Tank VS Tankless
What's the difference between a water heater tank and tankless? Let's start with a water heater tank. It is a 20 to 75-gallon tank of water, typically located in your garage, a closet, or on the exterior of your house. They most commonly run on gas or electricity, and they heat the water in the tank from either a fire under the tank or electric heating coils in the tank. When you turn on a water fixture, it takes the water from the top of the tank and refills the bottom with cold water.
A tankless water heater is much smaller and does not have any reservoir. They are fueled primarily by gas due to how much electricity an electric one would need. Cold water enters the heater and goes through a heat exchanger with a very hot fire. In just a few seconds the water in the heater will "Instantly" go from cold to hot. When you are not using hot water the heater does not use any energy.
When you hear tankless provides Instant hot water, it can be deceiving. You might think hot water will come out the second you turn on the sink. However this is not the case. Once you turn on the sink, the tankless heater will sense water flow, and turn on the heater, this process could take a second or two, then it has to flow through the pipes filled with already cooled water to your sink. In reality, a water tank will be slightly faster due to the water already being hot, but it still has to flow through the already cooled pipes. If you want hot water the second you turn on your sink a circulation pump is the way to go. Circulation pumps take water from the heater and pump it through the house (an extra pipe leading back to the heater is needed). This will lower the efficiency but can save you water.
The main pro of a tankless is that you never run out of hot water. With a water heater tank once the tank of hot water is fully replaced with cold water, you will have to wait for it to heat back up (which can take up to an hour) before you can get hot water again. With a large family, this could mean having to stagger your showers or limiting your time in the shower. With a tankless, this is not a concern because the water is being heated up as it flows through the heater.
The main advantage of a tank is its reliability. Most tankless water heaters will need gas to heat the water, and electricity to manage the computers in the heater. If you lose gas, or electricity to your house, you will not have hot water. If you have a tank that runs on gas, you can lose electricity and it will function fine. Even if you lose gas (or electricity in an electric tank) you will still have a full tank's worth of hot water. Even if you lose water to your house in a natural disaster, a tank can provide you with life-saving water for days. (Keep in mind the tank uses the city water pressure, it will not come out from a sink, but you can get it from the maintenance port at the bottom.)
Let's talk efficiency, (when I say efficient, I'm strictly referring to operation cost) tankless are marketed as being more efficient because when you're not using it they are not wasting any gas. While this is true, modern water heater tanks are very well insulated, and don't loose a lot of heat into the room, and they are only getting better. Also tankless is only practical for a residential installation with a gas connection, If your electric rate is cheaper then your gas, or you have solar to offset it, A tank may be the more efficient route. If pure efficiency is your goal, you can look into getting a heat pump water heater tank, which instead of producing heat, it takes the heat from the room it's in and puts it in the water heater. This has the added bonus of cooling the room it's in.
People think tankless requires less maintenance, but that could not be further from the truth. Tankless water heaters need to be flushed with a descaler (such as white vinegar) every 6 months to a year (depending on use). In addition a lot of them have screens that need cleaning. A plumber is recommended for this service, and will inspect the rest of the water heater system to make sure it's operating correctly.
A tank should be "flushed" once a year to remove the sediment at the bottom of the tank. This is done by emptying the tank, filling it up a drop, and emptying it back out, and just keep repeating till the water comes out clear. Also a good idea to check the anode rod (the anode rod is a sacrificial piece of metal That is designed to get corroded so the water heater doesn't. If it's fully corroded, the water heater will corrode faster). While the maintenance could be done on your own without special tools, it's better to have a plumber do it so he can inspect the rest of the heater.
Water heater tanks will typically last around 8 years, and tankless typically lasts around 10. Both are capable of lasting far longer if proper maintenance has been done.
If you're thinking of upgrading from a tank to tankless, You may run into some obstacles along the way. Being that tankless heats all the water at once, instead of over time, they need a large gas supply. If your current heater is electric , and you don't have gas run to your house, tankless may not be a feasible option. An electric tankless water heater will require 3/4 of the amount of power your whole house consumes (meaning if the water heater and AC turn on at the same time, you will overload your electrical system), making them not feasible. The gas supply running to your tank right now may not be large enough, and in some cases you may even need to upgrade your gas meter, and gas pipe from the city, which can get very costly.
In conclusion tankless water heaters are amazing for large families, and people who need a lot of hot water in a short amount of time. Water heater tanks are far more robust, and with newer technologies, like heat pumps, could be more efficient than tankless. They both have pros and cons, so when choosing, make sure to choose one that will work for your household's needs.