The list of water heating methods is a long one, natural gas, propane, electric resistance, wood burning, boiler, and passive solar, active solar, geothermal, and heat pumps.
For this article I’m not going to get into the more exotic water heating methodologies, I’ll stick to the more common methods of heating water for residential settings.
Solar Hot Water Heating
Solar water heating systems can be a great bargain, provide inexpensive or even free water heating, or on the other hand they can be some of the most expensive and high maintenance system types. There are passive solar systems and active systems that require pumps and controllers etc.
Passive solar systems do not have a pump. They just sit there and do their thing with no moving parts. Active systems use pumps, valves, and controllers. Active solar heaters are more efficient and more costly than passive solar systems.
Solar water heating is very location dependent. Passive systems are better if not used in areas that freeze often or have hard freezes. Active systems can drain the water or use other methods to protect the system from freezing trouble. There are different types of passive systems. Batch systems are one type that are basically a tank mounted in a tiny green house.
Some passive systems use solar collectors. Solar collectors mounted lower than the tank can use natural thermal siphoning action to transfer the hot water to the storage tank. There are of course many variations.
If you live where freezing is a problem then you need an active system of which there are many varieties. Some heat the water directly, some use an anti-freeze solution in the collectors, some drain the water out of the system when it approaches freezing etc.
Storage Water Heating
Standard storage type gas and electric water heaters are hard to beat. They are simple, require virtually no maintenance, and seldom have problems. There are high efficiency gas water heater models that cost a little more but rival tankless on efficiency. They still have storage standby losses though. Standby losses probably amount to less than $5.00 a month realistically.
If something breaks down with a storage heater it’s easy to find the parts and just about any plumber or handyman can fix it. It’s an easy project for most do-it-yourselfers. And tank type water heaters usually fail by way of a tank leak. When you see a puddle under your water heater you know it’s time to purchase a new one. It gives you time to get it replaced without having to live for a few days without hot water.
Storage or tank type water heaters provide the hot water faster than tankless heaters since the water is already hot unlike the tankless units.
Heat Pump Water Heaters
Heat pump water heaters are a highly efficient method of water heating. Heat pump water heaters don’t have to make the heat as with other types of water heaters. They don’t work well in cold climates due to an insufficient amount of heat available in the environment around the heat pump.
Heat pumps work just like your air conditioner. A heat exchanger inside the tank heats the water and another heat exchanger outside pulls the heat out of the outside air. There just has to be sufficient heat in the surrounding air.
Heat pump water heaters are slower to heat water than electric resistance heaters or gas water heaters. As the water temperature increases and/or the outside air temperature decreases the heat pump heats the water even slower.
Heat pumps are much more expensive than standard electric water heaters and will be harder to find service for but they are sometimes a perfect solution where gas water heaters are not practical.
Heat pumps usually have a backup resistance heating element in the tank for those times when the outside temperature drops too low for the heat pump to work effectively.
Tankless Water Heaters
Tankless water heaters are hot products lately, presumably because they are a green, reducing the amount of energy you use to heat your hot water. Tankless water heaters do not have the standby losses inherent with tank type water heaters. The monetary savings isn’t all that much. You can probably save about $5.00 a month.
Gas tankless water heaters are much more common than electric tankless heaters. To heat water rapidly as a tankless heater must do it takes a whole lot of electricity. For a decent sized tankless heater you will need to run a dedicated wiring run and you might even need to upgrade the wiring from the power lines to your home. It gets very expensive.
Gas tankless water heaters also need significantly larger diameter venting than storage heaters do, and often it needs to be stainless steel which is pricy. The gas tankless heaters require a larger gas line than storage water heaters as well. That too can be expensive.
In order to start a tankless water heater, and to keep it on, you have to draw a minimum flow rate of ½ to ¾ gallons per minute of hot water. If you draw less than the minimum flow rate the water heater shuts off. Forget being able to run a trickle of hot water.
Tankless water heater need to be installed correctly. Failure to do so can result in problems that might not show up for years. Improperly draining the condensate can destroy the heat exchange which is expensive and won’t be covered under the warranty due to the improper drainage.
Point-of-Use Water Heating
Point of use water heaters are simply water heaters located very close to the fixture or fixtures they serve. They can be small tankless water heaters designed for small uses like hand washing and making tea, or they can be full water heaters of any type as long as they are located very close to the fixture. This pretty much eliminates the wasted water run down the drain while waiting for the hot water since there is virtually no wait with such a plumbing system.
Combination Systems
One can combine some of these water heating methods to save more water and energy making them even greener. You can use both a tankless water heater and a solar water heater hooked up together. You have inexpensive water heating and you won’t run out on those occasions when you need a large volume. If you combine systems you should be sure to check with the manufacturers because the equipment might not be compatible.
With any type of water heating you should check your local building codes and regulations before you change the type of water heater you are using.
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