Six gallons is not six gallons of shower
Nobody showers in 130 degree water. The valve mixes tank water with cold until it reaches something like 105, and the proportion of each is fixed by the three temperatures. With a 130 degree tank, 55 degree inlet and a 105 degree shower, the hot fraction is 50 over 75, or two thirds. Every three gallons out of the head is two gallons of tank water and one of cold.
Run that backwards and the six gallon tank produces nine gallons of shower water. That is the mixing ratio shown in the results, and it is the reason people who divide tank gallons by shower flow always underestimate. At 1.5 gallons a minute, nine gallons is six minutes before recovery is counted at all.
The ratio is not a constant. It moves with all three temperatures, and it moves most with the inlet. Filling from a mountain spigot at 45 degrees rather than a sun-warmed hose at 75 changes the ratio substantially, which is why the same coach gives noticeably different showers in April and August with nothing altered.
Recovery, and why it matters less than you would hope
An RV water heater on gas recovers in the order of six gallons an hour. That sounds useful until it is expressed per minute: a tenth of a gallon. Against a draw of one gallon of hot water per minute, recovery is replacing ten percent of what you are using. It extends the shower from six minutes to about six and two thirds, which is real but not transformative.
Where recovery earns its keep is in the gaps. Fifteen minutes between two people puts a gallon and a half back, which is a quarter of a six gallon tank and often the difference between the second person getting a full shower and getting most of one. The same effect is what makes turning the water off while soaping so effective: the burner runs through the pause, so a four minute shower spread across eight minutes in the cubicle collects twice the recovery. That is the mechanism behind the advice, and it is a better reason than virtue.
The four levers, ranked
| Change | What it does | Catch |
|---|---|---|
| Lower flow shower head | Proportional — a third less flow is a half more time | Below about 1.0 gal/min it stops feeling like a shower |
| Water off while soaping | Cuts running minutes and adds recovery in the pause | Requires a head with a shut-off button |
| Higher thermostat set point | Raises the mixing ratio, so the tank stretches further | Scald risk rises at every tap, not just the shower |
| Bigger tank | Directly proportional | Weight, space, and a longer initial heat-up |
The first two cost almost nothing and are worth more than the second two. Raising the set point deserves a second look before it is adopted as a habit: it works, and the reason it works is that you are storing water that is more dangerous at the tap. Children and older people scald at lower temperatures and shorter contact times than most adults assume, and a coach has a galley tap and a bathroom tap fed from the same tank as the shower.
Tankless, and what changes
An on-demand heater has no stored volume, so the shower length is limited by nothing except water in the fresh tank and gas in the bottle. What limits it instead is flow at temperature rise: the unit can only heat so many gallons a minute by so many degrees, and cold inlet water in winter is exactly the case that pushes it past its limit. If a tankless unit is failing to hold temperature, the fix is a lower flow head rather than more gas.
To model one here, set the tank size to zero and put the unit continuous output into the recovery field as gallons per hour. The continuous minutes figure will then read as unlimited whenever recovery meets the draw, which is the correct answer, and will show a finite number when the head is asking for more than the unit can heat.
The last minute is not the same as the first
The arithmetic here treats the tank as a reservoir of water at the set point that empties evenly. Real tanks are worse than that. Cold water enters at the bottom as hot leaves the top, and although stratification keeps them somewhat separate, mixing is continuous. The practical result is that the temperature at the head starts drifting down before the calculated time is up, and the valve has to be turned toward hot to compensate — which raises the hot fraction and empties the tank faster still. Treat the number above as the point where it stops being a comfortable shower, not the point where the tank contains no warm water.
Related pages
The water leaving the head also fills the grey tank, which is worked on the RV holding tank duration calculator, and heating it burns propane on the RV propane consumption calculator. On electric, the heater is one of the larger loads on the RV shore power load calculator. The mixing arithmetic in general is on the shower mixing temperature calculator, and household sizing on the water heater sizing calculator.
Questions people ask
How many showers can I get from a 6 gallon RV water heater?
With a 130 degree tank, 55 degree inlet and a 105 degree shower through a 1.5 gallon per minute head, the tank gives about six and a half minutes of continuous running. That is one comfortable four minute shower with a little to spare, and a second full one if you leave fifteen minutes for recovery in between. Two people back to back with no gap will find the second one cut short. Every one of those numbers moves with the inlet temperature and the head flow, so run your own figures rather than adopting the six minute answer as a fact about six gallon heaters.
Why does the tank give more shower water than its own capacity?
Because the shower is mixed. Nobody stands under 130 degree water, so the valve blends tank water with cold to reach about 105, and with a 55 degree inlet that blend is two parts hot to one part cold. Six gallons of tank water therefore produces nine gallons at the head. The ratio is shown in the results and is worth watching, because it climbs when the inlet is cold and falls when it is warm — the same tank stretches further in summer than in winter for reasons that have nothing to do with the heater.
Does turning the water off while soaping really help?
It helps twice, which is why it works better than it sounds. It removes the running minutes directly, so a four minute shower uses four minutes of hot water rather than eight. And the burner keeps recovering through the pause, so the tank is fuller when you turn the water back on. On a six gallon tank with a six gallon per hour recovery, a four minute pause returns nearly half a gallon, which is eight percent of the tank recovered for free. Set the running share field above to 50 to see both effects.
Should I turn the thermostat up to get longer showers?
It does work, and the results above show by how much for your own numbers, but it is the lever with a cost attached. A hotter tank means a higher mixing ratio and more shower minutes, and it also means every tap in the coach delivers water that scalds faster. Children and older people are injured at lower temperatures and shorter exposures than adults expect, and RV taps are not always fitted with the mixing protection a house might have. If you do raise it, be deliberate about it rather than leaving it high permanently for a trip that turned out not to need it.
How do I find my heater recovery rate?
It is on the data plate, usually expressed as gallons per hour at a stated temperature rise. Gas recovers faster than electric on most RV heaters, and a unit that can run both together recovers faster than either alone. If the plate is unreadable, you can measure it: heat the tank fully, draw it down completely at a known flow, then time how long the burner runs before the thermostat satisfies. Tank gallons divided by that time in hours is your recovery rate, and it will be your real one rather than a nominal one.