So, you’ve got a tankless install on the schedule for tomorrow. You’ve got the unit, the venting, and the gas piping ready to go. But as you’re double-checking your truck stock, a question pops up that we hear a lot at the counter: Does a tankless water heater need a check valve?
It’s a common debate in the field. Some guys swear by them on every install; others say they’re just another point of failure. The short answer? In most cases, yes, a check valve is highly recommended, and in some setups (like recirculation systems), it’s absolutely mandatory.
Let’s break down the “why” and “how” so you can make the best call for your customer and ensure that install holds up for the long haul.
What is a check valve, anyway?
Before we dive into the specific application for tankless units, let’s do a quick refresher. You know what a check valve is—it’s the traffic cop of the plumbing world. Its job is simple but critical: allow water to flow in one direction only and stop it from backing up the wrong way.
Inside the valve, there’s usually a spring-loaded disc or a flap. When water pressure pushes against it in the correct direction, it opens up. If the pressure drops or reverses, that mechanism snaps shut, creating a seal that prevents backflow.
Why does this matter for a heater? Because plumbing systems are dynamic. Pressures fluctuate. When you have multiple fixtures running, a dishwasher cycling, and a shower going at the same time, water doesn’t always behave the way we want it to. A check valve ensures that once the water enters the heater (or leaves it), it doesn’t decide to take a U-turn.
Check valves and tankless water heaters: The big debate
Now, to the meat of the issue. Does a tankless water heater need a check valve?
If you check the manufacturer’s manual for a standard, straightforward installation—cold in, hot out, no fancy loops—you might not see a check valve listed as a strict requirement on the cold water inlet. However, “not required” doesn’t mean “not a good idea.”
Here is where the nuance comes in.
1. The Recirculation Loop Factor
If you are installing a dedicated return line for instant hot water, a check valve is 100% non-negotiable. Without one, the water will take the path of least resistance. When a homeowner turns on the cold water tap, the water might pull from the return line instead of the fresh cold supply.
This leads to “crossover,” where you get lukewarm water coming out of the cold tap, or cold water mixing into the hot line. It’s the number one reason for comfort complaints on recirc systems.
2. Thermal Expansion and Backflow
Even without a recirc pump, check valves are smart insurance. When water heats up, it expands. While tankless units hold very little water compared to a 50-gallon tank, there is still pressure fluctuation in the lines. A check valve on the cold inlet prevents hot water from migrating backward into the cold supply line due to thermal expansion.
3. Preventing Siphoning
In rare instances, a sudden drop in city pressure (like a water main break) could theoretically siphon water out of the home’s piping. While a tankless unit doesn’t hold a “tank” of water to drain, siphoning can damage the flow sensor or other internal components. A check valve stops this immediately.
Benefits of using a check valve
Since we are all about quality you can trust here at Van Marcke, let’s look at why adding that small valve is a win for your installations.
Preventing Backflow
This is the big one. You don’t want hot water creeping into the cold line. It’s not just a comfort issue; it can be an efficiency issue. If the flow sensor in the tankless unit gets confused by back-pressure or fluctuating movements of water, the unit might try to fire up when it shouldn’t, or fail to fire when it should.
Maintaining Consistent Pressure
Tankless heaters rely heavily on flow rates and pressure to determine how much gas to burn. By stabilizing the direction of flow, a check valve helps maintain a consistent environment for the unit’s sensors. This means more consistent temperatures for your customer—and fewer callbacks for you.
Protecting the Heat Exchanger
Cold water entering the outlet side? It sounds impossible, but in complex plumbing systems with pressure imbalances, cross-connection can happen. A check valve ensures the flow goes exactly where the engineers designed it to go: through the heat exchanger, not backwards into it.
Potential problems without a check valve
What happens if you skip it? You might save ten minutes and twenty bucks on parts today, but it could cost you a loyal customer tomorrow.
The “Cold Water Sandwich” effect
While usually caused by the unit flushing itself or a gap in usage, plumbing crossover can worsen temperature fluctuations. If cold water is pushing back against the hot outlet, the customer might get blasted with a burst of cold water in the middle of a shower.
Crossover Complaints
We mentioned this earlier, but it bears repeating. If you have a recirculation pump (either internal to the unit or external) and you forget the check valve, the system simply won’t work. The pump will push water into the cold line, heating up the toilet water and cooling down the shower.
Energy Waste
If the tankless unit detects flow because water is sloshing back and forth due to lack of a check valve, it might cycle on unnecessarily. This wastes gas and puts wear and tear on the igniter and fan assembly.
Installation Guidelines
Alright, you’re convinced. You’re putting one in. Here is a quick run-through on best practices for installing a check valve on a tankless system.
Tools and Materials:
- Properly sized check valve (usually 3/4″ for residential tankless)
- Thread tape/pipe dope
- Wrench set
- Your standard soldering or press kit (depending on your pipe material)
Step-by-Step:
- Locate the Cold Inlet: The check valve almost always goes on the cold water supply line, before it enters the heater.
- Check the Arrow: This is the rookie mistake that happens to the best of us when we’re rushing. Look for the flow arrow stamped on the valve body. It must point toward the water heater. If you install it backward, no water gets to the unit.
- Positioning: Install the check valve before the isolation valves (service valves). Why? Because when you flush the unit for maintenance, you don’t want the descaling solution pushing back into the house piping.
- Recirculation Lines: If you have a return line, you need a second check valve on that return line right before it connects to the heater or the pump inlet. This stops the cold supply from forcing its way down the return line.
- Test It: Once soldered or pressed in, turn the water on slowly and check for leaks. Then, run a hot tap and make sure you have full flow.
Pro Tip: If you are using a swing check valve, it usually needs to be installed horizontally. If you are in a tight vertical run, use a spring-loaded check valve, as they work in any orientation.
Maintenance Tips
Just like the heater itself, valves aren’t invincible. Hard water is a killer in Arizona, Nevada, and Texas. Scale buildup can cause a check valve to get stuck open (useless) or stuck closed (no hot water).
- Listen: If you hear a “chattering” noise near the heater when water is running, the check valve spring might be failing or the valve is undersized.
- Scale Check: When you go out for that annual flush service, take a look at the valve if it’s accessible. If the isolation valves are heavily scaled, there is a good chance the check valve is getting crusty too.
- Replacement: If you suspect a check valve is stuck, don’t try to hammer it loose. Just replace it. It’s a cheap part compared to the labor of coming back a second time.
Local Regulations
We can’t talk plumbing without talking code. The UPC (Uniform Plumbing Code) and IPC (International Plumbing Code) have different variations depending on your specific municipality.
In many jurisdictions, if a check valve is installed on the cold water supply line, you have created a “closed system.”
This is crucial: In a closed system, thermal expansion has nowhere to go. If you install a check valve, you must install a thermal expansion tank. If you don’t, the pressure buildup can blow the T&P relief valve or even rupture the heat exchanger.
Always check your local amendments. What flies in Phoenix might not fly in Las Vegas or Dallas. When in doubt, call your local Van Marcke rep—we keep our ears to the ground on local code changes.
Conclusion
So, does a tankless water heater need a check valve? If you want a system that is efficient, protects the equipment, and keeps your customer from calling you about “weird temperature issues,” the answer is yes.
It’s a small component that plays a massive role in the reliability of the system. Whether you are doing a new build or a retrofit, taking the extra time to install a check valve (and the accompanying expansion tank) is the mark of a pro.
Need to grab a few check valves for the truck? Head over to PlumbersOnly.com. You can check stock at your local branch, order online, and pick it up on your way to the job site. We’ve got the brand names you trust to help you get the job done right the first time.