How to Install a Gas Water Heater

A man is working on a hot water heater

Installing a gas water heater is no small undertaking, even for a professional. Because of their relatively high failure rates, (every 8 to 12 years) this is something that plumbers do routinely.

Here, find out how to install a gas water heater by learning all of the major steps, and finish up with some maintenance tips to extend the life of the tank.

Water Heater Selection

Gas-fired water heaters have very high output ratings compared to other types. The size of the heater is based primarily on the number of people and the number of bathrooms in a home. Most manufacturers offer sizing tools on their websites that make all those calculations easy.

You will likely need a much smaller tank type heater if you choose one that burns gas. Some of these heaters can heat the water so quickly, they don’t use a storage tank at all. For now, we will focus on the tank type heaters since they are the most common. Learn more about the different gas water heater venting options on our blog post.

Water Heater Location

Once an appropriate heater has been selected, one of the first specifications to be considered is the size of the burner. Burners are rated according to the quantity of gas that it consumes, and along with that, it needs a tremendous amount of air to burn properly. Most gas water heaters use the air in the room to supply this “combustion air” to the burner. If the installation is in a mechanical closet, that room must be ventilated to provide enough air to feed the burner. If the water heater is located in a basement, that room must be large enough to provide combustion air for all the appliances located in that space.

These guidelines are all spelled out in the International Plumbing Code, down to the number of square inches needed for an outside air ventilation duct. This, along with the manufacturer’s instructions, provides plumbers with all they need to verify a safe location for the heater.

Location Red Flags

Sometimes when replacing an existing water heater, we discover what was once an open basement has been finished out by the homeowner and now the appliances are stuffed into a tiny room where no one has to hear or see them. Now, you can barely stand in front of the heater, much less close the door behind you. They soundproofed the room making it airtight. This was a very bad idea. Although it was most likely done unknowingly, if the remedy requires running some shiny new ventilation ducts across the ceiling just above their giant flatscreen, somebody is not going to be happy.

Any water heater should be located in a room equipped with a floor drain or some means to drain the heater. It also comes in handy if there is ever a leak.

When replacing a water heater, draining it can be a challenge with or without a floor drain. Older heaters often collect a deep layer of crusty sediment in the bottom of the tank that will clog up the drain valve, leaving you with hours of nothing to do but watch the water slowly trickle out of the end of your hose.

An experienced plumber carries a pump on the truck, just for this occasion. These transfer pumps can draw down a 40-gallon tank in just a few minutes.

Removing an Existing Water Heater

Before draining the heater, our first step is always to disconnect the unit from its power source.

How to Turn Off the Electricity

Standard vent water heaters operate with a mechanical control valve, so there is no electricity to be concerned with. Power-vented heaters plug into a standard 115V receptacle, so it’s as simple as unplugging it to safely de-energize the unit.

How to Turn Off the Gas

If the gas piping was properly installed, you should have a gas shut-off valve within a few feet of the appliance. These are typically quarter-turn valves. If the valve handle is parallel to the pipe, gas is allowed to flow through, if perpendicular, the gas is shut off.

If you see that the valve body and handle are both made of brass, or if the handle is square or a raised rectangular shape, use a small adjustable wrench and slowly turn the valve to the off position. Sometimes these old brass valves are hard to move and easily break. Using a small wrench, and not a BIG one, will keep you from applying too much force to the valve and connecting piping.

How to Turn Off the Water

Be just as careful when turning off the water. There should be a valve on the cold water line, serving only the heater, and there should NOT be one on the hot side. If the cold water valve turns off and on easily, and it completely stops the water, keep it. If not, replace it.

If there is a valve on the hot side, you’re going to remove it and save the brass for scrap.

How to Drain the Water Heater

With the water, gas and electricity shut down, it’s time to drain the heater. Right now, the heater weighs about 500 pounds, so this is not a step you want to skip.

  • While your transfer pump is running, go to the kitchen sink and open up the hot side of the faucet to drain the hot water piping.
  • If your shoes are clean, find the furthest, highest fixture, and do the same. If you don’t drain the piping, that water will drizzle the whole time you are working.
  • If the venting system is galvanized steel pipe, you can remove the screws from the first few fittings and take it apart. If it’s plastic, there is likely a clamp at the top of the vent motor assembly that you can loosen up to remove the pipe.

When the heater is done draining, the next task is to disconnect the piping. If the original installation was done correctly, there will be fittings on the gas and water lines that allow you to disconnect the unit.

Once it’s free of all those connections, you can start to move it out of the way. Grab the top and “walk” the heater out far enough that you can put it on a cart to haul it away. If there are any stairs involved, you will need a helper. It can be done with one man, but you’re just showing off and it’s not worth it.

Installing the New Gas Water Heater

Keep the unit in the box while you are carting it. You will notice the new unit is much lighter, partly because of the 40 pounds of soaked sediment that was in the bottom of the old one. Follow these steps:

  1. Set the new one in place, level it and if it doesn’t have any legs on the bottom, put some rubber isolation pads under it to keep it off the floor. If the unit is in a garage, reuse the stand it was on. It’s there for a reason.
  2. Line up the vent and repipe the water lines. You may need to reconfigure the vent fittings to artfully ensure the flue gases have clear passage to the outside.
  3. Inspect the existing vent system and make sure it is intact and terminated properly.
  4. Verify there are no soot, rust or blockages of any kind.
  5. When it comes to the water piping, flexible connectors are your friend. If there is a slight height difference, the flex connectors can take up the slack. They also come with a built-in dielectric union connection that keeps you in compliance with code. I usually save the gas connection for last, deciding how to repipe it while the heater fills with water.

Pro Tip: If the existing unit had a flexible gas connector, be careful about reusing it. In most cases, since you already have all the fittings on hand, it’s easy enough to repipe a few black steel fittings. Even on a flexible pipe installation, the last few fittings should be steel so you meet the typical requirements for gas appliances of having a shut-off, drip leg and union.

  1. Once you test all the gas fittings, you are ready to fire the heater. Don’t forget to turn off any of the faucets you opened. When all the air is pushed out of the system and water is running steadily, you know the heater is full and safe to fire.
  2. If it’s a power-vented type, you will have to plug it in first, find the on/off switch and then figure out how to light it. You may need to bleed the air out of the gas line so you’re not wasting time waiting for the air to squeeze through a tiny pilot orifice.
  3. Once the heater fires, it will take about 45 minutes to get up to temperature. This is a good time to go through the installation checklist to be sure you didn’t miss anything.

Pro Tip: It’s a good idea to check the gas pressure both while the unit is running and while the burner is off. Verify that you have drafted through the flue. Exercise the relief valve. If you want to go the extra mile, perform combustion analysis on the unit to verify you have a clean burn with no carbon monoxide. If you skip these final steps, you may get a callback. Nobody likes callbacks.

Water Heater Maintenance

Annual maintenance is recommended by most manufacturers. This typically includes:

  • Flushing sediment
  • Exercising the relief valve
  • Cleaning the vent blower (if it has one)
  • Inspecting the flue
  • Checking the thermostat control

It’s also a great idea to inspect the anode rod. Find the biggest ratchet you own and a 1 1\16” socket; wrestle it out of the heater so you can inspect it. Do this while the heater is still full.

That 500 pounds of water will work to your advantage and keep you from twisting the heater and damaging all the fine work you did to install it. If the anode has disintegrated down to the wire, or is significantly corroded, replace it.

Pro Tip: If someone told you to remove it, even if you have smelly water, don’t do it! You have been misinformed. That “solution” creates a much bigger problem down the road.

In any profession, experts can make a job look easy — a one-handed grab in the end zone, stealing second base, a long putt on a wavy green or defending a corner kick. We see athletes performing amazing feats of physical prowess and we pretend we can do the same. Plumbers have a certain finesse with fluids and a penchant for piping that seems to go beyond normal human abilities. People see what we do and sometimes think they can do it themselves. The results might look embarrassing and could be quite dangerous in this case.

Instead, get started with SkillMill™ online plumbing courses to expand your knowledge of the trade.


Frank Garro

Interplay Learning Plumbing & HVAC Expert

Frank is Interplay’s resident plumbing and HVAC expert. While he began his career in the plumbing trade, his passion for HVAC led him to become an Associate Professor and HVAC Program Chair, at Ivy Tech Community College. Frank is a Licensed Mechanical, Plumbing and Electrical Contractor, and owns his own mechanical services company. As an educator, Frank is a firm believer in teaching the fundamentals and has a gift for simplifying complex concepts.