Your tankless water heater worked great for years. Now it’s leaking, underperforming, or just done. Before you panic, the good news is that a tankless water heater replacement is manageable if you know what you’re doing. Most homeowners can handle the basics with the right tools and a clear plan.
Learning how to replace a tankless water heater can save you hundreds in labor costs. This guide covers everything from warning signs to final testing. Follow each step carefully, and you’ll have hot water running again in no time.
Signs You Need to Replace a Tankless Water Heater
Not every problem means you need a full replacement. But some signs are hard to ignore.
Watch for these red flags:
- The unit is more than 15 to 20 years old
- You see rust or corrosion around the heat exchanger
- Water flow drops even after descaling
- Error codes appear repeatedly after repairs
- Repair costs are close to 50% of a new unit’s price
- You get lukewarm water no matter the setting
If you’re seeing two or more of these, it’s time to replace your water heater rather than patch it again.
Tools and Materials Needed
Before you start, gather everything. Stopping mid-job to run to the hardware store wastes time.
Tools:
- Pipe wrench and adjustable wrench
- Screwdrivers (flathead and Phillips)
- Voltage tester (for electric units)
- Pipe cutter
- Teflon tape and pipe thread compound
- Drill and drill bits
- Level
Materials:
- New tankless water heater (gas or electric)
- Flexible water supply lines
- Mounting hardware
- Gas line fittings (if gas unit)
- Venting pipes (if needed)
- Pressure relief valve
Having everything ready means a smoother job and fewer surprises.
Safety Precautions Before Replacement
This is not the section to skip. Rushing past safety steps causes injuries and damage.
For gas units:
- Turn off the gas supply valve completely
- Wait 10 minutes before disconnecting any lines
- Never use a flame near gas connections
- Use a gas leak detector spray on fittings after reconnecting
For electric units:
- Turn off the circuit breaker for the heater
- Use a voltage tester to confirm power is off
- Never assume the power is off without testing
For both:
- Shut off the main water supply
- Open a nearby faucet to release pressure
- Let the unit cool down before touching it
These steps protect you and your home. Don’t skip them.
Step-by-Step Guide: How to Replace a Tankless Water Heater
Here’s how to replace a tankless water heater from start to finish.
Step 1: Turn Off All Utilities
- Gas units: close the shut-off valve on the gas line
- Electric units: flip the circuit breaker off
- Shut off cold water supply to the unit
Step 2: Drain the Unit
- Connect a hose to the drain port
- Open the pressure relief valve
- Let all water drain out completely
Step 3: Disconnect the Old Unit
- Disconnect water lines (hot and cold)
- Disconnect the gas line or electrical connections
- Remove the venting pipe if it’s a gas unit
- Unmount the unit from the wall
Step 4: Prepare the Installation Area
- Inspect the wall for damage or mold
- Make sure the mounting area meets the manufacturer’s clearance requirements
- Check that your gas line or electrical panel supports the new unit’s load
Step 5: Mount the New Unit
- Use a level to make sure the unit is straight
- Secure it with the mounting brackets provided
- Follow the manufacturer’s specifications exactly
Step 6: Reconnect Water Lines
- Use flexible water supply lines for easier connection
- Apply Teflon tape to all threaded connections
- Connect cold water inlet and hot water outlet
Step 7: Reconnect Gas or Electrical
Gas:
- Connect the gas line using approved fittings
- Apply gas-rated thread sealant
- Turn on gas slowly and check for leaks using soapy water
Electric:
- Connect the wiring according to the wiring diagram
- Double-check wire gauge matches the unit’s requirements
- Turn on the breaker and test
Step 8: Reconnect Venting (Gas Units Only)
- Attach the vent pipe to the unit’s exhaust outlet
- Make sure all joints are sealed
- Check that the vent exits the building properly
Step 9: Flush and Test
- Turn on the cold water supply
- Open a hot water faucet to bleed air from the lines
- Turn on the unit and check for leaks
- Let hot water run for a few minutes to confirm proper operation
This is the full process for how to replace a tankless water heater correctly.
You can read about: How Much Does a Hot Water Heater Cost
How to Choose the Right Replacement Unit
Don’t just buy the same model. Your needs may have changed.
Key factors to consider:
Flow rate (GPM): If you’ve added bathrooms or appliances, you may need a higher GPM rating. A typical home needs 6 to 10 GPM.
Fuel type: Stick with your current fuel source unless you’re doing a bigger renovation. Switching from gas to electric (or vice versa) adds cost and complexity.
Energy factor (UEF): Look for units with a high Uniform Energy Factor. Higher means more efficient.
Brand reliability: Brands like Rinnai, Navien, Rheem, and Noritz have strong track records. Read real user reviews, not just specs.
Warranty: A good tankless hot water heater replacement comes with at least a 5-year warranty on parts and 10 years on the heat exchanger.
Replacing a Tank Water Heater with a Tankless: Key Differences
Some homeowners use this moment to upgrade from a tank to a tankless unit. That’s a smart move, but it’s a bigger job.
What changes:
- You’ll need to resize your gas line or upgrade your electrical panel
- Venting requirements are completely different
- You’ll remove the old tank and patch the water lines
- The installation location may change
The cost to replace a water heater this way is higher upfront. But you’ll save money on energy bills long-term. Tankless units use 24% to 34% less energy than tank heaters, according to the U.S. Department of Energy.
This type of switch is often best handled by a licensed plumber unless you have strong plumbing and gas/electrical experience.
Common Mistakes to Avoid
Even experienced DIYers make these errors. Learn from them before you start.
Skipping the flush: New units need to be flushed before use to remove debris and air pockets.
Wrong pipe size: Using undersized gas or water pipes reduces performance and can void your warranty.
Ignoring local codes: Most municipalities require a permit for water heater replace jobs. Check before you start.
Not testing for leaks: Always test every connection before closing walls or moving on.
Choosing the wrong size: A unit that’s too small won’t keep up with demand. A unit that’s too large wastes energy.
Not sealing vent joints: Improper venting on gas units is a serious carbon monoxide risk.
How Long Does Tankless Water Heater Replacement Take?
A straightforward swap of the same type of unit usually takes 2 to 4 hours.
If you’re switching from tank to tankless or changing fuel types, plan for a full day. Permit approval can add days or weeks depending on your location.
Professional plumbers typically finish a standard tankless water heater replacement in about 3 hours. DIY installs take longer, especially for first-timers.
When to Call a Professional Plumber
Some jobs are worth doing yourself. Others aren’t.
Call a pro if:
- You’re switching fuel types (gas to electric or vice versa)
- Your gas line needs resizing
- Your electrical panel needs an upgrade
- You’re unsure about local code requirements
- The installation requires new venting through walls or the roof
- You’ve never worked with gas lines before
A mistake with gas lines or high-voltage wiring can be life-threatening. If you’re not confident, don’t guess.
Professional Tankless Water Heater Replacement in Eagle Rock
If you’re in the Eagle Rock area, Derks Plumbing handles every aspect of Tankless Water Heater Installation in Eagle Rock. From pulling permits to final testing, the team takes care of it all.
Whether you’re doing a direct tankless water heater replacement or upgrading from a tank unit, Derks Plumbing brings licensed expertise to every job. No guesswork. No cutting corners.
Contact Derks Plumbing today for a free estimate on your water heater replacement project.
Conclusion
Knowing how to replace a tankless water heater puts you in control. You can handle a basic swap if you follow the steps and take safety seriously. For complex upgrades, bring in a pro.
Either way, act on the warning signs early. A failing unit that goes unaddressed leads to water damage and bigger repair bills. Replace it before it fails completely, and you’ll save time, money, and stress.
FAQs
Can I replace a tankless water heater myself?
Yes, if you’re replacing like-for-like and comfortable with basic plumbing and gas or electrical work. Complex changes are better left to a licensed plumber.
How much does a tankless water heater replacement cost?
DIY replacement costs $500 to $1,500 for the unit. Professional installation adds $500 to $1,500 in labor depending on complexity.
Do I need a permit to replace a water heater?
Most areas require one. Check with your local building department before starting.
How long do tankless water heaters last?
With proper maintenance, 15 to 25 years. That’s significantly longer than tank heaters.
What size tankless water heater do I need?
Calculate your peak hot water demand in gallons per minute. A family of four typically needs 7 to 9 GPM.
How often should a tankless water heater be serviced?
Once a year. Flush and descale the unit to prevent mineral buildup that reduces performance.
Is it hard to replace a water heater with a tankless unit?
The process is more involved than a tank-to-tank swap. You may need a gas line or electrical upgrades. Budget extra time and cost.
