How Long an RV Water Heater Takes to Heat Up

You switch the water heater on, and the question is how long until someone can shower. The honest answer depends on four numbers: tank size, burner or element rating, how cold the water going in is, and what temperature the thermostat stops at. Those are all known, so the time can be computed instead of guessed.

Short version from the spec-sheet math: a 6 gal Suburban on gas heats a cold tank in about 20 to 25 minutes, the same tank on the 120 V element alone takes about 45 to 50, and both together take about 15. A 10 gal tank on gas is closer to 40. Real tanks run a little longer than the arithmetic because the jacket loses heat and the first water out of the line has been sitting in cold pipe.

Everything in the tables below is computed from manufacturer input ratings, not measured on a heater. The method is shown so you can plug in your own data plate.

The physics, in one line

One BTU raises one pound of water one degree Fahrenheit. Water weighs 8.34 lb/gal. So the energy to heat a tank is:

BTU = 8.34 × gallons × temperature rise (°F)

A 6 gal tank going from 50 °F to 120 °F needs 8.34 × 6 × 70 = 3,503 BTU. Divide by the useful heat per hour the heater delivers, and you have the time in hours.

Useful heat is the input rating times efficiency:

• Gas: RV water heaters burn in an open combustion chamber with the flue out the side wall, and some heat goes out the vent. The figure used here is 78 percent, inside the 75 to 80 percent range typical for this design. Your heater could be a little better or worse, especially with a dirty flue.
• Electric: the element sits in the water, so nearly all its heat goes into the tank. The figure used here is 95 percent, allowing for jacket losses. A 1,440 W element is 1,440 × 3.412 = 4,913 BTU/h input, about 4,670 BTU/h useful.

The input ratings used

Heater	Gas input (spec)	Electric element (spec)
Suburban SW6DE, 6 gal	12,000 BTU/h	1,440 W
Suburban SW10DE, 10 gal	12,000 BTU/h	1,440 W
Atwood/Dometic GC6AA-10E, 6 gal	8,800 BTU/h	1,440 W

Ratings vary by model year and suffix. Read the data plate on your tank face before trusting a table, yours included. Some older 10 gal Atwood units are rated at 10,000 BTU/h, and a few Suburban electric elements are 1,000 W rather than 1,440 W.

Heat-up times, computed from spec ratings

Minutes to heat a full tank, assuming 78 percent gas efficiency and 95 percent electric efficiency. Not measured.

Configuration	Useful BTU/h	50 → 120 °F	50 → 130 °F	70 → 120 °F	70 → 130 °F
6 gal, gas (Suburban 12,000)	9,360	22 min	26 min	16 min	19 min
6 gal, gas (Atwood 8,800)	6,864	31 min	35 min	22 min	26 min
6 gal, electric only (1,440 W)	4,670	45 min	51 min	32 min	39 min
6 gal, gas + electric (Suburban)	14,030	15 min	17 min	11 min	13 min
10 gal, gas (Suburban 12,000)	9,360	37 min	43 min	27 min	32 min
10 gal, gas + electric (Suburban)	14,030	25 min	29 min	18 min	21 min

Read the 50 °F columns for spring and fall, or any time the fresh tank is cold. Read the 70 °F columns for summer city water. Winter well water can come in at 40 °F or below, which adds another 10 to 15 percent to the 50 °F numbers.

Add a few minutes for the real world: the tank, fittings, and burner box start cold, and the water in the line between heater and faucet has to be pushed out before hot water arrives at the tap.

Recovery rate: how many gallons per hour

Recovery rate is how many gallons per hour the heater can raise by a given amount while you’re drawing water. It matters more than heat-up time once the tank is hot and two people are showering back to back.

Gallons per hour = useful BTU/h ÷ (8.34 × temperature rise)

Configuration	Recovery, 70 °F rise (50 → 120)	Recovery, 50 °F rise (70 → 120)
Suburban 12,000 BTU/h gas	16 gal/h	22 gal/h
Atwood 8,800 BTU/h gas	12 gal/h	16.5 gal/h
1,440 W electric only	8 gal/h	11 gal/h
Suburban gas + electric	24 gal/h	34 gal/h

These are computed. Manufacturers often quote recovery at a 90 °F or 100 °F rise in sales literature, which gives smaller numbers for the same heater, so compare at the same rise.

A typical RV shower head flows 1.5 to 2.5 gal/min, and a low-flow head like an Oxygenics is often around 1.5 gal/min at RV pressure. At 1.5 gal/min, a 10-minute shower uses 15 gal of mixed water. No 6 gal tank keeps up with that from recovery alone, which is why the next sections matter.

Why gas and electric together cut the time so much

The two heat sources are independent. Each has its own thermostat and its own ECO on the tank, and the heat simply adds. On a Suburban 6 gal, gas delivers about 9,360 BTU/h useful and the element about 4,670, so together they deliver 14,030, 50 percent more than gas alone.

Against gas alone, that cuts heat-up time by about a third: 22 minutes becomes 15. Against electric alone it cuts it by two thirds: 45 minutes becomes 15. The common forum claim that running both “halves the time” is roughly right compared with the slower source and an overstatement compared with gas. Where it really pays off is recovery during back-to-back showers: 24 gal/h against 16.

The cost is propane plus 12 A of shore power for the element. On a 30 A pedestal with the air conditioner running, the element is one of the first things to trip the breaker.

Why the first shower is shorter in winter

Two effects stack. First, the tank takes longer to heat, because the rise from 40 °F to 130 °F is 90 °F instead of 60 °F. Second, and this is the one people notice, the hot water mixes with colder water at the shower valve.

To get a 105 °F shower from 130 °F tank water and 70 °F cold water, you need about 58 percent hot. With 45 °F cold water, you need about 71 percent hot. The same 6 gal tank produces about 10.3 gal of shower water in summer and about 8.5 gal in winter, before recovery. As you draw, cold water enters the tank and the hot water temperature falls, so the usable amount is a little less than these numbers in practice.

Hot fraction = (shower temp − cold temp) ÷ (tank temp − cold temp)

The mixing valve trick

A tempering (mixing) valve at the water heater outlet lets you store water hotter than you’d want at the tap and blends it down with cold before it leaves the heater. Hotter water stores more usable heat in the same 6 gal.

Using the same formula with 50 °F cold water and a 105 °F shower:

Tank temperature	Hot fraction	Shower water from 6 gal
120 °F	79%	about 7.6 gal
130 °F	69%	about 8.7 gal
140 °F	61%	about 9.8 gal

So a tank held at 140 °F and tempered at the outlet gives roughly 30 percent more shower water than one at 120 °F. Idealized, computed, but the direction is right.

Some Suburban and Dometic heaters now ship with a mixing valve option, and aftermarket tempering valve kits fit the 1/2 in outlet of most RV heaters. Two cautions. Don’t raise the storage temperature by modifying, jumpering, or bypassing the thermostat or ECO. Only use a heater or thermostat kit that is designed to run hotter, with the mixing valve installed. And water at 140 °F scalds in seconds, so the mixing valve has to be set and working before anyone uses an unmixed tap.

Thermostat and ECO settings

Most RV tank heaters have no temperature knob. The setpoint is fixed in the thermostat:

• Suburban gas thermostat: typically opens around 130 °F. The electric element has its own thermostat, typically a little lower or the same. Some newer Suburban models have an adjustable thermostat.
• Atwood/Dometic gas thermostat: typically opens around 140 °F on many models.
• ECO (energy cut-off): a separate safety switch on the tank that opens around 180 °F if the thermostat fails or the tank runs dry. It is not a temperature setting and must never be bypassed.

If the water comes out hotter than about 140 °F, the thermostat is suspect, and a water heater whose ECO trips with a full tank belongs in the stop box of the won’t light guide.

Tankless RV water heaters: a different limit

A tankless unit (Girard GSWH-2, Truma AquaGo, Furrion tankless, Dometic and Suburban tankless models) has no tank to heat, so there is no heat-up time beyond the few seconds to ignite. What limits it is how many gallons per minute it can raise by the temperature rise you need.

Max flow (gal/min) = useful BTU/h ÷ (500 × temperature rise)

(500 is 8.34 lb/gal × 60 min/h, rounded.)

The Girard GSWH-2 is rated around 42,000 BTU/h and the Truma AquaGo around 60,000 BTU/h; check your data plate. Computed at about 80 to 85 percent efficiency, to 120 °F at the outlet:

Unit input	70 °F inlet (50 °F rise)	50 °F inlet (70 °F rise)	40 °F inlet (80 °F rise)
~42,000 BTU/h	about 1.3 gal/min	about 1.0 gal/min	about 0.8 gal/min
~60,000 BTU/h	about 2.0 gal/min	about 1.5 gal/min	about 1.3 gal/min

In summer, either one keeps up with a low-flow shower head. In winter, a 42,000 BTU/h unit may not, and the shower runs lukewarm unless you cut the flow.

Three tankless behaviors that confuse people:

• Minimum flow to fire. The burner only lights above a minimum flow, typically around 0.5 gal/min. A faucet barely open will never get hot. Open it further.
• The cold-water sandwich. Turn the shower off to lather and back on, and a slug of cold water arrives between the hot water still in the pipe and the new hot water, while the burner relights. Units with a small buffer or a “comfort” mode reduce it. A tank heater has no sandwich.
• Pump pulsing. A demand pump that cycles at low flow can drop the flow below the minimum and cut the burner on and off. An accumulator smooths it; see the pump pulsing guide.

Measuring your own heater

To check whether your heater meets the computed numbers:

1. Start with a full tank that has sat overnight, gas and electric off. Read the cold tap temperature with a thermometer; that is your starting point.
2. Switch on gas only and start a timer.
3. When the burner shuts off (you’ll hear it stop at the exterior door, or see the burner go out), note the time.
4. Run the hot tap into a cup and read the temperature.

A Suburban 6 gal that takes 35 minutes from 50 °F to 130 °F on gas is running well below its rating. Look for a dirty burner or flue, low gas pressure, or a scaled tank. A tank full of scale holds less water and transfers heat worse; draining and flushing it is in the drain and anode guide.

Symptom → likely cause

What you see	Most likely	Check
Takes far longer than the table on gas	Dirty burner or flue, low gas pressure	Flame color, stove test, flue
Takes far longer on electric only	Element failed or 120 V switch off	Element resistance, breaker, rocker switch
Hot water runs out fast in winter	Cold inlet water, more hot in the mix	Expected; run both modes, mixing valve
Water only lukewarm, never hot	Bypass valve partly open, thermostat	Bypass valve positions, thermostat
Tankless runs lukewarm in winter	Flow above unit capacity at that rise	Reduce flow, low-flow head
Tankless won’t fire at a trickle	Below minimum flow	Open the faucet further

Parts and tools mentioned

• Kitchen or probe thermometer
• Gallon measuring bucket and a stopwatch (for flow rate)
• Tempering (mixing) valve kit sized for a 1/2 in water heater outlet
• Low-flow shower head (around 1.5 gal/min)
• Data plate on your heater, for the real input ratings

Questions people ask next

Can I leave the water heater on all the time? Yes, the thermostat cycles it. Standby losses on a 6 gal tank are modest, and many people leave the electric element on whenever they’re plugged in and use gas only for a quick recovery. Turn it off when the tank is drained or the rig is winterized; never run the element or burner dry.

Is it cheaper to heat water on gas or electric? At the campground, electric is usually free in the site fee. Off-grid, gas is the only practical option; a 1,440 W element pulls about 12 A at 120 V, too much for most inverter and battery setups.

Why does my 10 gal tank heat in about the same time as my old 6? If the old 6 gal was an Atwood at 8,800 BTU/h and the new 10 gal is a Suburban at 12,000 BTU/h, the times are closer than the tank sizes suggest: about 31 minutes against 37 by the math.

Do I need to wait for the full heat-up before showering? No. Hot water at the top of the tank is available sooner. You just get less of it.