I have a RV that I live in full time and I added a tankless in line with the factory tanked hot water heater (for trips or future plans I am leaving the tanked). I am having an issue of I can use up the 6 gallon tank of hot water, but the tankless fill it back with cold or barely warm water. So my question is how can I get the tankless to put out hot water before it needs to top off the tank as I use it? I have a couple ways in mind but they are very redneck and waste a bunch of water. Thanks for the help!

  • 1
    Clearly, the tankless heater cannot keep up with the flow of water. You need to restrict the flow, or use a more powerful heater. Oct 24, 2022 at 23:15
  • Sounds like they are in the wrong order, and that the tankless is not big enough (or not getting enough gas.)
    – Ecnerwal
    Oct 25, 2022 at 0:02

2 Answers 2


Clearly, the tankless heater cannot keep up with the flow of water. You need to restrict the flow, or use a more powerful heater. In an RV, it might not be possible to provide enough current for a large electric heater, though.

However, by keeping both tank and tankless heaters, you're getting the worst features of both. If you can get the tankless heater to keep up with the needed flow, and then hold that water in a tank, you're letting it cool off after all that energy was used to heat it!


Problem: my tanked heater has far less run-time than I'd prefer.

Solution: ??? throw random parts at the problem and pray.

Yeah, we get this "Imma connect a tankless to a tanked" question all the time. And most of the time we steer people away before they sink $ into it. But your experience is exactly what we expect.

Any tankless has a BTU rating, which amounts to the BTUs per hour it can put into the water. So you get an inverse graph between temperature rise and flow. The faster the flow, the less temperature rise. The slower the flow, the more temperature rise.

You are drawing water too fast for your heater to keep up. The answer is "reduce flow". This is the classic sizing error made by many people who contemplate tankless heaters. I gather you were unable to power a larger tankless due to gas or electric limits, e.g. the 50A available in a large RV, or the flow of gas possible with RV tanks.

Combining simply does not work - with one exception

You now know what goes wrong. When you are flowing water, it is going much too fast for the tankless heater to do any appreciable heating. Thus, the only heater doing anything at all is the tanked heater, and you have the same exact limitations as you did before. It doesn't matter whether you put the tanked before or after the tankless - same problem either way.

The net result is the same as the tanked heater; the undersized tankless is useless. (unless you reduce flow enough for the tankless to become effective).

If you do make the tankless big enough to be useful, then it's big enough to do all the heavy lifting, and you don't need the tanked heater at all! Goodbye tanked heater. Hello more space.

Remember this. The first rule of combining tanked and tankless is the tankless must be big enough to do the job alone, which means you don't need the tank.

The unhelpful-for-you exception is if you have surplus solar power, you can dump that into a tanked water heater ahead of the tankless. The tankless still needs to be big enough to do all the heavy lifting by itself. But having its inlet water pre-heated will let it the tankless save some fuel while the solar-heated water holds out. However this must be done carefully to avoid spreading disease or scalding people. And it's no use to you. It doesn't fix an "undersized" problem.

Option: Stay with the tankless.

In that case you will need to dramatically reduce flow. Install ultra-low-flow shower heads and aggressive flow restrictors as needed. Many people foolishly buy cheap tankless on eBay or Amazon, which are simply a Chinese knock-off of a British "electric shower". One should buy proper gear for fire-safety reasons, but a proper British electric shower comes with an ultra-low-flow showerhead reasonably matched to the unit.

Note that the flow reduction is positively painful for Americans used to the big blast of high-flow heads. A rule of thumb is 8-10,000 watts for every 1 gallon per minute you want.

Option: stick with the tanked heater

In this case you need to learn the RV method for taking showers. This is: turn it on long enough to get yourself wet. Turn it off and lather up. Turn it back on to rinse. Standing under the stream going "Ahhhhh" is not a thing, unless you want a larger heater tank.

With all tanked heaters, once the tank empties you are done. Recovery is very slow. This is normal.

"Power needed" and "tank size" have little to do with each other. So if you want a huge tank on a little heating element size (due to electrical or gas limits), that's not a problem as long as the RV frame can handle the weight of the extra water. The tank will simply take longer to recover back to temperature after use.

For instance a 20-gallon water heater with a 4500W element will take about 40 minutes to fully recover after running out. If that same heater is fed 120V, it will be 1150W, and will take 160 minutes to recover after running out. But it will recover.

Just don't break your RV's frame - water weighs 8.3 pounds per gallon!

  • in addition to solar, in an RV engine heat may also be an option for warming a tanked heater,
    – Jasen
    Oct 25, 2022 at 1:59
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    @Jasen Good point.... oh right, it's still the 2020s and electric hasn't reached RVs yet. Never seen a better use-case for battery electric - $300,000+ vehicles so a 400kWH battery is not a cost barrier. The lifestyle is laid back, so stopping for an hour at a 350kW DC fast charger is no big deal. Loads of roof space for solar. Now I want to build one lol. Oct 25, 2022 at 2:14
  • Thank you for the fast and very informative replies! I'm sorry I should have put more info in my post, I have a Bosch electric tankless that says 2.7gpm. The shower head is rated 1.8gpm. I run my camper at 22psi when in use. What seems like is happening IMO is that cold water is going through the tankless as it's ramping up to heating water and it gets stored in the tank and used before the tankless can properly do its job. The shower gets cold, get dressed, and water is hot. I'm just wondering if there is some sort of loop pump or some sort of bypass to preheat the tankless before showers? Oct 25, 2022 at 2:25
  • @BrandonSparks I think it's time you edit your question to include links or model numbers for tank and tankless heater. Because your facts simply do not match up to physical possibility. Methinks you don't have what you think you have. Oct 25, 2022 at 3:16

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