Nobody likes an ice-cold shower. Seriously. You are halfway through washing your hair and BAM—freezing water. If you are sick of that happening, you have probably looked into those tankless water heaters everyone keeps talking about.
But here is the thing.
Installing one of these isn’t like screwing in a new showerhead. It is a major project. You are dealing with gas lines. Upgraded electrical wiring. Complex venting systems. Getting it right means endless hot water. Getting it wrong? That is a massive safety hazard.
So let’s break down exactly what goes into putting one of these in. We will look at what you can actually tackle yourself to save some cash, and when you absolutely must call in a plumber. If you are wondering how to install a tankless water heater from start to finish, grab a cup of coffee. Let’s get into it.
Best Safety Disclaimer
Safety Disclaimer: Installing a tankless water heater can involve gas lines, electrical wiring, water pressure, exhaust venting, and local building code requirements. Gas, electrical, and venting work should always be completed or inspected by a licensed professional according to local code and the manufacturer’s installation manual. If you are not fully qualified, use this guide for planning and preparation only, and hire a licensed plumber or contractor for the installation.
First Off, What is a Tankless Water Heater?
Before we start ripping pipes out of your wall, let’s cover the basics. What is a tankless water heater?
Think about your old water heater. It is essentially a giant metal pot sitting in your basement or garage. It holds 40 or 50 gallons of water and keeps it hot 24 hours a day, 7 days a week. Even when you are sleeping. Even when you are on vacation. You are paying for that energy.
A tankless system changes the game. There is no tank. It is just a box on the wall. When you turn on the hot water tap in your bathroom, cold water flows into the unit. A sensor detects the flow and instantly fires up a massive gas burner or an electric heating element. The water gets flash-heated as it passes through the heat exchanger. You get hot water right then and there. When you turn the tap off, the heater shuts down. No wasted energy.
Tankless vs Tank Water Heater: Quick Comparison
| Feature | Tankless Water Heater | Traditional Tank Water Heater |
|---|---|---|
| Hot water supply | Heats water on demand | Stores 40–80 gallons of hot water |
| Energy use | Usually more efficient because it only runs when needed | Uses energy all day to keep stored water hot |
| Upfront cost | Higher equipment and installation cost | Lower initial cost |
| Installation difficulty | More complex; may need gas, venting, or electrical upgrades | Usually simpler replacement |
| Space needed | Wall-mounted and compact | Large floor-standing tank |
| Lifespan | Often 15–20+ years with proper maintenance | Usually 8–12 years |
| Maintenance | Needs regular flushing, especially in hard-water areas | Needs flushing, anode rod checks, and tank maintenance |
| Best for | Homes wanting efficiency, space savings, and long-term performance | Homes needing lower upfront cost and simpler replacement |
Read More: How to Build a Simple Floating Shelf: Step-by-Step Guide
What is the downside of a tankless water heater?
We have to be honest here. These things are amazing, but they are not flawless magic boxes. So, what is the downside of a tankless water heater?
First, the upfront cost. They are expensive to buy and even more expensive to install. Second, retrofitting an older home is a nightmare. Your existing gas lines are probably too small. Third, there is something called the “cold water sandwich.” If you are doing dishes, turn the water off for a few seconds, and turn it back on, a small slug of unheated water slips through. You get hot water, then two seconds of cold, then hot again. It is annoying, but most people get used to it quickly.
How to Size Your Tankless Water Heater
What size tankless water heater do I need?
Do not guess on this. If you guess wrong, you will be miserable. So, what size tankless water heater do I need? It comes down to two numbers: flow rate and temperature rise.
Flow Rate (GPM)
GPM stands for gallons per minute. You need to add up the water you plan to use at the exact same time. A standard showerhead uses about 2 GPM. A kitchen sink uses about 1.5 GPM. If you want to run the shower and wash dishes simultaneously, you need a unit that can handle at least 3.5 GPM.
Temperature Rise
Where you live really matters here. If you live in Florida, the groundwater coming into your house might be 70 degrees. The heater barely has to work to get it up to 120 degrees. But if you live in Minnesota? That water might be 40 degrees in the winter. The heater has to work incredibly hard to heat that freezing water. If you live up north, you need a unit with a much higher BTU rating. Always go a little bigger than you think you need.
Pro Tip: Always size up slightly when choosing your unit. If you calculate that you need exactly 3.5 GPM, buy a unit that handles 4.0 or 5.0 GPM. Having a little extra capacity is much better than running out of hot water on a freezing morning.
The Big Question: DIY or Pro?
Can I install a tankless water heater myself?
This is the most common question on internet forums. Can I install a tankless water heater myself? Or, phrased another way, can I install a tankless water heater on my own?
The short answer is: mostly no.
Unless you are a licensed plumber or an extremely advanced DIYer who has run gas lines before, you should not do the whole job.
Here is what you can do yourself. You can drain the old tank. You can remove it. You can build the mounting frame on the wall. You can even hang the new unit. But running a new high-capacity gas line? Drilling through your roof to run category III stainless steel exhaust venting? That is where things get dangerous. A bad water connection ruins your drywall. A bad gas connection takes down your house. Carbon monoxide is not a joke.
Why are plumbers against tankless water heaters?
You might have heard rumors about this. Why are plumbers against tankless water heaters?
The truth is, they aren’t against them at all. Plumbers love them. What they hate is dealing with homeowners who bought a cheap unit online and expect a $200 installation. Plumbers know how hard it is to retrofit these into older homes. They know that undersized gas lines will make the unit throw error codes constantly. They push back because they want to do the job right, and doing it right costs money.
How much should a plumber charge to install a tankless water heater?
Let’s talk numbers. How much should a plumber charge to install a tankless water heater? For just the labor, expect to pay between $800 and $2,500. It depends heavily on where you live and how hard the job is. Does the plumber need to run 30 feet of new gas pipe? Do they need to cut a new hole in your roof for the vent? Do they need to pull city permits? (Yes, they do).
By the time you buy the unit, pay for the permits, and pay for the labor, a full professional install usually runs between $3,000 and $4,500.
Getting Your House Ready
Before the physical box goes on the wall, your house needs some upgrades.
- The Gas Line Upgrades
Your old tank heater probably ran on 40,000 BTUs. A standard half-inch black iron gas pipe feeds that just fine. A new tankless unit demands anywhere from 150,000 to 199,000 BTUs. It is a massive jump. Your old half-inch pipe will literally starve the unit of gas. You will need a plumber to run a new 3/4-inch or even 1-inch gas line from your meter straight to the heater.
- New Water Line Installation
You also have to reroute your plumbing. A new water line installation is required to connect your existing hot and cold copper pipes to the bottom of the new unit. This involves cutting your old pipes, cleaning them up, and soldering new fittings.
More importantly, you have to install isolation valves. Do not skip this. These are special valves that go on the hot and cold lines right under the heater. They let you shut off the water to just the heater so you can perform maintenance later without turning off the water to your whole house.
Tools & Materials Checklist
Before you begin installing a tankless water heater, make sure you have the right tools and materials ready. Having everything prepared will make the job safer, faster, and easier to manage.
Tools You May Need
- Pipe wrench
- Adjustable wrench
- Pipe cutter
- Drill and masonry anchors
- Screwdriver set
- Level
- Measuring tape
- Propane torch, if soldering copper pipes
- Bucket and garden hose
- Safety gloves and eye protection
- Spray bottle with soapy water for gas leak testing
Materials You May Need
- Tankless water heater unit
- Mounting bracket or plywood backing board
- Hot and cold water isolation valves
- Pressure relief valve
- Gas shutoff valve
- Flexible gas connector
- Gas-rated pipe tape or pipe dope
- Vent pipe approved by the manufacturer
- Copper, PEX, or approved water supply piping
- Pipe fittings and connectors
- High-temperature silicone sealant
- Electrical outlet or GFCI connection if required
The Process: Installing a Tankless Water Heater
Okay, let’s get our hands dirty. If you are doing the prep work, or just want to understand what your plumber is doing all day, here is how the process works.
Step 1: Kill the Power and Drain the Dinosaur
First things first. Turn off the gas valve next to the old heater. Turn off the cold water supply valve above it. Go to your breaker box and shut off the electricity to the unit.
Now, grab a garden hose. Hook it up to the drain valve at the bottom of the old tank. Run the other end of the hose to a floor drain or out into the driveway. Go to a nearby bathroom and open the hot water tap. This breaks the vacuum inside the pipes and lets the water flow out.
Be patient. It takes forever. And do not try to move the tank while it has water in it. Water is incredibly heavy.
Step 2: Rip It Out
Once it is completely empty, use two pipe wrenches to disconnect the gas line. Cut the hot and cold water pipes with a pipe cutter. Disconnect the old exhaust vent. Put the tank on a hand truck and wheel it out of your life.
Step 3: Mount the New Box
Pick a spot on the wall. You want it close to your new gas line, close to your water lines, and preferably on an exterior wall so venting is easier.
These units are heavy. Do not just screw them into drywall. If you have a concrete basement wall, drill in some masonry anchors, attach a piece of 3/4-inch plywood to the wall, and then screw the heater’s mounting bracket straight into that thick plywood. Hang the unit. Make sure it is perfectly level.
Step 4: The Venting Nightmare
This is the hardest part of installing a tankless water heater. You cannot use your old brick chimney. The exhaust from a tankless unit is cooler than a tank unit. That means it creates condensation. That condensation is highly acidic and will literally eat through your brick and mortar over time.
You need direct venting. This means running a pipe straight out the side of your house or straight up through the roof. If you have a non-condensing unit, you have to buy an expensive Category III stainless steel vent pipe. If you bought a condensing unit, you can usually use schedule 40 PVC.
Cut the hole in the wall. Run the pipe. Seal the absolute heck out of the exterior hole with high-temp silicone so bugs and rain stay out. Keep the vent pipe as straight as possible. Every elbow you add reduces how far the fan can push the exhaust.
Safety Warning: Improper exhaust venting is one of the leading causes of carbon monoxide poisoning in homes. Never use your old masonry chimney for a tankless heater. If you are unsure about running direct vent pipes, hire a licensed professional for this specific step.
Step 5: Gas Connections
Run the new 3/4-inch gas line to the bottom of the heater. Right before the unit, you need to install a sediment trap. This is just a small downward-facing pipe that catches dirt and rust before it gets sucked into your expensive heat exchanger.
Add a gas shutoff valve, then use a flexible gas line to make the final connection to the unit. Use yellow Teflon tape or gas pipe dope on all threaded connections.
When it is tight, turn the gas on. Spray every single joint with soapy water. Watch closely. If you see bubbles forming, you have a gas leak. Turn the gas off immediately and tighten the fitting. Never skip the bubble test. Ever.
Safety Warning: Never skip the bubble test. Mix a little dish soap and water in a spray bottle and soak every single threaded gas connection. Even a microscopic bubble means you have a dangerous gas leak. Shut it off and retighten it immediately.
Step 6: Water Lines and Pressure Relief
Now for the water lines. Solder your copper pipes to the isolation valves we talked about earlier. Clean the copper pipe ends with sandpaper until they are shiny. Apply flux paste. Put the pieces together, heat the joint with a propane torch, and touch the solder to the pipe. It will suck right in.
Attach the cold water line to the right side and the hot water line to the left side.
Finally, install a pressure relief valve on the hot water outlet. Plumb a small pipe from that valve down toward the floor. If the pressure inside the heater ever spikes, this valve will pop open and shoot water out. You want that water hitting the floor, not spraying you in the face.
Step 7: Plug It In
Most of these run on standard 120-volt electricity. They just need to power the computer board and the spark igniter. Plug it into a grounded outlet. If it is in a basement or garage, make sure it is a GFCI outlet so it trips if it gets wet.
Getting Things Running: Tankless Water Heater Setup
Your tankless water heater setup is almost done. But do not turn the power on yet.
You have to purge the air out of the pipes. If you fire up the heater while there is air in the heat exchanger, it will burn the internal components instantly. It is an expensive mistake.
Open the cold water valve to let water flow into the unit. Go around your house and open every hot water faucet. Sinks, showers, tubs. Let them run. At first, it will spit and sputter violently as the trapped air gets pushed out. Let it run for about five minutes until the water stream is perfectly solid and smooth.
Shut off the faucets. Now you can hit the power button on the heater. Set the temperature to 120 degrees. Anything hotter is a scalding risk, especially if you have kids.
Go turn on a shower and listen to the heater. You should hear the fan kick on, followed by a slight tick-tick-tick of the igniter, and then a quiet hum as the burner fires up.
How long does a water heater take to heat up?
People always get confused by this. They turn on the tap and expect boiling water in one second. So, how long does a water heater take to heat up?
The unit itself heats the water in about two seconds flat. But that hot water still has to travel from your basement all the way up to your second-floor bathroom. It has to push all the cold water currently sitting in your pipes out of the way first.
It takes the exact same amount of time to reach your showerhead as your old tank heater did. It is not a slow heater. It is just basic plumbing physics.
Maintenance Is Not Optional
If you ignore your new heater, it will die in five years. If you take care of it, it will easily last twenty. Hard water is the enemy. The minerals in your city’s water get baked onto the inside of the tiny copper tubes in the heat exchanger. This blocks water flow and destroys efficiency.
How to flush a tankless water heater
You need to do this once a year. Maybe twice if your water is incredibly hard. Here is how to flush a tankless water heater.
Remember those isolation valves you installed? This is why you need them. Turn off the gas and power to the unit. Shut off the hot and cold isolation valves to isolate the heater from the rest of the house.
Get a 5-gallon bucket and a small submersible sump pump. Hook a washing machine hose from the pump to the cold water service port. Hook another hose from the hot water service port back into the bucket.
Dump four gallons of undiluted white vinegar into the bucket. Drop the pump in. Plug the pump in and let it circulate that vinegar through the heater for a full hour. The acid in the vinegar will eat away all the calcium and scale buildup inside. After an hour, dump the vinegar, flush the unit with clean water for a few minutes, and you are done.
How to clean tankless water heater filters
There is one more tiny task. People always forget this one. Here is how to clean tankless water heater filters.
Right where the cold water enters the unit, there is a small cap you can unscrew. Behind it is a little mesh screen. This screen stops dirt, rust, and pebbles from getting inside the heater. Over time, it gets clogged up, which drops your water pressure.
Turn off the water, unscrew the cap, pull the screen out, and rinse it under a sink. Pop it back in. It takes roughly two minutes and makes a massive difference in how well your shower runs.
Frequently Asked Questions
Do tankless water heaters need permits?
In most areas, yes. A tankless water heater installation usually requires a permit because it may involve gas lines, electrical work, venting, water connections, and pressure relief safety requirements.
From a plumber’s point of view, this is not something to skip. Permits help make sure the system is installed safely and inspected properly. Rules vary by city, so always check your local building department before starting the job.
Can I replace a tank water heater with a tankless water heater?
Yes, but it is not always a simple swap. A traditional tank heater and a tankless unit work very differently.
In many homes, the gas line may need to be upsized, the venting system may need to be changed, and the water lines may need to be rerouted. Electric tankless units may also need a major electrical panel upgrade.
So yes, replacement is possible, but a licensed plumber should inspect the home first to confirm what upgrades are needed.
How often should a tankless water heater be flushed?
Most tankless water heaters should be flushed once a year. If you live in an area with hard water, flushing every 6 months may be better.
Minerals like calcium and magnesium can build up inside the heat exchanger. Over time, that reduces efficiency, lowers water flow, and can shorten the life of the unit. Regular flushing is one of the best ways to protect your investment.
Is gas or electric tankless better?
For most whole-house installations, gas tankless water heaters are usually the better choice because they can heat more water at a higher flow rate.
Electric tankless heaters can work well for small homes, apartments, point-of-use sinks, or places where gas is not available. But for multiple showers, laundry, and kitchen use at the same time, gas usually performs better.
The best choice depends on your home’s fuel source, electrical capacity, hot water demand, and installation budget.
Final Thoughts
One of the best upgrades you can make to your home is installing a tankless water heater. Unlimited hot water. Lower energy bills. It will last for at least 20 years with proper care.
But don’t underestimate the dangers of the labor. The gas line, the venting, the electrical work— those are difficult jobs that take skill and risk. Talk with yourself about the tasks you can safely undertake.
This guide will help you if you know your way around gas and plumbing work. If you don’t have a lot of experience, go ahead and do it yourself: clear space, build the mounting frame, plot the vent route, and hire a professional to do the gas and electrical work. In either case, you’ll end up with a system that makes your old tank heater look ancient.
Final Editorial Note: This guide was reviewed with plumber-level expertise to help ensure the installation, safety, and maintenance advice is practical, accurate, and homeowner-friendly. Always follow local code, manufacturer instructions, and licensed professional guidance for gas, electrical, and venting work.
