RV Water Heater Not Getting Hot Enough
The Short Answer: Lukewarm RV water is almost never “buy a 10-gallon.” Bypass valves left halfway mix cold into hot. Dual-fuel tanks lose gas or electric and still feel “on.” A tempering valve or a packed anode makes 6 gallons of 140°F feel like 105°F. Ice-cold is a different job. Name the heat path, then hire an RV tech if it is not the valves.
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Search Intent & The Real Problem
You have water. It is not hot. Forums sold a bigger Suburban and a vinegar jug. A 6-gallon Atwood/Dometic or Suburban SW6DE/SW10DE that never gets past tepid is mixing, one dead heat source, scale, or a setpoint — not family-of-four math. Ice-cold with a fault lamp or click-no-light is ignition, not this page. Tankless in the same hole — Girard, AquaGo, Suburban Nautilus — is flow and lockout, not a 6-gallon anode.
Coach skins (Forest River, Keystone, Thor, Grand Design, Jayco) are still Suburban or Atwood inside. Read the door badge.
What other guides get wrong
They start with “is 6 gallons enough for a family of four.” That is a sizing argument. This search is tepid right now. They skip the winterize valves, so you buy a board for a tank that is still in bypass. They mash the red ECO with the tank empty. They paste house dip-tube articles onto an aluminum Atwood that does not have one. And they tell you to crank the thermostat to scald as a test.
Step-by-Step Troubleshooting
Step 1: Lukewarm vs short vs ice-cold
| What you feel | Likely cause | Next |
|---|---|---|
| Always tepid at every fixture, tank jacket only warm | Bypass mixing, mixer, low setpoint, scale | Step 2–4 |
| Kitchen hot, shower tepid | That cartridge or a long run, not a dead tank | Cartridge vs mixer |
| Hot for a minute, then tepid, recovers if you wait | One heat path dead (gas or electric), or 6 gallons used up | Step 3 |
| Ice-cold, click or red fault LED | Ignition / DSI — wrong page | Fault light, fires up then out |
| Scalding then ice | Mixing cartridge failed | Cartridge, not a new heater |
- What to look for: A probe thermometer at the nearest hot faucet and a hand on the tank jacket (warm = something is heating). 120°F at the tank and 105°F at the shower is mixing, not a “weak heater.”
- Required tools: Contact thermometer, flashlight, phone photo of the door badge.
Step 2: Bypass / winterize valves — the free save
- What to look for: Three-valve or two-valve bypass left in bypass or halfway after winterize. Cold city water or the pump is teeing around the tank into the hot line. You get a trickle of heat from residual, then tepid forever. Handles should be normal / tank in circuit — both tank ports open, the crossover closed. Confirm against the sticker on that bay, not a forum photo of a different manifold.
- Required tools: Eyes on the valves. A bucket if you crack a drain plug later.
A tank you just filled after storage can take 20–40 minutes on LP, longer on a 120 V element. Time it once with bypass proven normal before you condemn the box.
Step 3: Prove gas and electric as two machines
Dual-fuel tanks have independent heat paths. Electric can be dead while propane still makes a lukewarm shower, and the reverse.
| Path | Pass | Fail |
|---|---|---|
| Interior electric switch + shore / generator / inverter | Jacket gets hot in 15–30 min, no LP | Pedestal 105 V, ECO, element, bypass |
| Interior LP switch, bottles open, other appliances light | Burner proves, jacket hot | DSI, igniter, ECO, empty tank |
| Both “on,” jacket still cool | Empty, bypass, or dual ECO | Fill, then one ECO press with power off |
- What to look for: Suburban electric is a different thermostat than Atwood/Dometic. Gas-dead: Suburban on gas, Atwood clicking. Electric-dead: Suburban electric, Atwood electric. Coach-brand trees: Forest River, Keystone electric.
- Required tools: Interior switches, a hot faucet, ears at the exterior door. Shore unplugged before covers.
Do not hit electric on a tank you cannot prove full — Suburban reset only on a full tank.
Step 4: Mixer, P&T, and city-water pressure
- What to look for: A tempering / anti-scald valve on the hot outlet set to 105°F. City pedestal over ~80 psi with no regulator — the 6-gallon becomes the expansion tank and the P&T dribbles, stealing heat and water. Lever someone lifted to “test.” Typical relief 150 psi / 210°F. Never cap it. Full drip path: RV P&T.
- Required tools: Hose-bib / city-inlet gauge if you have one. Eyes on the P&T tube.
A mixer failed closed-to-hot feels like a dead heater. A mixer failed open scalds. Do not crank the tank thermostat to 160°F to beat a mixer.
Step 5: Scale, anode, and when the tank is the limit
- What to look for: Suburban steel tanks with a never-changed anode, popping for years, lukewarm even on a long LP cycle — sediment blankets the bottom and the element or burner heats mud. Atwood/Dometic aluminum usually has no anode; scale still skins the element. Tankless boxes: flow below activation or a dirty inlet filter looks like “not hot enough.” Wet bay, rust at the seam, drain-plug bead: stop chasing temperature — bottom leak.
- Required tools: Paper towel. Door open, LP and electric off. Do not Flex Seal a pressure vessel.
A 6-gallon that does hit 120°F at the jacket and still feels short after two showers is capacity. That is not this diagnostic. This page is water that never gets hot.
When DIY Ends: Calling a Licensed Pro
You may set bypass to normal, confirm bottles and shore power, time one heat cycle, and photograph a fault lamp. Stop at propane odor, water in the burner box, an ECO that trips twice, a wet jacket, and any board or gas-valve swap. Element, DSI module, and mixer work in a tight bay is RV-tech territory. Same-day licensed diagnosis beats another weekend of lukewarm showers and a melted harness.
Frequently Asked Questions
Q: Why is my RV water heater not getting hot enough? A: Bypass valves mixing cold into hot, only one of two heat paths working, a tempering valve, low setpoint, or scale. Ice-cold with a fault light is ignition, not lukewarm. Feel the tank jacket before you buy a heater.
Q: How long should an RV water heater take to heat up? A: On LP, a full 6-gallon is often 15–30 minutes; 10-gallon longer. Electric is slower. If the jacket is still cool after that with bypass in normal, the heat path is dead — not “it needs more time.”
Q: Can I just turn the RV water heater thermostat up? A: Only after you prove it is not mixing. Cranking a working tank to scald hides a bypass valve and burns the next person. If the jacket is already 120°F and the shower is 105°F, fix the mixer or the valves.