RV Water Heater Calculator

How long does a 6-gallon RV water heater last in the shower?

About 5.3 minutes with the water running, on spring water through a normal RV showerhead. The usual answer of five to ten minutes spreads that wide because of two numbers, the inlet temperature and the showerhead. The tank holds 130 °F water but you shower at 105, so the valve mixes: on 55 °F inlet water the mix is 67% hot, and a 1.5 gpm showerhead pulls 1.00 gpm from the tank. Only about 70% of a tank comes out hot before the incoming cold has mixed the rest lukewarm, 4.2 gallons of a 6, and the burner is putting 0.20 gpm of hot back while you stand there. So 4.2 ÷ (1.00 − 0.20) = 5.3 minutes, 7.9 gallons of shower. By inlet water:

1.5 gpm showerhead, tank at 130 °F, shower at 105 °F, 70% of the tank usable, 12,000 BTU/h burner at 63% putting hot back during the shower. A navy shower is 3 gal of mixed water. On the 6-gallon tank at 55 °F, a 2.0 gpm head cuts the running shower to 3.7 minutes and a 1.0 gpm low-flow head stretches it to 9 minutes.

The navy-shower column is the boondocking number: two people can shower back to back from a hot 6-gallon tank on spring water, and the tank is ready for a third after 9.9 minutes on gas. The boondocking-days calculator counts the fresh water those showers drink.

How long does an RV water heater take to heat up?

A gallon needs 8.34 BTU per degree. Six gallons from 55 to 130 °F is 3,753 BTU. Suburban's 6-gallon SW6D burns 12,000 BTU/h and gets about 7,560 of it into the water; the 1,440 W element on the SW6DE delivers 4,569 BTU/h, and with both on they add:

Efficiencies from Suburban's tank water heater specifications, which list the 12,000 BTU/h SW6D at 10.1 gal/h on gas and the 1,440 W element at 6.1 gal/h (the 10-gallon SW10D burns the same 12,000). The sheet doesn't state the temperature rise; at the 90 °F rise this page assumes, those work out to 63% and 93%. Colder inlet water adds minutes in proportion to the rise.

That covers most of the gas-or-electric question. The element is slower, free on a bundled pedestal, and 12 A of a 30 A service that the air conditioner also wants. Gas is faster and costs 0.022 gal of propane a navy shower. Both together is fastest. On hookups run both; off-grid the element has nothing to run on.

Will an RV tankless water heater keep up with a shower?

A tankless unit never runs out, but it can only lift the water so many degrees at a given flow: BTU/h = gpm × 60 × 8.34 × rise. The Girard GSWH-2 and Furrion F2GWH are both rated 42,000 BTU/h. At 80%, this page's assumption since neither maker publishes a thermal efficiency, that delivers 33,600 BTU/h. To reach 105 °F from 55 °F water that allows 1.34 gpm, less than the showerhead, so at 1.5 gpm the water arrives at 99.8 °F. The unit is working as designed; it is temperature-limited, and turning the tap down is the fix:

80% thermal efficiency is this site's planning figure; the makers publish input ratings only. 105 °F shower. Girard and Furrion both state a minimum of 0.80 gpm per hot faucet (Girard's controller starts at about 0.90), and the Truma AquaGo lights above about 1.5 liters a minute, 0.40 gpm. Below that the flame drops out, so a navy shower's "water off, soap up" restarts the burner each time, and the cold slug that follows is the "cold sandwich."

So at 42,000 BTU/h a tankless unit is a summer-and-south appliance, and at 55,000 an all-season one, as is a 42,000 unit with a low-flow head. Truma's AquaGo modulates from 20,000 to 60,000 BTU/h, so at full fire it does a little better than the 55,000 column, and it aims for a 120 °F outlet. On the propane side a 42,000 unit burns 0.46 gal/h only while water runs, 0.061 gal for an 8-minute shower, and nothing on standby.

How much propane does an RV water heater use?

A navy shower on gas is 0.022 gal of propane at 55 °F inlet. The other ways to shower, and a crew of four over a week:

55 °F inlet, tank at 130 °F. Standby: no RV maker publishes a standby loss, so this page borrows the federal ceiling for commercial gas storage water heaters in 10 CFR 431.110, Q/800 + 110√V BTU/h, as a stand-in. For the 12,000 BTU/h, 6-gallon tank that is 284 BTU/h lost from the water, replaced at 63%: a gallon of propane every 8.4 days for a heater left lit. A 20 lb cylinder is 4.7 gal; the propane calculator adds the furnace, fridge and stove.

Should you run an RV water heater on gas, electric, both or tankless?

On a pedestal, run the element all the time and add gas when a queue forms, after checking the pedestal. The element is 12 A continuous, and on 30 A with a 13,500 BTU air conditioner it is the pair that trips the breaker when the microwave starts. The result panel runs that check against your rig, and the shore-power calculator runs it with everything else on the circuit. Off-grid: gas, lit before the showers and off after. Tankless: buy the BTU for the coldest water you camp in rather than the length of the rig, and fit a 1.0 gpm showerhead, which on 55 °F water raises a 42,000 unit's outlet by 22 °F for the price of a showerhead. When the season ends, the tank gets drained and bypassed before any antifreeze goes in; the winterizing calculator shows what skipping that step costs.