Just about every day we get this question: “Which tankless water heater will work for my RV / food truck / boat / 120V outdoor bar/sink?” The answer is probably none will work. Instead, the solution is probably a mini-tank. You may have ruled out getting a mini hot water tank due to size. But actually the 1.3 gallon, 2.5 gallon and 4 gallon models are surprisingly small. Read on for the deets:
First let’s abbreviate “incoming water temperature” to IWT. IWT is your cold water from the outside water supply/utility. Makes this article easier to read and shorter.
First key question: How much electrical power you can provide for the water heater?
If it’s 120V and 20 amps maximum, which is a common in RVs, food trucks, boats, and outdoor bars, then any/every* tankless water heater will heat your IWT by about 33°F at a flow rate of 0.5 GPM. And if your sink flows faster than 0.5 GPM, then the IWT increase will be proportionately less than 33°F. Right…not much. And no chance to get hot water for a shower.
Mini-tanks are surprisingly small and can even fit in boats, RVs and food trucks.
Example using a 120V and 20 amp tankless water heater:
If your IWT is 60°F, then your output temp will be 93°F, which only feels warm…not hot at all. Now extrapolate (calculate) that to your situation. In other words, your IWT must be higher than 72°F to get 105°F hot water for a sink.
* I wrote “any/every“. You see, electric tankless water heaters are 99% efficient. So if one tankless model, which uses 120V and 20 amps, can’t make enough hot water, then no other manufacturer or tankless model will.
Can you provide 220V? Better, but 20 amps is still not enough for a shower. A low-flow showerhead uses 1.5 GPM. So, with a 220V 20 amp tankless unit, the IWT increase would be only about 21°F! With 220V, a shower probably will work with 30 or 40 amps.
That’s why the only practical way to get hot water for a sink or shower in an RV, food truck, food stand, small boat, or outdoor bar, is a mini-tank. They run on only 120V 12 amps. The water temperature inside the tank can be set to 140°F. Since 140°F is too hot to touch, mix the cold and hot water with the faucet handle(s) to get a skin-comfortable output temperature of about 105°F (not scalding-hot 120°F for example). This will lengthen the time before the hot water runs out.
How long before the hot water runs out?
That’s the key question! The answer depends on a few factors:
• the flow rate of the point of use. For example, if you are using it with a kitchen sink, then the flow rate is typically 1.25 GPM. If bathroom sink, then the flow rate is typically 0.5 GPM, less than half of a kitchen sink, and makes a big difference.
• your IWT
• the tank temperature setting (which can be set from 60°F to 140°F)
• how much cold water you are mixing in at the faucet to get your preferred output temperature
• mini-tank capacity/size
Example 1 for a kitchen sink:
≈ kitchen sink faucet flow rate of 1.25 GPM
≈ IWT is 70°F
≈ minitank water temperature is 140°F
≈ minitank capacity is 2.5 gallons
Given all that, the hot water will last about 2 ½ minutes. Note that’s with the water constantly running. If you shut the water off when not immediately needed, then the hot water will last longer, because the mini-tank has time to reheat the water.
If any of these ≈ variables (especially a lower IWT, mini-tank size, flow rate, etc.) have different values, then change the numbers for your situation, and redo the calculation.
Example 2 for a low-flow showerhead:
≈ showerhead flow rate of 1.5 GPM
≈ IWT is 70°F
≈ minitank water temperature is 140°F
≈ minitank capacity is 6 gallons
Given all that, the hot water will last about 5 minutes. Note that’s with the shower constantly running. If you just have it on to get wet and rinse, and stop the water while soaping up (the “navy shower”), then the hot water will last much longer, because the mini-tank has time to reheat the water.
Will this water heater fit?
Dimensions of this 2.5 gallon mini-tank are only 11.75″ wide by 14.5″ high by 10.375″ deep. A 1.3 gallon mini-tank is a little smaller, and a 4 gallon mini-tank is a little larger.
GadgetsGo.com has been selling mini-tank and tankless water heaters since 2010, so we can give expert advice to you get hot water.