Heating Household Water

If you’re a typical resident in the industrial world, 15-30% of the energy you use in your home goes for one purpose, to heat (and keep heated) the water you use for bathing showering, and washing. The petroleum era solution is a 30-80 gallon tank that stays full of water heated to 120 degrees or so, equipped with a thermostat that reheats it when the temperature of the water falls below the target level. This means, of course, that you’re paying to heat water not only when you’re showering and washing but also when you’re sleeping, working, on vacation, and just sitting around playing with the kids. 24/7. Heating, Cooling off. Reheating. Cooling off. Reheating. You get the idea.

In the post-petroleum era, we’ll have to look for more efficient ways of heating our water. We’ve gathered here the various choices available for domestic hot water, along with the relative advantages and disadvantages of each. Unlike your petroleum era solution consisting of one energy-guzzling appliance, the solution for domestic hot water in the post-petroleum world will likely be a suite of at least two means of heating water, one designed to “take the chill off,” and the other to finish the job and heat the water the last 5-30 degrees so it feels nice and hot when you call for it in your lavatory.

No Change in Equipment

Let’s be honest. Many of us will not have the option to upgrade our equipment to more efficient machinery, at least not right away. So we will struggle to reduce the energy use of our existing water heaters. Here’s how we will do it:

  • We will insulate the water heater and the output pipes to cut down on heat “leakage” as much as possible.
  • We will turn the thermostat down. You can probably lose 15 degrees without sacrificing much comfort at all.
  • We will avoid baths, which use 25 gal. or so of water, in favor of showers, which use 10 gal. or so.
  • We will get in the habit of turning our water heater off when we leave for vacation, and maybe even when we’re going to be away all day or asleep all night. Those of us who are particularly hard pressed (and there will be many of us in this group) may develop a habit of heating the water to temperature and then turning the heater off for 2-4 days. Only when the water has become too tepid for comfortable bathing and washing will we turn it back on just long enough to heat it to temperature again, and we will repeat that cycle indefinitely. Because the majority of the energy used by a residential water heater is for keeping the water hot (as opposed to making it hot), this simple technique will save a surprising amount of energy.
  • If and when our water heater needs to be replaced or is in need of major repair, we will replace it with one or more of the solutions below.

Active Solar

active solar directActive solar water heating relies on a pump to move water or an antifreeze solution between a solar heat collector and a holding tank. In areas where the temperature falls below freezing on a regular basis, the pump will move an antifreeze solution (this is called an indirect system). In areas where the temperature rarely or never falls below freezing, the actual water will move back and forth (this is called a direct system).

If you’re using a direct system, you will be passing the actual water through the thermal collector. That will work fine as long as the temperature stays above 32 degrees Fahrenheit (0 degrees Celsius). If the temperature is ever forecast to fall below freezing overnight, however, you’ll need to evacuate the thermal collector to make sure the water doesn’t freeze inside it. So if you live in an area where the temperature often falls below freezing (remember, it just takes one mistake on the night of a good hard freeze to create havoc), opt for an indirect system instead.

As an alternative to an indirect system, you could opt for a system that keeps water moving through the collector when the temperature falls below freezing. This will use energy, and it’s vulnerable if the pump ever breaks down, but this approach to active solar water heating is quite popular.

The most oft-recommended system today for most homes is an indirect version of an active solar system called a Solar Drainback Water Heater.

Passive Solar Thermosiphon

passive solar thermosiphon2Passive solar thermosiphon water heating operates on the same principle as active solar, with one significant difference. Instead of a pump to move the water or fluid from the collector to the tank and back, passive solar depends on convection. The tank must always be above the collector. The fluid in the collector heats up from the thermal energy of the sun and reaches a temperature above that of the fluid in the tank. The hot fluid expands and becomes lower in density and therefore lighter, and so it rises, displacing the cooler fluid and pushing it down to the collector. When the heating in the collector stops, so does the convection.

The biggest advantage of passive solar water heating is its simplicity and reliability. Because there are no pumps or switches, there’s precious little to go wrong. The disadvantage of passive solar is that it doesn’t use the sun’s heat quite as efficiently as an active solar system would. Another disadvantage of thermosiphon systems is that they work only when the temperature stays above freezing. If the temperature drops below freezing and stays there for long, a thermosiphon system will need to be either drained or kept warm somehow. For this reason, thermosiphon systems sometimes come equipped with a small circulating pump that turns on in very cold weather. The pump moves water through the system and keeps it from freezing in the exposed collector.

Integral Passive Solar

integral passive solarEven simpler than the passive solar thermosiphon system is an “integral” passive solar system. This means that there is no separate tank. The hot water supply actually sits in the solar collector. Because there’s 40-80 gallons of water sitting in the collector, they tend to be heavier than other solar collectors. Many integral passive solar water heaters actually sit on the ground beside the house rather than on the roof. Integral passive solar water heaters tend to be do-it-yourself projects, using a couple of old water heater cores from the junkyard, for example. Put a couple of old cores under a couple of thicknesses of glass and expose them to the southern sun, and it’s amazing how much of your hot water needs you can satisfy at low cost.

Because there’s not a great deal of equipment to buy for an integrated passive solar system to work well, there’s less profit for manufacturers, so relatively few companies try to sell them. Those who want to sell solar water heating tend to want to sell active systems or thermosiphon systems.


There are two kinds of geothermal water heating systems, those that are dedicated to the heating of water for use with a radiant heating system and those that are incidental to a geothermal heat pump. The use of a geothermal heat pump for the heating of water is extremely efficient, but not nearly so efficient as the use of passive solar for space heating. And radiant heating systems tend to be more expensive than the other alternatives we are describing here, so we won’t have much to say here on LettheSunWork about dedicated geothermal water heating. We’ll focus instead on the opportunities for water heating as an incident to a geothermal heat pump.

A geothermal heat pump works on the same principle as a traditional heat pump. The difference is that the geothermal heat pump uses the earth, or a water source, to discharge the extra heat in the summer and to take on heat in the winter. The opportunity for water heating comes from attaching a device called a “desuperheater” to the geothermal heat pump. During the cooling season, it collects a significant portion of the heat from the air before the heat is discharged to the ground loop and uses it to heat water. Even during the heating season the desuperheater can provide some water heating.


Except in the warmest climates, all solar water heaters and geothermal water heaters have one thing in common: they don’t always deliver water at a reliable 120 degrees F, which is the temperature at which most of us would call water “really hot” when it runs over our skin. So most of those installing these systems will opt to have some other kind of water heater that can provide the last few degrees of “lift” on cold days or when the sun isn’t shining brightly. Enter the tankless water heater.

Unlike a conventional water heater, which uses energy 24/7 to keep water hot, a tankless water heater turns on instantly when you call for hot water. A tankless water heater is particularly appropriate for solar and geothermal backup, because of its unique ability to run flat-out if needed or just add a tiny lift to the water temperature if needed. When the tankless heater is running, of course, it’s guzzling power (or natural gas). But it’s running only while you’re calling for hot water.

The capacity of a tankless water heater is all about “lift,” the ability of the heater to increase the temperature of the water. When the manufacturer of a tankless water heater describes its capacity, as in 3.6 gallons per minute, the manufacturer is making the assumption that the temperature of the water will need to be “lifted,” or increased, by 45 degrees F. The average ground water temperature in the U.S. is about 60 degrees F. If your solar or geothermal system has succeeded in lifting the temperature of your standing water to 90 degrees or so (which will almost always be the case), you will need a “lift” of only 30 degrees. This will improve your flow rate and perhaps allow you to size the tankless water heater at a slightly smaller level.