RV Tankless Water Heater Sizing and Safety Kit

    A plain-language kit for RVers, van dwellers, campers, and off-grid cabin owners. Use it to estimate the flow rate and temperature rise a portable tankless water heater needs, and to run through the propane, ventilation, and scald safety checks before you use one.

    A portable outdoor shower running warm water beside an RV at a lakeside mountain campsite, fed by a wall-mounted propane tankless water heater, with temperature, water, ventilation, and safety icons.

    How to Size a Tankless Water Heater for an RV

    Sizing comes down to two numbers. The first is flow rate, measured in gallons per minute (GPM): how much hot water you want to run at once. The second is temperature rise, the difference between the temperature of the water coming in and the temperature you want coming out. A tankless unit has a fixed heating capacity, so those two numbers pull against each other.

    Incoming water temperature matters more than most people expect. Summer water from a warm campsite spigot might arrive near 70°F, while winter groundwater in a cold region can be 40°F or lower. If you want a 105°F shower, that is a 35°F rise in the first case and a 65°F rise in the second. Because a unit can only add so much heat per minute, a bigger temperature rise means fewer gallons per minute of hot water.

    This is why the large “maximum GPM” number on the box can be misleading. That figure is usually measured at a small temperature rise. In cold weather, or with cold groundwater, the flow you can actually get is lower, sometimes much lower. As a general rule, tankless units supply roughly 2 to 5 GPM, and correct sizing depends on both peak flow and the required temperature rise. When you compare models, read the flow rate off the manufacturer's performance chart at your expected temperature rise, not the single headline number.

    To estimate simultaneous demand, add up the fixtures you would realistically run at the same time. In most RVs and campsites that is one fixture at a time, which is what portable units are built for. If two people might shower back to back, or someone runs the galley sink while another showers, add those flows together and size for the total.

    Estimated Flow by Use Case

    Use this as a starting point only. Every number below is an estimate for planning, and real deliverable flow depends on your incoming water temperature and the specific unit. Verify against the manufacturer's performance chart before you buy.

    Use caseEstimated flowNotes
    Hand washing / one faucet0.5 to 1.0 GPMA single low-flow tap. The lightest demand.
    One low-flow RV shower1.0 to 1.5 GPMMost RV and camp shower heads are built for low flow.
    Shower plus one faucet1.5 to 2.5 GPMA shower running while someone uses the galley sink.
    Two simultaneous fixtures2.5 to 3.5 GPMTwo showers, or a shower plus a kitchen fixture at once.

    Estimates are for typical low-flow RV and camp fixtures. Your fixtures may differ, and colder incoming water lowers what any unit can deliver. Confirm with the manufacturer's performance chart.

    Sizing Calculator

    Enter your own numbers below. The calculator estimates the temperature rise and the combined flow rate to look for. It does not recommend a product, and it cannot account for everything that affects real performance.

    Sizing Calculator

    Enter your numbers to estimate the temperature rise and the flow rate a unit would need to deliver. Everything updates as you type.

    Groundwater is colder in winter and in northern climates, which raises the temperature rise your unit must produce.

    A comfortable shower is often near 105°F. The CPSC recommends setting water heaters to about 120°F to limit scald risk.

    Most low-flow RV and camp shower heads run near 1 to 1.5 GPM.

    Add a second fixture only if it would run at the same time as the shower.

    50°F
    Calculated temperature rise
    1.5 GPM
    Estimated required flow

    To cover this use, look for a unit that can deliver at least 1.5 GPM at a 50°F temperature rise. That is the number to read off the model's performance chart, not the single headline GPM figure.

    This is a planning estimate only. It does not recommend a specific product. Real-world output is also affected by elevation, the propane supply pressure, a unit's electrical or battery ignition, and the manufacturer's own specifications. Confirm any choice against those before you buy.

    Portable Propane Safety Checklist

    Portable propane tankless heaters are safe when used the way the manufacturer intends. The points below cover the risks that matter most. They are general guidance: your unit's manual is the authority on exact clearances, pressures, and limits, and it overrides anything here.

    Outdoor use and carbon monoxide

    • Use the unit outdoors only, in open air. It is a combustion appliance that produces carbon monoxide, a colorless, odorless gas. The CPSC warns against running fuel-burning heaters in enclosed spaces such as tents, campers, and vehicles.
    • Keep the burner and exhaust in open air, well clear of doors, windows, vents, and air intakes so exhaust cannot drift into an enclosed space. Follow the clearance distances in your manual.
    • Keep a working carbon monoxide alarm where you sleep, and know the early symptoms of CO exposure: headache, dizziness, weakness, nausea, and confusion. If you feel them, get to fresh air.

    Propane, water, and connections

    • Use the regulator and hose supplied with the unit. Inspect them for cracks or wear before each trip.
    • Check connections for leaks with a soapy-water solution, never a flame. Watch for bubbles, and tighten or replace as needed.
    • Shut the propane off at the tank when the unit is not in use, and disconnect for transport and storage.
    • Stay within the water-pressure range in your manual. Many portable units are built for low-pressure RV or gravity-fed supplies, and too much pressure can damage them.

    Scalding, freezing, and when to call a pro

    • Guard against scalds. The CPSC recommends setting water heaters to about 120°F (49°C). Test the water by hand before use, since a portable unit's output can swing as flow and incoming temperature change.
    • Protect against freezing. Drain the unit before storing it in cold conditions, following the manufacturer's draining steps, so trapped water cannot freeze and crack internal parts.
    • Follow the manufacturer's instructions first and always. For a permanent installation, a new gas line, or any fixed indoor unit, follow local codes and use a qualified professional where one is required.

    Optional: Free Summary for Website Owners and Editors

    This section is for website owners, editors, and writers, not something a reader needs to size or safely use a unit. If you publish content about RV or portable water heaters, you are welcome to reuse the summary below. It gives your readers a concise sizing explanation and the five safety checks that matter most. Copy it as plain text, or grab the self-contained HTML version, which has no scripts and no tracking.

    You may quote, adapt, or republish this information for free. Attribution to Tankless Geek is appreciated but not required. A backlink is not a condition of use.

    Plain-text version

    HTML version

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    Want a copy for the road? Use the button below to print the kit or save it as a PDF. The print layout drops the site navigation and interactive controls and keeps the sizing guide, table, and safety checklist.

    Keep Reading

    Sources

    The safety facts on this page are drawn from the following primary sources. Specific points, such as the 120°F setting and the warning against running fuel-burning units in enclosed spaces, are attributed to them above. Sizing figures are planning estimates: always confirm them against the manufacturer's performance chart.

    Frequently Asked Questions

    How many GPM does an RV tankless water heater need?+
    As a rough estimate, one low-flow RV shower runs comfortably at about 1 to 1.5 GPM, and a shower plus a galley faucet at the same time is closer to 2 to 2.5 GPM. These are starting points, not guarantees: the flow a unit can actually heat drops as the incoming water gets colder. Always check the figure against the manufacturer's performance chart for the temperature rise you expect.
    Why does cold incoming water lower the flow rate?+
    A tankless heater has a fixed heat output, so the colder the incoming water, the harder it has to work to reach your target temperature and the less water it can heat per minute. The large "maximum GPM" number on the box is usually measured at a small temperature rise. In cold weather, or with cold groundwater, the flow you can actually get is lower. Size against the manufacturer's GPM-at-temperature-rise chart rather than the headline number.
    Can I use a portable propane tankless heater inside my RV, van, or tent?+
    No. Portable propane units are combustion appliances that produce carbon monoxide, and the U.S. Consumer Product Safety Commission warns against using fuel-burning heaters in enclosed spaces such as tents, campers, and vehicles. Use them only outdoors in open air, follow the manufacturer's instructions, and keep a working carbon monoxide alarm where you sleep.
    What water temperature is safe for showering?+
    The CPSC recommends setting water heaters to 120 degrees Fahrenheit (about 49 degrees Celsius) to reduce the risk of scald burns, which matter most for children and older adults. Test the water by hand before use, and remember that a portable unit's output can swing with changes in flow and incoming temperature.
    Do I need a professional to install one?+
    A portable outdoor unit that hangs on a hook and connects to a propane cylinder is generally set up by the owner. A permanent installation, a new gas line, or a fixed indoor unit is different work: follow local codes and use a qualified professional where one is required. See our installation tips for the wider picture.