How to Install a Recirculating Pump on Tankless Water Heater

You know the drill. You turn on the bathroom faucet and stand there, watching your phone, waiting for the water to warm up. With a tankless heater, that wait can stretch to two or three minutes depending on how far the fixture is from the unit. Meanwhile, gallons of perfectly good water go straight down the drain. A recirculation pump fixes that by keeping hot water moving through your pipes so it’s ready the second you open a tap.

But installing one on a tankless water heater isn’t the same as slapping a pump on a tank unit. Tankless heaters fire only when they detect a minimum flow rate, and they heat on demand. That changes everything about how you set up the recirculation loop, what controls you use, and how you avoid the dreaded cold water sandwich. This guide walks you through the full installation, including the bypass kit integration most generic tutorials skip.

You’ll learn what tools you need, how to wire the controls, how to purge air from the lines, and how to troubleshoot short-cycling. By the end, you’ll have a system that delivers hot water in about 15 seconds instead of minutes, without wrecking your heater’s efficiency.

If you’re looking for a complete kit that includes everything you need for a DIY install, the Egmiku Hot Water Recirculating Pump Kit pairs a cast-iron pump with two brass sensor valves and all necessary supply lines. It’s designed to work with tankless units and handles the return path without requiring a separate recirculation line.

Why Tankless Recirculation is Different (and Tricky)

With a storage tank heater, a recirculation pump just pushes water through the loop. The tank keeps a reservoir of hot water ready, so the pump can cycle as often as it wants without stressing the heater. Tankless units don’t have that luxury.

A tankless heater fires up only when water flows through it at or above a minimum rate, usually between 0.5 and 1.0 gallons per minute depending on the model. If the flow drops below that threshold, the burner or heating element shuts off. This creates two problems when you add a recirculation pump.

First, if the pump pushes water too slowly, the heater never fires, so the water in the loop never gets hot. Second, if the pump cycles on and off rapidly, the heater short-cycles, firing and shutting down repeatedly. That wastes energy and wears out the unit’s components faster.

That’s why the bypass kit matters. A proper tankless bypass kit, like the ones made by Navien or Rinnai, integrates with the heater’s internal recirculation logic. It includes a check valve and a temperature sensor that tells the pump when to run. Without it, you’re just guessing.

The other issue is thermal expansion. When you close the loop, water heats up and expands inside your pipes. On a closed system with a backflow preventer, that pressure has nowhere to go. You’ll need an expansion tank on the cold water line to absorb the extra volume. Most codes require it.

Tools and Materials: The Tankless Bypass Kit Checklist

Before you start, gather everything. Running to the hardware store mid-install is a pain, and some of these parts are specific to tankless setups.

Tools You’ll Need

• Adjustable wrench or two (one for holding, one for turning)
• Pipe wrench for stubborn fittings
• Teflon tape or pipe thread sealant
• Tube cutter if you’re working with PEX or copper
• Bucket and rags for draining lines
• Voltage tester to confirm power is off
• Drill with hole saw if you need to mount the pump to a wall

Materials Specific to Tankless Installations

The pump itself is only half the story. You also need a bypass kit designed for your heater’s brand. Navien and Rinnai sell proprietary kits that connect to the unit’s internal recirculation port. Some newer units have a built-in recirculation pump, but if yours doesn’t, you’ll use an external kit.

You’ll also need a check valve to prevent hot water from flowing backward into the cold line, and a temperature sensor or flow sensor depending on your control strategy. If your home doesn’t have a dedicated return line, you’ll install a crossover valve under the farthest sink. That valve connects the hot and cold lines under the sink and lets the pump push water through the cold pipe back to the heater.

For the pipe connections, you have options. Copper and PEX both work. SharkBite push-to-connect fittings make the job easier if you’re not comfortable soldering. Just make sure everything matches the pipe size you’re working with, typically 3/4-inch.

One more thing: check your local code before you start. Many jurisdictions require a permit for recirculation work, and some have specific rules about backflow prevention. A quick call to your building department saves you a headache later.

Step 1: Shut Off Power, Gas, and Water Supply

This step is non-negotiable. Tankless heaters run on either electricity or gas, and both can kill you if you mess with them while they’re live.

For an electric unit, flip the dedicated circuit breaker to the off position. Confirm with a voltage tester at the unit before touching any wiring.

For a gas unit, turn the gas valve to the closed position. The valve handle should be perpendicular to the gas line when it’s off. Also shut off the water supply to the heater. There should be a shutoff valve on the cold water inlet line. Close it, then open a hot water faucet somewhere in the house to relieve pressure and drain the lines.

Wait a few minutes for the heater to cool down. The internal components get hot during operation, and you don’t want to burn yourself working near them.

Step 2: Locate the Recirculation Inlet and Install the Bypass Kit

Look at your tankless heater’s plumbing. You’ll see the cold water inlet and the hot water outlet, usually labeled. Some units also have a dedicated recirculation inlet, often capped off with a brass plug. That port is where the return line from your recirculation loop connects.

If your unit has that port, remove the cap and install the bypass kit’s fittings. The kit typically includes a check valve and a temperature sensor that threads into the port. The check valve ensures water flows only one way, from the return line into the heater, never backward.

If your unit doesn’t have a dedicated recirculation port, you’ll install the bypass kit on the cold water inlet line. The kit creates a connection point where the return line ties in. This is a common setup for older tankless models, and it works fine as long as the check valve is installed correctly.

Wrap the threads with Teflon tape before you tighten anything. Use two wrenches, one to hold the fitting steady and one to turn the nut. Over-tightening cracks brass fittings, so stop when it feels snug.

Step 3: Mounting the Pump and Connecting the Return Line

Now you’ll mount the recirculation pump. Pick a spot on the wall near the heater, close enough that the supply lines reach without kinking. The pump should be oriented with the arrow on its housing pointing in the direction of flow.

For a tankless system, the pump usually sits on the cold water inlet side, pushing water from the return line into the heater. That keeps the pump out of the hot water stream and reduces wear on the impeller.

Use the mounting bracket that came with your pump. Mark the hole locations, drill pilot holes, and secure the bracket with screws. Make sure the pump is level and has clearance around it for airflow and maintenance.

Connect the pump to the return line using flexible supply lines or PEX tubing. The Egmiku kit includes supply lines and adapters, so you won’t need to buy extra pieces. Tighten the connections with a wrench, but don’t overtighten.

If you’re using a crossover valve under a sink instead of a dedicated return line, install that now. The valve goes on the hot and cold supply lines under the farthest fixture from the heater. It’s a small brass fitting with a check valve inside, and it threads onto the existing shutoff valves.

Step 4: Wiring the Pump and Setting the Controls (Timer vs. Sensor)

Here’s where tankless installations get interesting. The control strategy determines how often the pump runs, and that directly affects your heater’s efficiency and lifespan.

Timer Mode

A timer runs the pump at set intervals, say every 30 minutes, for a few minutes at a time. This works well if you have predictable routines. You wake up at 7 AM, so the pump runs at 6:45 AM to have hot water waiting. The downside is that the pump runs even when nobody’s using water, which means the heater fires more often than necessary.

Temperature Sensor Mode

A temperature sensor monitors the water temperature in the return line. When it drops below a set point, usually around 98°F, the pump kicks on and pushes that water back to the heater. The heater fires, heats the water, and the pump shuts off when the sensor reads the target temperature.

This is the smarter choice for tankless units. The pump only runs when the water actually cools down, so you’re not cycling the heater needlessly. The Egmiku kit’s sensor valves do exactly this, activating on temperature drop and shutting off at 98°F.

Wire the pump to a 110V outlet. If you’re not comfortable with electrical work, hire an electrician for this part. The pump should be plugged into a GFCI-protected outlet if one’s available.

Set the built-in timer if your pump has one. Start with the factory defaults and adjust based on your household’s usage patterns. You can always fine-tune it later.

Step 5: Flushing Air and Testing for Leaks

With everything connected, it’s time to fill the system and purge the air. Open the cold water shutoff valve to the heater. Open a hot water faucet in the house and let it run until water flows steadily without sputtering. That clears air from the lines.

Now turn on the recirculation pump. Listen for unusual noises. A grinding or rattling sound usually means air is still trapped in the pump housing. If that happens, turn the pump off, open the bleed valve on the pump (if it has one), and let the air escape. Close the valve and restart the pump.

Check every connection for leaks. Feel under each fitting with a dry paper towel. A slow drip might not be visible right away, so give it a few minutes. Tighten any fitting that weeps, but remember to shut off the pump first.

Once everything is dry, let the system run for a full cycle. Time how long it takes for hot water to reach the farthest faucet. It should be significantly faster than before, usually under 30 seconds.

If your heater has a digital display, watch it during the cycle. The burner should fire and stay on for a reasonable duration, not flicker on and off.

Troubleshooting: Fixing Short-Cycling and the Cold Water Sandwich

Two issues plague tankless recirculation setups: short-cycling and the cold water sandwich. Both are fixable, but you need to understand what’s causing them.

Short-Cycling

Short-cycling happens when the pump turns on and off rapidly, causing the heater to fire and shut down in quick succession. This usually occurs when the pump’s flow rate is too low for the heater’s minimum GPM requirement.

Check your heater’s manual for the minimum flow rate. Most tankless units need at least 0.5 GPM to fire. If your pump is set to a low speed, the heater might not detect enough flow and will refuse to ignite. Increase the pump speed until the heater fires consistently.

Another cause is a faulty temperature sensor. If the sensor reads the water as hot before it actually is, the pump shuts off too early. The heater fires, heats a small volume of water, then shuts down. Replace the sensor if adjusting the pump speed doesn’t help.

The Cold Water Sandwich

The cold water sandwich is that annoying burst of cold water you feel between two hot water uses. It happens because the recirculation loop keeps hot water in the pipes, but the heater itself takes a second or two to fire when you open the tap.

Here’s the fix: install a delay timer on the pump. A delay timer keeps the pump running for a set period after the last hot water use, usually 5 to 10 minutes. That maintains pressure in the loop and prevents the heater from cycling off and on.

Some pumps have this built in. If yours doesn’t, you can buy a separate delay timer and wire it inline. Set it to match your typical usage patterns. If you take a shower, then brush your teeth a few minutes later, a 5-minute delay covers that gap.

You can also install a mixing valve at the heater outlet. That tempers the hot water to a safe temperature and reduces the temperature swing when the heater cycles.

When to Call a Professional (Gas Lines and Venting)

Most of this installation is DIY-friendly, but there are limits. If your tankless heater runs on natural gas or propane, the gas line connections and venting are not something to experiment with. A gas leak can cause an explosion, and improper venting can lead to carbon monoxide poisoning.

You should also call a pro if your installation requires modifying the heater’s internal components. Some bypass kits require opening the unit’s casing and connecting to the control board. That’s beyond what most homeowners should attempt.

If your water pressure is too high or too low, that’s another reason to get help. A recirculation system needs a certain pressure range to work correctly. A plumber can test your pressure and install a pressure reducing valve if needed.

Finally, if you’re unsure about any step, hire a licensed plumber. The cost is worth the peace of mind, especially when you’re dealing with gas lines and high-temperature water.

FAQs: Recirculation Pumps on Tankless Heaters

Does a recirculation pump damage a tankless water heater?

It can if it’s installed incorrectly or set to cycle too frequently. The main risk is short-cycling, which wears out the ignition system and heat exchanger. A properly configured system with a temperature sensor or delay timer won’t damage the heater. It actually reduces wear by keeping the unit at a more consistent operating temperature.

What’s the minimum flow rate for a tankless recirculation pump?

Most tankless heaters need between 0.5 and 1.0 GPM to fire the burner. Your pump needs to move water fast enough to trigger the heater’s flow sensor. Check your heater’s manual for the exact number, then adjust the pump speed accordingly. If the flow is too low, the heater won’t fire and you’ll get lukewarm water.

Can I use a regular recirculation pump on a tankless heater?

A standard pump will physically work, but it won’t integrate with the heater’s controls. You’ll need to add a temperature sensor or flow sensor and a check valve to make it function correctly. Proprietary bypass kits from brands like Navien and Rinnai are easier because they’re designed for the unit’s specific logic.

Do I need a return line for a recirculation pump?

No. If your home doesn’t have a dedicated return line, you can install a crossover valve under the farthest sink. That valve connects the hot and cold pipes under the sink, letting the pump push water through the cold line back to the heater. It’s a common retrofit solution and works well for most homes.

How much energy does a recirculation pump use?

A typical pump draws between 20 and 40 watts when running. That’s less than a standard light bulb. The bigger energy cost comes from the heater firing more often to keep the loop warm. Using a temperature sensor instead of a timer reduces that cost because the pump only runs when the water actually cools down.

What You Should Know Before You Start

• Check your heater’s manual for the minimum flow rate and any specific recirculation requirements before buying a pump.
• Buy a bypass kit designed for your heater brand. Proprietary kits integrate with the unit’s controls and prevent short-cycling.
• Install a check valve on the return line to prevent hot water from flowing backward into the cold supply.
• Add an expansion tank to your cold water line if your system has a backflow preventer. Closed-loop systems need somewhere for expanding water to go.
• Use a temperature sensor or delay timer instead of a simple timer to reduce unnecessary heater firing and save energy.
• Purge all air from the lines before running the pump. Trapped air causes noise and can damage the impeller.
• If you’re not comfortable with gas lines or electrical wiring, hire a licensed professional. The DIY savings aren’t worth the risk.

Installing a recirculation pump on a tankless heater is a solid weekend project for someone with basic plumbing skills. The key is respecting the heater’s minimum flow requirements and using the right controls. Do that, and you’ll get hot water on demand without the wasted gallons or the cold water sandwich. For a complete kit with sensor valves and all the fittings you need, check the current price on Amazon for the Egmiku recirculating pump kit. If you’re also planning a full heater install, our gas tankless install guide covers the basics, and this recirculation pump explainer breaks down the theory in more detail.