You know the drill. You turn on the shower, step back, and wait. And wait. For a tankless water heater, that wait can stretch to 30 or 40 seconds while cold water in the pipes clears out. It wastes water, and it tests your patience every single morning.
A recirculating pump fixes that by keeping hot water moving through the pipes, so it’s ready the second you open a faucet. This guide walks you through the full installation process, from choosing the right pump type to wiring it correctly. You’ll leave knowing exactly what to buy, where to put it, and how to avoid the mistakes that trip up most DIYers.
Before you start cutting pipe, grab a copy of Ultimate Guide to Home Repair and Improvement, 3rd Updated Edition. It has clear diagrams for plumbing layouts and electrical connections that make this project far less stressful. The photos alone are worth it when you’re staring at a wall of copper pipes.
Know Your Pump Options Before You Buy
Not all recirculating pumps work the same way, and picking the wrong one means a wasted weekend. You have three main choices.
Dedicated return line pump. This setup requires a third pipe running from the farthest fixture back to the water heater. It’s the most efficient option, but retrofitting that return line into existing walls is a major project. Most homes don’t have one, so this usually only makes sense during new construction or a full remodel.
Integrated recirculation pump. Many modern tankless units, like Rinnai and Navien models, have a built-in pump and a dedicated return port. If yours has that, installation is simple: connect the return line to the marked port and set the timer on the unit’s control panel. No external pump needed.
Cross-over valve kit. This is the most common retrofit solution. You install a small pump at the water heater and a crossover valve under the sink farthest from the heater. The valve connects the hot and cold lines, so the pump pushes hot water into the cold line, which sends it back to the heater. It’s not as efficient as a dedicated return line, but it works without tearing open walls.
For most people, the crossover valve kit is the practical answer. It costs less, installs in an afternoon, and solves the cold-shower problem. The trade-off is that cold water lines get warm when the pump runs, which can be a minor annoyance during summer.
What You Need Before Starting
Gather these tools and materials first. Running to the hardware store mid-project gets old fast.
- Recirculating pump kit (includes pump, crossover valve, and timer)
- Pipe wrench and adjustable wrench
- Teflon tape or pipe thread sealant
- Pipe cutter (if you need to trim copper or PEX)
- SharkBite fittings or soldering gear, depending on your pipe type
- Voltage tester
- Bucket and rags for water cleanup
- Safety glasses and work gloves
Check your tankless heater’s manual for the exact inlet and outlet port sizes. Most use 3/4-inch connections, but some compact models use 1/2-inch. Getting this wrong means a trip back to the store.
Also, verify your heater’s minimum flow rate. Tankless units only fire up when water flows at a certain rate, usually around 0.5 gallons per minute. The recirculating pump must generate enough flow to trigger the heater, or you’ll get lukewarm water instead of hot. Most quality pumps handle this fine, but cheap ones sometimes fall short.
Step-by-Step Installation for a Crossover Valve Kit
This process assumes you’re installing a pump at the heater and a crossover valve at the farthest sink. It works for copper, PEX, or CPVC pipes.
Step 1: Shut Off Water and Power
Turn off the cold water supply valve to the heater. Then shut off the gas or electricity to the unit. For electric heaters, flip the breaker. For gas, turn the valve to the off position. Open a hot water faucet somewhere in the house to relieve pressure in the lines.
Wait a few minutes. The pipes should be cool to the touch before you start cutting.
Step 2: Install the Crossover Valve Under the Sink
Go to the sink farthest from the heater. Look under the sink and find the hot and cold supply lines. The crossover valve has two inlets and one outlet. You’ll connect it between the hot and cold lines, right where they come out of the wall.
Cut the hot water line and install a tee fitting. Do the same for the cold line. Then connect the crossover valve between the two tees. Wrap Teflon tape around all threaded connections and tighten them firmly with a wrench. Don’t overtighten—you can crack the valve body.
Step 3: Mount the Pump at the Water Heater
Find a spot on the hot water outlet pipe near the heater. The pump needs to be oriented so the arrow on its body points in the direction of water flow, which is away from the heater and toward the house.
Cut the hot water pipe and install the pump using the included fittings. If you have copper pipes, you can solder the connections or use SharkBite push-to-connect fittings. SharkBites are easier for beginners and don’t require a torch, but they cost more per fitting.
Secure the pump to the wall with the mounting bracket if your kit includes one. A pump that hangs only on the pipe puts stress on the connections and can cause leaks.
Step 4: Wire the Pump to Power
Most recirculating pumps plug into a standard 120-volt outlet. If you have an outlet near the heater, just plug it in. If not, you’ll need to run a new circuit or hire an electrician.
Some kits come with a timer or a thermostat that controls when the pump runs. Wire that between the pump and the outlet according to the manufacturer’s instructions. A timer is worth the extra effort—running the pump 24/7 wastes electricity and keeps water warm even when nobody’s home.
Use a voltage tester to confirm the outlet is dead before you touch any wiring. Never assume the breaker is off just because you flipped it.
Step 5: Test and Adjust
Turn the water supply back on and check all connections for leaks. Tighten any that drip. Then restore power to the heater and the pump.
Open a faucet and let it run for a minute. You should feel hot water arriving much faster than before. If the water is warm but not hot, the pump might not be generating enough flow to trigger the tankless heater’s burner. Check the pump’s flow rate and adjust the crossover valve if it has an adjustment knob.
Set the timer so the pump runs during peak usage hours, like 6 AM to 9 AM and 5 PM to 10 PM. Some smart pumps learn your habits automatically, but most basic models just run on a schedule you set.
Comparing Pump Types and Methods
| Method | Effort Level | Cost Range | Best For | Drawback |
|---|---|---|---|---|
| Dedicated return line | High (major plumbing) | $$$$ | New construction or full remodel | Requires extra pipe, invasive |
| Built-in pump (tankless) | Low (no extra pump) | $$ (already in unit) | Modern tankless with return port | Limited to specific models |
| Cross-over valve kit | Medium (afternoon project) | $$ | Retrofit in existing homes | Warms cold lines slightly |
| Point-of-use heater | Low (plug-in) | $$$ | Single remote bathroom | Only fixes one location |
The crossover valve kit hits the sweet spot for most homes. It’s not perfect, and you’ll notice the cold water in your kitchen gets a bit warm during the summer months. But the convenience of instant hot water at every faucet usually outweighs that quirk.
Common Mistakes and How to Avoid Them
People mess up this installation in predictable ways. Here’s what to watch for.
Ignoring the flow rate requirement. Your tankless heater needs a minimum flow to turn on. If the pump doesn’t deliver enough, the burner never fires. Check both specs before buying. A pump rated for 3 gallons per minute usually works, but verify against your heater’s manual.
Installing the pump backwards. The arrow on the pump body must point away from the heater. It sounds obvious, but it’s the most common installation error. Water flows out of the heater, through the pump, and into your pipes. Reverse that and the pump just spins uselessly.
Skipping the timer. Running the pump continuously wastes electricity and keeps hot water circulating through your pipes all night. A simple timer cuts operating costs by 50 to 70 percent. If your kit doesn’t include one, buy a separate outlet timer for about fifteen bucks.
Forgetting to bleed air from the lines. After installation, air gets trapped in the pipes. That air can cause the pump to run dry, which damages the impeller. Open all hot water faucets for a few minutes to purge the air before running the pump.
Frequently Asked Questions
Will a recirculating pump damage my tankless water heater?
No, provided you meet the flow rate requirement and install it correctly. The pump pushes water through the heater, which is exactly what it’s designed to handle. Just make sure the pump doesn’t run when the heater is off, or you’ll get cold water circulating. Most timers handle this automatically.
Can I install a recirculating pump myself, or do I need a plumber?
A competent DIYer can handle this in an afternoon if they’re comfortable with basic plumbing tools. The crossover valve kit avoids soldering if you use push-to-connect fittings. If you’ve never touched a pipe wrench, hire a plumber. The risk of a leak that damages drywall isn’t worth saving a couple hundred bucks.
How much electricity does the pump use?
A typical recirculating pump draws 20 to 40 watts, similar to a small light bulb. Running it 24 hours a day costs about $2 to $4 per month. With a timer, that drops to under a dollar. The water savings from not running the tap while you wait usually offsets the electricity cost.
Does the recirculating pump work with a smart thermostat?
Yes, but you’ll manage them separately. The pump has its own timer or thermostat. Some smart pumps connect to Wi-Fi and integrate with home automation systems, but those cost significantly more. A basic timer works fine for most households.
What if my water heater is on the opposite side of the house from the bathroom?
That’s exactly the situation where a recirculating pump helps most. The longer the pipe run, the longer you wait for hot water. A pump cuts that wait from 45 seconds down to 2 or 3 seconds, regardless of distance.
What to Do After Installation
- Test all connections for leaks over the next 24 hours, not just right after installation.
- Set the timer to match your family’s morning and evening routines.
- Listen for unusual noises from the pump. A grinding sound means air in the lines or a damaged impeller.
- Check the pump’s filter screen every few months and clean it if you have hard water.
- Keep the pump’s manual with your water heater paperwork for future reference.
- If you notice lukewarm water in the cold lines, reduce the pump’s run time or install a thermal expansion tank.
Installing a recirculating pump on a tankless water heater is a solid weekend project that pays off every time you step into a warm shower without waiting. The key is matching the pump to your heater’s flow requirements and setting a sensible schedule. Do that, and you’ll wonder why you didn’t do it years ago.
