Hi,
The general concnesis is that tankless might work fine with city water supply, and in a campground, with warm ground water (at least 55F). However with a small 2 GPM water pump, variable water flow of a RV, not the ability to run endless amounts of water to get the temperature right, and not the ability to run the water heater at less than 1/4 gallon per minute, most tankless water heaters will not meet the demands of a average RV. And if you happen to camp up north with 40F ground water, you might have either a trickle of water or a really cold shower. Most water heaters only warm the water by about 60F when at 1.2 gallons per minute.
Some RV tankless water heaters have fixed gas valves, that say will heat 1 GPM at 60F and to get warmer water, you must slow the flow, to get it cooler, increase the flow. Yet if you have a tank of 70F water, then the resulting 130F water needs to be sped up more than 2.5 GPM to prevent scalding. So turn on the cold water? Probably not going to work, as turning on the cold water will decrease flow through the hot water heater and then it will go over 140F and shut off. So you go from really hot shower to insta-cold one!
That said, I bought a $120 L5 water heater from Amazon.com. I use that water heater to warm the water going into my 100 gallon fresh water tank. I had a problem with 45F water going into my RV in the winter, and the 6 gallon tank only lasts for about 3 minutes. A really short shower. By heating the water to 80F in the fresh tank, I can take a 15 minute shower and it is still warm!
The L5 water heater is really simple. It does have spark ignition, with a pair of D batteries to run it. The gas valve is something that is turned for full heat, and to the right to get less heat. You would adjust the output heat by reducing or increasing flow, and turning the valve. There is a 130F overtemp safety.
What to look for in a higher quality tankless built in water heater. Needs at least 40,000 Btu maximum heat output. Needs to have three temperature sensors, one at the inlet, mid heat exchanger, and one at the outlet. This will provide feedback to the controller and change the variable gas valve to open or close it as temp changes. Accurate flow monitor, so the heater can detect changes in flow to cause it to increase or slow heat output, to match the load. Perhaps even a mixing valve at the outlet to prevent scalding.
Fred.
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