With the proliferation of DIY websites and how-to TV shows, and the need for people to save money on home repairs and upgrades, it's no wonder that many try to install or replace water heaters on their own. However, this really isn't a job for the DIY set, especially if the water heater runs on gas. Even if you're just replacing one water heater with an identical water heater, let professional contractors do it -- your new water heater, your home, and your safety will be the better for it.
Leaks and Insecure Connections
One risk you take if you try to install the water heater yourself is the risk of a leak. This can be a water leak from an insecure connection or a gas or carbon monoxide leak once you start the fuel flowing on gas-run heaters. You're not going to have the tools to check for these leaks (though hopefully you have a CO detector if you have gas appliances), and you might not notice the smell of gas or a water leak until it's too late to prevent damage.
Depending on what you're doing (replacing an identical water heater, switching fuel types, switching between tank and tankless types, and so on), you may need a permit to install the new heater. Sometimes the work is very involved and requires moderate construction and line re-routing. Contractors will be aware of the requirements in your city and should be able to apply for the permit on your behalf. Even though you could apply for one, you could end up answering questions incorrectly and delaying permission to install the new water heater.
Accidental Line Cuts
If you are trying to install a heater that uses a different fuel type (such as switching from gas to solar) or switching to a different tank style, you may have to sever and reconnect several lines. If you don't know what you're doing, you could end up cutting the wrong line and losing hot water or setting off a gas leak.
Complete Lack of Warranty on the Work
When you install an appliance yourself, you are responsible for anything that goes wrong. Even if the actual appliance has a warranty on parts, any problem with the connection is all your responsibility -- and you risk voiding the appliance warranty if you do the installation work incorrectly. That's a costly mistake and one you can avoid by having professional contractors do the installation.
Play it safe and spend a little more money to have the experts install the water heater. You'll be glad you did when that heater continues to work well and safely for years.