Heating water

Posted Tuesday, March 4, 2014 in Sustainable Maine

Heating water

by Paul Kando

Heating water for our household needs is the largest single year-round energy user in most houses. There are many ways to do it. Old standbys include tanks of water heated either by gas or electricity, an internal “on-demand” heat exchange-coil in the heating system’s oil-fired boiler, and an external “boilermate” tank hooked to the boiler much like another heating zone. Each of these have their pluses and minuses. 

An electric water heater is least expensive to buy but uses expensive power to maintain the temperature in a whole tank of water whether we use any of the water or not. It uses electric resistance elements that get hot the same way as a toaster does. A conventional gas water heater is more expensive to purchase, but it is heated with somewhat less expensive fuel gas. It, too, has to maintain that tankful of water. Heating water using the heating system’s boiler seemed like a great idea when oil was dirt cheap – which it is no longer. The in-boiler heat exchanger and the boilermate both require the boiler to come on even outside the heating season and the boilermate must also maintain the tank temperature. The in-boiler heat exchanger turns the boiler on even to heat just a few gallons of water. Furthermore, both boiler-based water heaters involve inefficient boiler operation.

In recent years other water heating options have become available. Tankless  water heaters, powered by gas or electricity, heat water on demand. They do so by means of small diameter water pipe coiled around a high temperature heating element. Whenever a hot water faucet is opened, a sensor fires up the heating element, which heats the water as it passes through the coil. Tankless water heaters come in two varieties: small, point-of-use models serve a single faucet, shower or washer; while large, whole house models serve a whole house or apartment. Tankless heaters cost more to buy than heaters with storage tanks, but they are more economical to operate, precisely because they don’t have to maintain the temperature of a tank full of water. A drawback of tankless heaters is that  hard water can clog the small diameter pipe-coil with mineral deposits. Another is their relatively complex electronics, for which local servicemen are difficult to find.

A relative newcomer to the water heater market is the “hybrid" or heat pump water heater. The heat pump takes energy from its surroundings and, by means of compression, increases a heat transfer fluid’s temperature to the level suitable to heat the water.  Whenever any form of energy is conveyed to matter, the target matter’s temperature rises.  In this case the energy to heat water is conveyed in the form of pressure.  In place of a heating element, an electric motor drives a compressor. Modern heat pumps work down to below-freezing ambient temperatures. An unheated basement, for example, has plenty of heat energy to draw on, at a pretty steady temperature, in the mid-50ºs to 40º range.

According to the 2nd  law of thermodynamics, energy's capacity to do work diminishes with use. But the energy is still there because, according to the 1st  law of thermodynamics, energy cannot be lost or destroyed. Regardless of temperature, there is some heat energy present every place we humans encounter. (Actually this is true all the way down to absolute zero –  3rd law, beyond our concern here). The heat pump takes this energy and adds to it by compression, which  increases the temperature. The colder the ambient temperature, the less efficient the heat pump becomes –  but never less than the 100% electricity-to-heat conversion rate of an electric resistance heater. With heat pumps, the term for efficiency is "coefficient of performance" (COP), and a COP of 1 equals 100%. State of the art heat pumps have COPs as high as 4 –  i.e. their electricity-to-heat conversion can be 4 times better than that of a straight electric resistance water heater.

The main advantage of heat pump water heaters is their high efficiency. Disadvantages include relatively high initial cost, and the need to maintain a tank temperature, whether the hot water is needed or not. Also heat pump water heaters are mechanical devices with complex moving parts . In contrast, an old fashioned electric water heater has two screw-in heating elements and no complex subsystems or moving parts. That is why these “hybrid systems” cost more to buy and maintain.

Bottom line: the least expensive water heater  to purchase is an old fashioned electric one. The next cheapest to buy (but not necessarily to operate) is probably an old-fashioned gas (propane) heater. But the most efficient -- and on the long run "cheapest to operate" (not counting solar) is either a tankless or a heat pump water heater. Heat pump water heaters are winners in my book because they are very efficient, the technology is familiar to local service people and the energy they use is electricity –  the easiest form of energy to generate from multiple,  notably renewable,  sources. It is also the cleanest, easiest to transport energy form we have available. With solar photovoltaic panels on your roof, heat pump water heaters are a great environment- and pocketbook-friendly choice.  I will explore solar water heating on another occasion.

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