RV Tankless Water Heaters Explained: How They Work, Why They Go Cold, and Should You Upgrade

Few things in camper life spark more confusion (and more mid-shower yelping) than the tankless water heater. I get questions about these constantly: "How does it even work?" "Why won't mine get hot?" "Why did it go cold for three seconds and scare the daylights out of me?" "Should I upgrade my old tank to one of these?" So let's do a proper deep dive and answer all of it.

I'm Joe — Squatch to most folks — and today on Camping with Squatch we're getting to the bottom of the RV tankless water heater once and for all. Whether you've got one and you're fighting with it, or you're thinking about upgrading, by the end of this you'll actually understand the thing — including the quirks that make people think it's broken when it's working exactly as designed. Grab a coffee (a hot one, while you can) and let's dig in.

New to the RV life? Don't go it alone. This post lives inside my RV Beginners Guide, where I've rounded up everything I wish someone had handed me on day one.

First, How Is It Different From a Regular RV Water Heater?

To get tankless, you've got to understand what it replaced.

The traditional RV water heater is a tank — usually 6 or 10 gallons — that heats up a stored supply of water with a propane burner (and sometimes an electric element) and keeps it hot. The upside: simple and reliable. The downside: you get a finite amount of hot water. Run the tank dry with a long shower, and you're waiting 20-plus minutes for it to reheat the next batch. Anybody who's taken the "second shower" knows the betrayal of sudden cold water.

A tankless (or "on-demand") water heater throws the tank out entirely. Instead of storing hot water, it heats the water as it flows through, right when you need it. No tank, no waiting to reheat, and — the headline feature — endless hot water as long as you've got propane and water. It's also lighter and takes up less space, which RV makers and weight-conscious campers both love.

How a RV Tankless Water Heater Actually Works

Here's the simple version of what happens when you turn on the hot tap:

1. Water starts flowing through the unit.

2. A flow sensor detects that movement and tells the unit, "Hey, somebody wants hot water."

3. The igniter lights the propane burner, which fires up around a heat exchanger.

4. Water passes through that hot heat exchanger and comes out the other side toasty.

5. You turn the tap off, flow stops, and the burner shuts right off.

That's the whole magic trick — it only burns propane when water's actually moving, which is exactly why it's more efficient than keeping a tank hot all day. But that same on-demand design is also behind the two quirks that drive people nuts. Let's talk about those, because understanding them is the difference between "my heater's broken" and "oh, that's just how it works."

The Two Quirks Everybody Panics About

Quirk #1: The "Cold Water Sandwich." You're showering, hot water's great, you shut it off to soap up, then turn it back on — and BAM, a slug of cold water hits you before it goes hot again. That, my friends, is the famous cold water sandwich. Here's why it happens: when you stop the flow, the burner shuts off. When you restart, there's a brief lag while the burner refires, and the not-yet-heated water already sitting in the line comes out cold first. It's not a malfunction — it's physics. (Some newer units minimize it, but most do it to some degree.)

Quirk #2: It needs enough water flow to fire up. This is the big one, and it's behind a huge share of the "why won't it get hot?!" complaints. A tankless unit needs a minimum flow rate to trigger that flow sensor and light the burner. If you trickle the water too gently — which campers do all the time when conserving water while boondocking — the unit never fires, and you get cold water and a whole lot of confusion. The fix is often hilariously simple: open the tap up more. More on that below.

"Why Won't It Get Hot?!" — The Troubleshooting Run-Down

Okay, this is the section you came for. If your tankless is blowing cold, run through these in order, because the fix is usually simpler than you fear:

1. Turn the water up. The number one culprit. Too little flow = the burner won't ignite. Open that tap and give it something to work with.

2. Check your propane. Are you actually getting gas? Make sure the tank isn't empty and the valve's open. If you just swapped propane tanks, there may be air in the line — run a stove burner for a minute to purge it. If your other propane appliances won't light either, it's a propane problem, not a heater problem.

3. Check the power. Most tankless units need 12-volt power to run the igniter, control board, and fan. If your battery's dead or a fuse is blown or the unit's switch is off, it won't fire. (See how this ties back to my [RV batteries] post? It's all connected.)

4. Did you just de-winterize? If the water heater bypass valve is still set to "bypass," water isn't even going through the heater. Easy to forget in spring.

5. Check the temperature setting. Many units have a dial or panel — make sure it's not turned down low.

6. Cold incoming water + too much flow. In winter, frigid incoming water is harder to heat. If it's lukewarm at full blast, try slowing the flow a bit so the unit has time to heat it. (Yes — too little flow OR too much can both cause trouble. It's a Goldilocks thing.)

7. Clogged inlet screen or low water pressure. A gunked-up inlet filter or weak pressure can starve the unit. Clean the screen.

8. Look for an error/fault code. Many units blink a code when something's wrong — check your manual to decode it.

Nine times out of ten, it's the flow rate or the propane. Start there and you'll save yourself a service call.

Should You Upgrade to a Tankless Water Heater?

The big question. Here's my honest take, because — say it with me — the right answer depends on how you camp.

An upgrade is probably worth it if:

• You're tired of running out of hot water, or you've got a family doing back-to-back showers.

• You camp often and value the endless hot water and the weight/space savings.

• You like the efficiency of only heating water when you need it.

It might NOT be worth it if:

• You're a casual weekender and your current tank heater works just fine.

• You're on a tight budget — the unit plus installation isn't pocket change, and a light user may never recoup it.

• You don't love fiddling — tankless has a small learning curve (managing flow) and more electronics that can fail down the road.

And a reality check on expectations, because these myths cause buyer's remorse:

• "On-demand" doesn't mean "instant." You still wait for hot water to travel through the pipes to your faucet, same as any system. On-demand means it heats on demand, not that it teleports hot water to the showerhead.

• "Endless" isn't "unlimited." You'll still run out if the propane runs dry or, when boondocking, your fresh water tank empties.

• It won't fix low water pressure or other unrelated plumbing gripes.

Plenty of newer rigs come with tankless from the factory now, so if you're shopping, it's a feature worth asking about either way.

Don't Forget Maintenance

A tankless unit is low-fuss, not no-fuss:

• Winterize it properly. No tank to drain, but the lines still need antifreeze or to be blown out before a freeze, or you'll crack something expensive.

• Descale/flush it periodically. Minerals build up in the heat exchanger, especially with hard water, and that hurts performance. A vinegar flush every so often keeps it happy.

• Clean the inlet screen now and then.

• Keep the exterior vent clear. Remember the critter post? Wasps and spiders love to nest in water heater vents. A clogged vent messes with how the unit runs, so check it. (Insect screens, my friends.)

Squatch Tips: Living Happily With a Tankless

Here's what I tell folks so they actually enjoy the thing:

• When in doubt, turn the water UP. Most "it's not getting hot" panics are just too little flow to fire the burner. This one tip solves the majority of complaints.

• Expect the cold water sandwich and roll with it. It's normal. Don't flick the water on and off rapidly during a shower if you hate the cold surprise.

• Find your sweet spot. Every unit has a flow-and-temperature combo where it runs steady. Spend one shower dialing it in and you'll have it figured out for good.

• Flush it before it needs it. A regular descale beats waiting until performance tanks. Hard water is sneaky.

• Mind the power and propane together. No gas or no 12-volt power means no hot water. Check both before you assume the worst.

That's the heart of Camping with Squatch — understanding your rig so the quirks stop being scary and start being just... how it works.

Print This: Tankless "No Hot Water" Troubleshooting Checklist

Tape it inside a cabinet near the heater for those frustrating mornings.

Start Here (most common)

• [ ] Turn the water flow UP — it needs enough flow to ignite

• [ ] Confirm propane is on and flowing (do other propane appliances work?)

• [ ] Confirm 12V power is on (battery good, switch on, fuse intact)

Then Check

• [ ] Water heater bypass valve NOT in winterize/bypass mode

• [ ] Temperature dial/panel set high enough

• [ ] In winter: try slowing flow so it can heat cold incoming water

• [ ] Clean the inlet screen / check water pressure

• [ ] Look for a blinking error code (check the manual)

Normal Behavior (not broken!)

• [ ] Brief cold burst when restarting flow = "cold water sandwich," totally normal

• [ ] Slight wait for hot water to reach the tap = normal for any system

Upkeep

• [ ] Descale/vinegar flush periodically (especially hard water)

• [ ] Keep the exterior vent clear of wasp/spider nests

• [ ] Winterize the lines before any freeze

Let's Get You Sorted

Tankless water heaters are a genuinely great piece of tech once you understand their personality — endless hot water and a lighter rig, in exchange for learning a couple of quirks. Know how they work, manage your flow, keep up the maintenance, and that thing will treat you right for years.

And if you're shopping for a rig and want somebody to show you whether it's got a tank or tankless setup — or talk through whether upgrading your current camper is actually worth it for the way you travel — come find me at A&L RV Sales in Christiana, just outside Murfreesboro. Give me a call or text at 615-653-7561, or follow along with Camping with Squatch for more no-hype straight talk. I'll help you figure out what you really need. No pressure, ever — I just want you camping happy (and your showers hot).