Installing a gas tankless water heater is a complex but doable project for a skilled DIYer. The first two sentences of this guide answer the core question: you install a gas tankless water heater by carefully planning venting, gas supply, and condensate drainage, then mounting the unit and connecting water, gas, and electrical lines in compliance with local codes. This guide walks you through each step, but remember that gas and venting mistakes can cause carbon monoxide poisoning or fire, so if you feel unsure at any point, call a professional plumber.
Before you begin, understand that a tankless water heater heats water on demand, which means it needs a larger gas line and proper venting than a traditional tank. Many homeowners underestimate the importance of venting: most gas tankless units are power-vented, meaning they use a fan to push exhaust through a dedicated PVC or stainless steel pipe. You cannot reuse the old tank’s vent. Also, check your local building codes—most jurisdictions require a permit and inspection for gas water heater installation. Consult the U.S. Department of Energy’s water heating guide for efficiency basics, and review your manufacturer’s manual for specific clearances and venting tables.
Tools and Materials You’ll Need
Gather everything before you start to avoid delays. Here’s a checklist:
- Gas tankless water heater unit (condensing or non-condensing)
- Gas pipe (black iron or flexible gas line approved by local code), pipe dope or Teflon tape rated for gas
- Venting materials: PVC Schedule 40 for intake/exhaust (condensing) or stainless steel for non-condensing, plus primer and cement
- Water supply lines (PEX or copper), shut-off valves, and a pressure relief valve
- Condensate drain tubing (if condensing) and a neutralizer kit (often required)
- Mounting bracket, screws, and a level
- Adjustable wrench, pipe cutter, tubing cutter, drill with hole saw, screwdrivers, and a manometer (to test gas pressure)
- Carbon monoxide detector for nearby living areas
Step-by-Step Installation Process
1. Plan the Location and Venting Route
Choose a location close to the gas line and main water lines, but also consider venting. The vent must terminate outside, at least 12 inches above grade and away from windows or doors. For condensing units, you’ll need to slope the PVC vent downward toward the unit to allow condensate to drain. Measure the total vent length and check the manufacturer’s maximum equivalent length—usually 100–150 feet depending on the model. If your run is too long, you’ll need a larger diameter pipe or a power vent kit.
2. Turn Off Utilities and Drain the Old Water Heater
Shut off the gas valve and the water supply to the existing tank. Drain the tank by connecting a hose to the drain valve and running it to a floor drain or outside. Once empty, disconnect the gas line and water lines. Cap the gas line temporarily to prevent debris from entering. If you’re uncomfortable working with gas, stop here and call a licensed plumber—this is a critical safety step.
3. Mount the New Unit
Use the mounting bracket provided with your tankless water heater. Mark the stud locations and drill pilot holes. Make sure the unit is level and secure. The unit should be mounted with enough clearance for service access—typically 24 inches in front and 12 inches on the sides. Check the manual for exact clearances.
4. Install the Venting System
For a condensing unit, use PVC Schedule 40 pipe for both intake and exhaust. Dry-fit the entire vent run first, then glue with primer and cement. Slope the exhaust pipe downward toward the heater at a 1/4-inch per foot. For non-condensing units, use stainless steel venting rated for high temperatures. Terminate the vent with a screen to prevent rodents from entering. Never use a vent from an old water heater—it’s not rated for tankless temperatures and could fail.
5. Connect the Gas Line
Gas tankless water heaters require a higher BTU input than tank models, often 150,000–200,000 BTU/h. That means your existing gas line may be too small. Consult a gas pipe sizing chart (based on distance and BTU load) to determine if you need to upgrade to a larger diameter pipe. Use black iron pipe or an approved flexible gas connector. Apply pipe dope or Teflon tape to all threaded connections. After connecting, turn on the gas and test all joints with a soap-and-water solution—bubbles indicate a leak. Use a manometer to verify the gas pressure is within the manufacturer’s range (typically 7–10 inches water column).
6. Connect Water Lines
Install the cold water inlet and hot water outlet lines. Most tankless units have 3/4-inch connections. Use a union for easy future service. Install a shut-off valve on the cold side and a pressure relief valve on the hot side (if not pre-installed). For condensing units, connect a condensate drain line from the unit to a floor drain. If local code requires a neutralizer, install it inline—this raises the pH of the acidic condensate before it enters the sewer.
7. Connect Electricity and Set Up
Most gas tankless water heaters need a 120V electrical connection to run the fan and control board. Plug the unit into a dedicated GFCI outlet. Turn on the water supply and check for leaks. Then turn on the gas and power. Follow the manufacturer’s startup procedure: set the temperature, purge air from the gas line, and run hot water to verify the burner ignites. Check the vent for proper airflow and ensure no exhaust leaks into the house.
Important Safety and Code Considerations
This project involves combustion gas, so safety is paramount. Always follow the International Fuel Gas Code and your local amendments. Obtain a permit before starting—many municipalities require inspection. Never operate the unit without a working carbon monoxide detector nearby. If you smell gas, evacuate immediately and call your gas company. For more on water safety, the CDC’s Healthy Water site offers guidelines on safe water temperatures and preventing scalding.
Cost and Time Estimates
Typical installation costs for a gas tankless water heater in the U.S. range from $1,500 to $3,500, including the unit and labor, but prices vary by region and complexity. If you’re upgrading from a tank, you may also need to upgrade the gas line and venting, adding $500–$1,000. DIY installation can save labor costs, but mistakes can be expensive. Most installations take 6–10 hours for a professional, longer for a first-time DIYer.
Comparison: Tankless vs. Tank Water Heaters
| Feature | Gas Tankless | Storage Tank |
|---|---|---|
| Energy efficiency (EF) | 0.82–0.98 (condensing) | 0.60–0.70 |
| Lifespan | 20+ years | 10–15 years |
| Space required | Wall-mounted, compact | Floor space, large tank |
| Hot water supply | Unlimited (on demand) | Limited to tank capacity |
| Upfront cost | Higher | Lower |
| Operating cost | Lower | Higher |
According to EPA WaterSense, tankless heaters can be more efficient, but actual savings depend on your usage. If you use less than 41 gallons per day, a tankless can be 24–34% more efficient than a tank, per the DOE.
Frequently Asked Questions
Can I install a gas tankless water heater myself?
Yes, if you have experience with gas piping and venting, and you follow local codes. However, many municipalities require a licensed plumber to do gas work. If you’re not confident, hire a professional—incorrect installation can be dangerous.
Do I need a permit to install a gas tankless water heater?
In most U.S. jurisdictions, yes. A permit is usually required for any gas appliance installation. Your local building department will inspect the work to ensure it meets code.
What size gas line do I need for a tankless water heater?
Most gas tankless units require a 3/4-inch or 1-inch gas line, depending on the BTU input and the distance from the meter. Use a gas pipe sizing chart or have a professional calculate the correct size.
Can I vent a tankless water heater through the roof?
Yes, but only if the manufacturer’s venting table allows it. Condensing units can vent through PVC horizontally or vertically. Non-condensing units require special high-temperature venting and may have different clearance requirements.
How often should I flush my gas tankless water heater?
Manufacturers recommend flushing the unit with vinegar or a descaling solution every 6–12 months, depending on water hardness. This removes mineral buildup that reduces efficiency.
What happens if my gas tankless water heater doesn’t ignite?
Check the gas supply, ensure the shut-off valve is open, and verify that the vent is not blocked. Also, check the condensate drain—if it’s clogged, the unit may shut down as a safety measure.
When to Call a Professional
If you encounter any of these issues, stop and call a licensed plumber: gas line sizing uncertainties, venting runs over the maximum length, local code requirements you don’t understand, or if you smell gas. A professional can also handle tricky retrofits, like replacing a tank in a tight closet. For existing plumbing issues, we offer water heater repair, drain cleaning, leak detection, and pipe repair services. If you need help with other systems, check our sump pump and toilet repair guides.
Conclusion
Installing a gas tankless water heater is a rewarding project that can reduce energy bills and provide endless hot water. However, it’s not a beginner-level DIY task. Follow this guide, consult your manufacturer’s manual, and don’t hesitate to call a professional if you’re unsure. Proper installation ensures safety and efficiency for years to come.
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