirements on the current listing and in the manual for the exact model.

The short answer: who a Rheem tankless heater suits

Tankless heaters tend to suit households that value a continuous supply, want to free floor space, or are replacing an aging tank and have the utilities to support a switch. They are less appealing when the home has limited electrical or gas capacity, long pipe runs from the heater to the fixtures, or very hard water with no softening plan. Because Rheem sells several families of tankless units, the right choice depends on matching fuel, flow rate and location to your house rather than choosing by name alone.

Types of tankless water heater

Manufacturers, Rheem included, generally organize tankless products along a few lines. Check the current catalog and documentation, since model families and features change over time.

Gas versus electric

Gas-fired units burn natural gas or propane to heat a heat exchanger. They can reach high flow rates, but they need a gas supply that can deliver enough volume, plus exhaust venting. Electric units use heating elements and avoid combustion, which simplifies venting, but they draw large amounts of current, so wiring, breakers and panel capacity become the main constraint.

Condensing versus non-condensing

Condensing gas models extract extra heat from exhaust gases and typically use plastic venting, because the exhaust is cooler and acidic condensate forms; a condensate drain is needed. Non-condensing models vent hotter exhaust through metal venting. The trade-offs usually involve price, efficiency ratings and installation details. Verify what the listing states.

Indoor versus outdoor

Outdoor-rated gas models mount on an exterior wall and avoid indoor venting, but they need freeze protection in cold areas. Indoor models save you from weather exposure but need a vent path through a wall or roof. Which is easier depends on your house layout.

Whole-home versus point-of-use

A whole-home unit serves the entire plumbing system. A small point-of-use electric unit sits near one sink or shower, which can shorten waits for hot water at a distant fixture, though it will not supply the whole house.

What makes tankless appealing, and where it falls short

The main attractions are on-demand heating, a smaller wall-mounted footprint, and no standby tank to keep warm. Many buyers also like that the supply is not limited by a tank’s volume. Published efficiency ratings can look favorable compared with tanks, but real savings depend on your usage, fuel prices and the unit’s condition, so treat any savings number as an estimate.

The downsides are equally concrete. Installation is often more complex and costly than a straight tank swap, especially when gas lines, venting or electrical service must be upgraded. Units need a minimum flow to ignite, so very low flows from a tap can produce lukewarm or fluctuating water. Mineral scale shortens life when maintenance is skipped. And since the heater is a single point of failure, a breakdown affects the whole system until it is repaired.

Sizing: flow rate and temperature rise

Two numbers drive sizing. The first is flow rate in gallons per minute (GPM), which is how much hot water you want at the same time. The second is temperature rise, which is the difference between the incoming cold water and your desired output temperature. A heater delivers fewer GPM as the required rise grows, so the same unit will perform differently in Florida and in Minnesota in winter.

A worked sizing example with stated assumptions

The following is an illustration, not a recommendation. Assume a household of four that sometimes runs a shower at about 2.0 GPM and a second shower at 2.0 GPM at the same time, while a kitchen faucet draws 1.5 GPM. The peak simultaneous demand is then 5.5 GPM. Suppose the coldest incoming water in winter is 45 degrees F and the target is 120 degrees F, which is a rise of 75 degrees F.

At a 75 degree rise, many whole-home units deliver noticeably fewer GPM than their headline figure, because the headline number is usually quoted for a smaller rise. You would therefore look at the maker’s chart for the 75 degree row and confirm that the unit reaches at least 5.5 GPM there. If it falls short, options include a larger unit, two units, or accepting a rule that two showers should not overlap with the kitchen tap.

Note that the fixtures’ flow ratings differ by product, and the household’s habits matter. This example uses round numbers only to show the method.

Installation: venting, gas, electrical and codes

Water heater installation involves gas, electricity, water and exhaust gases. Mistakes can cause carbon monoxide exposure, fires, leaks or scalding. Follow the manufacturer’s instructions and local codes, and have a qualified, licensed professional handle gas lines, electrical connections, venting and plumbing. Many areas require permits and inspections.

Ask an installer to confirm the following in writing: the gas line size and meter capacity for gas units; the breaker and wire requirements for electric units; the vent material, length and route; condensate drainage for condensing models; the location of a pressure relief valve and its discharge; and any isolation valves for flushing. Also ask how the work affects your warranty, since many warranties are conditional on proper installation.

Install location matters for noise, freeze risk and service access too. Leave clearances the manual specifies, and do not store flammable items near a gas appliance. Consider a carbon monoxide alarm near any fuel-burning appliance, as recommended by safety authorities.

Maintenance, water quality and lifespan

Hard water leaves mineral scale on the heat exchanger. Scale reduces heat transfer and can cause error codes or noise. Manufacturer documentation commonly recommends periodic descaling or flushing with a suitable solution and pump, with the frequency depending on water hardness. Inlet filter screens should be checked and cleaned as directed.

If your water is hard, consider a conditioner or softener as part of the plan. Reading your municipal water quality report, or using a basic hardness test kit, will tell you where you stand. You may also want to review options for treating water at the point of entry, such as the whole house carbon water filters we cover elsewhere, though note that carbon filtration addresses chlorine and taste rather than hardness.

Lifespan claims vary by model and by how well the unit is maintained. Treat any number in marketing as a typical expectation rather than a promise, and keep the registration and warranty paperwork where you can find it.

What to check before buying

• Exact model number, fuel type and whether it is rated for indoor or outdoor use
• Flow rate at your temperature rise, from the manufacturer’s chart
• Minimum activation flow versus your low-flow fixtures
• Existing gas line size, meter capacity, or electrical panel capacity
• Vent type, route and clearance requirements
• Condensate handling for condensing units
• Warranty terms, including labor coverage and installer requirements
• Descaling method and the cost of isolation valves for flushing
• Permit rules and whether your insurer or utility has requirements
• Written installation quote that lists parts, labor and any panel or gas upgrades

Because the rated numbers on a product page are seller or manufacturer claims, confirm them in the official documentation. If the listing is unclear or contains typos in a spec, treat that as a reason to ask questions before purchase.

Buyer questions to ask the installer

A written quote is the best way to compare options. Ask whether the price includes new venting, a gas line upgrade, a condensate neutralizer, isolation valves and permit fees, and ask who handles the inspection. Ask how the installer calculated your flow requirement and whether they used your local winter water temperature. Ask what happens if the unit cannot keep up, for example whether a second unit or a recirculation setup is an option.

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Frequently asked questions

How do I size a tankless water heater for my home?

Add up the flow rates of the fixtures you expect to run at the same time, in gallons per minute, then work out the temperature rise your climate needs by subtracting the incoming water temperature from your target temperature. A heater’s output in gallons per minute falls as the required rise increases. Compare that calculation with the maker’s published flow-versus-rise chart rather than a rule of thumb.

Do tankless water heaters give endless hot water?

They heat water on demand, so there is no stored supply to run out, but there is a maximum flow they can heat to your target temperature. If simultaneous demand exceeds that limit, the water gets cooler or the flow is restricted. So the supply is continuous rather than unlimited. Sizing to your busiest realistic moment is what prevents disappointment.

Is a gas or electric tankless heater better?

Neither wins everywhere. Gas units generally deliver higher flow rates but need a suitable gas supply, venting and often a larger gas line. Electric units are simpler to vent but can require significant electrical capacity and dedicated circuits, which may mean a panel upgrade. Your existing utilities, local energy prices and the climate you live in usually decide it. Have a qualified professional assess the house.

Do tankless heaters need maintenance?

Yes. In areas with hard water, mineral scale can build up on the heat exchanger and reduce performance, so periodic descaling or flushing is commonly recommended in maker documentation. Inlet screens may also need cleaning. Follow the maintenance schedule in the manual for your exact model, and have a qualified technician handle any gas, electrical or venting work.

Can I install a tankless water heater myself?

Gas, electrical and plumbing connections on a water heater carry real risks, including gas leaks, carbon monoxide and scalding. Many jurisdictions require permits and licensed installers, and improper installation can void a warranty. Even confident DIYers should check local codes and the manufacturer’s instructions first, and most households are better served by hiring a qualified professional for the installation.