Introduction: Why Tankless Water Heater Installation Deserves Extra Attention
Transitioning from a traditional tank-style water heater to a modern tankless system is a smart home improvement move for many homeowners. Tankless water heaters offer endless hot water on demand, greater energy efficiency, and extra space in your utility area. However, the installation process is more complex than swapping out a standard tank. Many enthusiastic DIYers and even some professionals underestimate the unique requirements of tankless systems. Mistakes during installation can lead to disappointing performance, costly repairs, or even dangerous conditions. In this in-depth guide, we’ll explore the most common pitfalls homeowners encounter when installing a tankless water heater and provide step-by-step advice on how to avoid them. Whether you’re considering a DIY install or planning to hire a contractor, understanding these crucial factors is the key to enjoying reliable, efficient hot water for years to come.
Understanding How Tankless Water Heaters Work
Key Differences from Traditional Water Heaters
Tankless water heaters, also known as on-demand water heaters, heat water only when you need it. Unlike traditional storage tanks that keep 40–80 gallons of water hot around the clock, tankless units use high-powered burners or heating elements to instantly heat water as it flows through the device.
- Compact Design: Wall-mounted, saving valuable floor space.
- Energy Efficiency: No standby heat loss, resulting in lower utility bills.
- Endless Hot Water: As long as the unit’s flow rate isn’t exceeded, you never run out.
- Specialized Installation: Requires upgraded gas lines or electrical service, dedicated venting, and careful sizing.
Common Application Mistakes
One of the first mistakes is assuming any tankless model will suit your home. Sizing, fuel source compatibility, and installation location are all critical. Failure to account for these before purchase creates headaches during installation.
Mistake #1: Improper Sizing for Household Demand
Why Sizing Matters
Tankless water heaters are rated by their maximum flow rate (gallons per minute, or GPM) and the temperature rise they can deliver. If you undersize your unit, you’ll experience lukewarm water or pressure drops when multiple fixtures are used simultaneously.
How to Size Properly
- Calculate Peak Demand: List all hot water fixtures and appliances you may use at the same time (showers, dishwashers, washing machines, sinks).
- Sum GPM Requirements: For example, two showers (2.5 GPM each) and a dishwasher (1.5 GPM) = 6.5 GPM.
- Check Temperature Rise: Subtract your incoming cold water temperature from your desired hot temperature (usually 120°F). Northern climates may require more powerful units.
- Consult Manufacturer Specs: Choose a model that can handle your flow rate and temperature rise simultaneously.
Common Sizing Pitfalls
- Ignoring peak usage times (e.g., morning showers).
- Not accounting for future renovations or additional bathrooms.
- Assuming advertised GPM is always achievable (it drops with higher temperature rise).
Mistake #2: Skipping Necessary Upgrades to Gas or Electrical Systems
Gas Supply Considerations
Tankless gas water heaters often require a larger gas line than your old tank unit—commonly ¾ inch or even 1 inch, compared to the ½ inch found in many homes. Undersized lines cause insufficient fuel supply, leading to poor performance or dangerous combustion issues.
- Always check manufacturer’s gas supply specifications.
- Hire a licensed plumber or gas fitter to assess and upgrade gas lines as needed.
- Verify that your gas meter and main supply can handle the additional load.
Electrical Requirements
Electric tankless models draw much more power than traditional tanks—sometimes requiring up to three 40-amp breakers. Many homes need a panel upgrade to accommodate these demands.
- Have a licensed electrician evaluate your electrical panel before purchase.
- Install dedicated circuits as recommended by the manufacturer.
- Ensure proper grounding and GFCI protection as per code.
Mistake #3: Inadequate Venting and Air Supply
Why Proper Venting Is Critical
Tankless gas water heaters produce hot exhaust gases that must be vented safely to the outdoors. Unlike standard tanks, most tankless units cannot share a vent with other appliances. Inadequate or incorrect venting can cause carbon monoxide buildup, fire hazards, or premature unit failure.
Venting Essentials
- Follow manufacturer’s instructions for vent pipe material (often stainless steel), size, and length limits.
- Use only venting systems approved for your model (direct vent, power vent, etc.).
- Ensure adequate clearance from combustibles and other building materials.
- Install vent termination points per code—never under windows, near doors, or close to air intakes.
- For indoor models, provide proper combustion air (sometimes via dedicated intake pipes or louvered doors).
Common Venting Mistakes
- Improvising with incompatible vent pipe.
- Routing vents too long or with excessive elbows, reducing airflow.
- Neglecting to seal joints, leading to gas leaks.
Mistake #4: Poor Placement and Installation Location
Choosing the Best Location
While tankless units are compact, they shouldn’t be installed just anywhere. Location impacts efficiency, safety, and maintenance access.
- Proximity to Fixtures: The closer to kitchens, bathrooms, and laundry, the less heat lost in transit.
- Accessibility: Leave enough space for servicing, flushing, and future repairs.
- Protection from Freezing: Avoid exterior walls or unconditioned spaces in cold climates unless the unit is rated for outdoor use and protected accordingly.
- Compliance: Follow all local codes for clearance, mounting height, and seismic strapping where required.
Installation Location Mistakes
- Mounting in tight closets or crawl spaces with inadequate airflow.
- Ignoring manufacturer’s clearance recommendations.
- Placing the unit too far from high-use fixtures, causing long wait times for hot water.
Mistake #5: Incorrect Plumbing Connections
Water Line Upgrades
Tankless water heaters may require dedicated isolation valves, pressure relief valves, and expansion tanks—components not always needed with old tank-style systems.
- Install full-port isolation valves on hot and cold lines for easy maintenance.
- Use approved dielectric unions to prevent corrosion between copper and steel connections.
- Add a pressure relief valve per code—never skip this safety feature.
- Consider a small expansion tank if your system is closed-loop or subject to high pressure.
Common Plumbing Errors
- Cross-connecting hot and cold lines.
- Using the wrong pipe materials (e.g., PEX not rated for high temperatures near the heater).
- Failing to flush lines before connecting, allowing debris to enter the unit.
Mistake #6: Overlooking Water Quality and Filtration Needs
Why Water Quality Matters
Tankless water heaters have narrow heat exchangers that are especially sensitive to scale buildup and sediment. Hard water can drastically shorten the lifespan of your unit and void warranties.
Protecting Your Investment
- Test your water hardness before installation (above 7 grains/gallon or 120 ppm is considered hard).
- Install a whole-house water softener or a dedicated scale inhibitor filter if necessary.
- Flush the unit regularly (annually or as recommended) to remove mineral deposits.
Filtration and Water Treatment Mistakes
- Skipping pre-filtration in areas with sediment-laden or hard water.
- Using incompatible filters that restrict water flow below the heater’s minimum requirements.
Mistake #7: Ignoring Local Codes, Permits, and Manufacturer’s Instructions
Why Compliance Matters
Tankless water heater installations are subject to strict plumbing, gas, and electrical codes. Failing to obtain permits or follow manufacturer’s instructions can result in failed inspections, denied insurance claims, or dangerous conditions.
How to Stay Compliant
- Contact your local building department before starting any work.
- Ensure all work is performed by licensed professionals where required.
- Keep all manuals and documentation for inspections and warranty claims.
- Never skip the final inspection; it protects your home and investment.
Common Compliance Mistakes
- Assuming a permit isn’t needed for a ‘simple’ swap.
- Using unlicensed installers in regulated jurisdictions.
- Failing to register the unit for warranty coverage.
Mistake #8: Neglecting Routine Maintenance After Installation
Maintenance Basics
Unlike traditional tanks, tankless units must be flushed regularly to keep the heat exchanger clear of scale and debris. Neglecting this step leads to reduced efficiency, error codes, or total failure.
- Annual Flushing: Use a vinegar solution and a pump (most manufacturers sell kits) to clean the heat exchanger.
- Filter Cleaning: Clean or replace inlet screens and filters per the manufacturer’s schedule.
- Check Venting: Inspect vent pipes for blockages, leaks, or corrosion.
- Monitor for Leaks: Regularly check all plumbing and gas connections for drips or corrosion.
Maintenance Mistakes
- Forgetting to flush the system (set a calendar reminder!).
- Using harsh chemicals not approved by the manufacturer.
- Ignoring error codes or warning lights on the unit.
Professional vs. DIY Installation: Making the Right Choice
When to Hire a Pro
While many handy homeowners can handle some aspects of installation, tankless water heater setups often require specialized skills:
- Running new gas or electrical lines safely and to code.
- Ensuring proper venting and combustion air supply.
- Handling complex plumbing connections and water treatment.
Improper installation voids most warranties and can create safety hazards. If you’re unsure about any step, it’s wise to consult a licensed plumber, electrician, or HVAC contractor with tankless experience.
Step-by-Step Checklist for a Successful Tankless Water Heater Installation
Pre-Installation
- Assess household hot water needs and size your unit appropriately.
- Verify compatibility with your home’s gas or electrical service.
- Inspect water quality and plan for filtration if needed.
- Obtain all required permits and schedule inspections.
Installation
- Mount the unit securely, following manufacturer’s clearance and access guidelines.
- Connect gas or electrical supply lines using proper materials and fittings.
- Install venting per manufacturer’s instructions, sealing all joints.
- Plumb hot and cold water lines with isolation valves and pressure relief valve.
- Install any required expansion tank or water treatment system.
- Check all connections for leaks before powering up.
Post-Installation
- Flush and purge the unit according to the manufacturer’s procedure.
- Test for proper operation at all fixtures (check for adequate hot water and pressure).
- Schedule annual maintenance reminders.
- Keep installation manual, warranty info, and inspection records in a safe place.
Conclusion: Enjoying Reliable Hot Water—The Right Way
Installing a tankless water heater is a significant upgrade that can dramatically improve your home’s comfort and energy efficiency. But as we’ve explored, it’s not a plug-and-play project. Each step—from sizing and location selection to venting, gas or electrical upgrades, plumbing, and maintenance—demands careful attention to detail. The most common mistakes aren’t just technical missteps; they’re often the result of underestimating the complexity involved. Cutting corners or skipping critical upgrades might save time or money in the short term, but it risks long-term headaches, expensive repairs, or even safety hazards. By understanding and avoiding these frequent pitfalls, you’ll ensure your tankless water heater performs as promised—delivering endless, efficient hot water day after day. Whether you tackle the installation yourself or hire a qualified professional, take the time to plan thoroughly, use quality materials, and follow all codes and manufacturer’s instructions. Your reward: years of reliable hot water, lower energy bills, and the peace of mind that comes from a job done right. Don’t let avoidable mistakes undermine your investment—approach your tankless water heater installation with the care and expertise it deserves.
Joan Millton
I noticed you mentioned that tankless water heaters typically need upgraded gas lines or electrical service compared to traditional tanks. How can a homeowner tell if their current utilities are sufficient for a new tankless system, and what sort of costs might be involved in making those upgrades?
To determine if your current utilities are sufficient for a tankless water heater, check the heater’s manufacturer requirements for gas line diameter or electrical amperage. Compare those with your home’s existing setup. If you’re unsure, a licensed plumber or electrician can assess your system. Upgrading gas lines often costs $300–$800, while electrical upgrades can range from $500–$2,000, depending on complexity and local rates.
Since the installation process seems much more complex than for a traditional unit, is it realistic for a busy parent with basic DIY skills to attempt this, or would you strongly recommend hiring a professional even if it adds to the budget?
While a busy parent with basic DIY skills might handle some home projects, installing a tankless water heater is quite involved. It often requires electrical, plumbing, and sometimes gas line modifications, which can be risky if not done correctly. For safety, code compliance, and to protect your investment, hiring a professional is strongly recommended, even if it means a higher upfront cost.
If someone installs a tankless water heater and experiences disappointing performance, what troubleshooting steps should they take first to determine if installation mistakes are the cause before calling a professional?
Start by checking if the unit is matched to your home’s hot water demand and that the gas or electrical supply meets the manufacturer’s requirements. Confirm that the water lines are connected correctly and that there’s adequate water pressure. Also, review the venting setup for proper installation. If these basic checks don’t reveal the problem, then it’s wise to contact a professional.
Could you clarify what is meant by ‘careful sizing’ when selecting a tankless water heater? Are there specific calculations or guidelines for determining the right size based on household usage?
Careful sizing means choosing a tankless water heater that can handle your household’s peak hot water demand. To do this, estimate how many fixtures (like showers or faucets) might be used at the same time and add up their flow rates in gallons per minute (GPM). Also, consider the temperature rise needed—how much you need to heat your incoming water. Manufacturers usually provide sizing charts to help match your needs with the right unit.
After installing a tankless water heater, what are the most common performance issues people run into if something goes wrong? Are there early warning signs I should look out for?
Some frequent performance issues after installing a tankless water heater include inconsistent water temperature, low water pressure, and error codes on the display. Early warning signs can be longer wait times for hot water, water suddenly turning cold during use, or unusual noises from the unit. If you notice any leaks, fluctuating temperatures, or error messages, it’s best to address them promptly to avoid bigger problems.
When budgeting for this upgrade, should homeowners expect significant hidden costs related to new venting or electrical work not usually needed with tank-style water heaters, and are there ways to estimate those in advance?
Yes, homeowners should be aware that tankless water heater installations often require new venting systems or upgrades to electrical wiring, which can add to the overall cost compared to tank-style units. To estimate these costs, it helps to have a professional assess your home’s current setup before purchase. They can check if your existing gas line, electrical capacity, and venting meet the new unit’s requirements, giving you a clearer idea of potential extra expenses.
You mention that installation may require upgraded gas lines or electrical service. Is this typically a major additional expense, and how can I estimate those costs before purchasing a tankless unit?
Upgrading gas lines or electrical service can be a significant additional expense, depending on your home’s current setup. Costs can vary widely, from a few hundred to several thousand dollars. To estimate accurately, check your existing gas line size or electrical capacity and compare with the tankless unit’s requirements. A licensed plumber or electrician can provide a detailed quote before you commit to purchasing the unit.
Since installation mistakes can lead to costly repairs, do you have advice for someone trying to budget for both the purchase and proper installation of a tankless water heater? Are there parts or costs people usually don’t expect up front?
When budgeting for a tankless water heater, remember that you may need more than just the unit itself. Unexpected costs can include upgraded gas lines or electrical work, venting materials, permits, and possibly water treatment to prevent scaling. Get quotes from licensed installers, and ask them to detail all possible extra charges. It’s smart to set aside a contingency fund, typically 10-20% above initial estimates, to cover surprises.
It says that venting is a specialized requirement for tankless water heaters. Are there particular venting materials or layouts that work better for certain home types, or is it pretty standard across the board?
Venting requirements for tankless water heaters can vary depending on the model (gas or electric) and your home’s layout. Gas units typically need stainless steel or special PVC vent pipes, and the vent run should be as short and direct as possible. Homes with limited space or unusual layouts may use concentric vents or side-wall venting. Always follow the manufacturer’s guidelines and local building codes for the best fit.
When it comes to choosing the right tankless water heater, you talk about proper sizing and fuel compatibility. Are there any online calculators or easy rules of thumb homeowners can use to estimate what size will work for their families?
Homeowners can use online tankless water heater sizing calculators provided by most major manufacturers. These tools typically ask for the number of bathrooms, appliances, and the average simultaneous hot water use. As a simple rule of thumb, a family of four often needs a unit that delivers at least 7 to 10 gallons per minute (GPM). However, exact sizing depends on your climate and usage patterns, so using a calculator or consulting with a professional is the safest approach.
If I were to hire a contractor for this, what specific questions should I ask to make sure they understand the unique requirements of tankless water heater installation?
Ask the contractor about their experience with tankless water heaters specifically, not just standard water heaters. Check if they assess your home’s gas, electrical, and venting setup before installation. Ask if they handle permitting and follow local building codes. Also, confirm whether they size the unit based on your household’s hot water needs and if they offer support for future maintenance or repairs.
I’m trying to budget for this upgrade and wonder if the extra cost of upgrading gas lines or electrical service is usually significant. Do you have any ballpark figures or tips for keeping installation costs under control?
Upgrading gas lines or electrical service can add a significant amount to your installation costs—often anywhere from $500 to $2,000 or more, depending on your home’s current setup and local rates. To keep costs in check, get quotes from several licensed installers, ask if your existing lines are adequate, and look for any local rebates or incentives that might offset expenses.
The article states that tankless units are more complex to install and may even create dangerous conditions if mistakes are made. Could you elaborate on what those dangers might be and how to ensure my installation stays safe and up to code?
Tankless water heaters require precise gas, water, and electrical connections. Mistakes like improper venting, undersized gas lines, or faulty wiring can cause gas leaks, carbon monoxide buildup, fire hazards, or water damage. To keep your installation safe and up to code, always follow the manufacturer’s instructions, use the correct materials, and have the work inspected. Hiring a licensed professional is strongly recommended, as they’ll ensure permits are obtained and local codes are met.
Is there typically a big cost difference between retrofitting an older home for a tankless water heater versus installing one in new construction? I’m concerned about unexpected expenses.
There is usually a noticeable cost difference. Retrofitting an older home can be more expensive because it may require upgrades like new gas lines, venting, electrical work, or plumbing adjustments to fit the tankless system. In new construction, these needs can be planned for in advance, making installation smoother and often less costly overall. Factoring in these potential upgrades can help you avoid unexpected expenses.
How long does a full tankless water heater installation usually take for a first-time DIYer, and are there particular steps that tend to slow people down? I’m trying to estimate if this is a realistic weekend project.
For a first-time DIYer, installing a tankless water heater typically takes 8 to 12 hours, depending on your plumbing and electrical experience. Steps that often slow people down include rerouting pipes, updating gas or electrical connections, and securely mounting the unit. If you need to upgrade venting or deal with older home infrastructure, plan for extra time. For many, it’s possible to finish over a weekend, but having an extra day as a buffer is smart.
I’m thinking about hiring a contractor, but I want to avoid disappointments or unsafe setups. What questions should I ask a professional installer before committing to a tankless water heater project?
When speaking to a potential installer, ask if they are licensed and insured, and how much experience they have with tankless water heaters specifically. Request references from past installations. Find out which brands and models they recommend and why, and ask if they handle permits and code compliance. Also, clarify their process for sizing the unit and warranty coverage on both labor and equipment.
You mentioned that some people pick the wrong size tankless water heater for their house. What’s the best way to size one properly if I have a large household with multiple showers? Are there calculators or guidelines you recommend?
For a large household with multiple showers, you’ll want to calculate both your peak hot water demand (measured in gallons per minute, or GPM) and the required temperature rise. Count how many showers and appliances might run at once, add up their GPM rates, and use that total to guide your selection. Many manufacturers provide online calculators on their websites, which are quite helpful. You can also consult sizing charts that match typical usage scenarios to heater models.
I’m curious about the ‘careful sizing’ you talk about. If my family sometimes has guests over, should I choose a tankless model that’s slightly larger than what we usually need, or is it better to stick with the average household usage?
When sizing a tankless water heater, it’s wise to consider occasional peak usage, such as when guests are staying over. Choosing a model with a slightly higher capacity than your average needs can prevent running out of hot water during those busy times. Just make sure not to go excessively large, as this can lead to unnecessary costs. Estimating your maximum simultaneous hot water demands will help you pick the best size.