Building Science 101: Water heaters
By Michael Maines
FIRST, LET ME GET THIS OUT OF THE WAY: It’s a water heater. You could call it a cold-water heater. Many people call it a hot water heater, but if the water is hot, why do you need to heat it? (You should hear what else gets my hackles up….)
Maine homes may or may not have hydronic heating—a boiler supplying hot water (or sometimes steam) to radiators of some sort—for space heating. But virtually all homes have domestic hot water, or hot water used for cleaning, cooking and other purposes. What is the greenest way to heat water for domestic use? As usual in the building science world, the answer is, “it depends.”
Going way back, water was heated in a kettle with a stove, fireplace or campfire. Beginning in the 20th century, most homes have one of three ways to make hot water, which is then stored in insulated tanks that usually hold 40 to 60 gallons (though they range from 20 to 85 gallons or more). These three methods—electric resistance water heaters, gas- or propane-fired water heaters and indirect water heaters—are all viable options today.
Types of water heaters
Electric resistance water heaters have two heating elements, one near the top and one near the bottom of the tank. They are simple, requiring little maintenance.
Gas- or propane-fired water heaters heat water with a flame. The exhaust may be vented atmospherically, which means there is a chimney near the heater but not directly connected to it. Better for health, safety and performance are sealed-combustion units. The highest efficiency (and highest cost) are condensing units, which more completely burn the fuel than non-condensing units do.
Indirect water heaters are tanks that hold water heated by the same boiler that creates hot water for space heating. With old, oversized, inefficient boilers, this is one of the least efficient ways to heat water—like firing up a dump truck for a trip to the grocery store. But if you have a newer, high-efficiency boiler for heating your house, it may be a smart move to use it for domestic hot water as well.
Another approach still seen in older homes is a tankless coil—the same boiler that heats the house heats domestic hot water directly, whenever hot water is called for. In the winter, when the boiler is running anyway, this can be efficient, but in warmer months this approach wastes energy due to the oversized equipment.
Voltex® hybrid electric heat pump. PHOTO COURTESY OF AO SMITH.
Less common but still effective is solar hot water—using the sun’s energy to heat water, usually via a heat exchanger, and storing it in a well-insulated tank.
There are also various ways to heat water with firewood, but those are unusual, and to reduce our carbon emissions we should be minimizing how much wood we burn. While all these older options are viable, a few more have come onto the scene since the late 20th century, like tankless water heaters, also called on-demand water heaters or demand water heaters. Their advantages include small size and no “standby losses”: water sitting in a tank, even a well-insulated tank, cools off over time and needs to be replaced with more hot water ready to go. Tankless water heaters can be electric or gas- or propane-fired.
Water heater efficiency
Gas- or propane-fired tankless heaters can be pretty efficient; only the water that’s used gets heated, with little waste—efficiencies of 90% are typical for condensing-style units, or about 80% efficient for non-condensing types. That means that 80–90% of the fuel burned goes directly toward heating the water.
Electric tankless heaters sized for a whole house draw enormous amounts of electricity—sometimes enough to dim your neighbors’ lights! While their total energy draw can be lower than that of a tank-style system, depending on various conditions, most electrical systems can’t handle a tankless electric water heater. However, smaller units can be a way to get hot water in smaller quantities, such as at a guest bathroom far from the main water heater or at a seasonal camp.
Heat pump water heaters
Heat pump water heaters (HPWHs) use the magic of phase change technology, just like the heat pumps proliferating for space heating. Most HPWHs look similar to a regular water heater tank, with a mechanical device sitting on top—the heat pump. It pulls air from the space around the water heater, extracts the heat and moisture, and sends dry, cool air back into the room. The energy efficiency of these units is impressive, using less than half of the annual energy needed for an electric resistance water heater. They cost significantly more than electric resistance water heaters but usually pay for themselves in just a few years, and many Maine homes could use the “free” dehumidification. Plus, many qualify for rebates from Efficiency Maine that bring the price down to the same or less than a traditional unit. (At press time, Efficiency Maine was offering an $850 rebate on heat pump water heaters.)
Downsides include complexity; having something akin to a refrigerator or air conditioner attached to your water tank increases the chance of a service call; the refrigerant used is a potent greenhouse gas if and when it leaks; and some people complain about the noise—though if running properly, it shouldn’t be louder than a refrigerator or dishwasher. Perhaps the biggest hurdle is the amount of air they need; because they extract heat from the air you have already paid to heat, in winter they can be accused of being an “energy vampire.” They cycle through a lot of air and usually need at least 1,000 cubic feet of air over 50°F to work properly. Most new basements are good locations, but older, uninsulated basements may get too cold in winter, and you can’t just tuck a HPWH into a closet the way you can with other types of water heaters. Going with an all-electric house is usually a greener approach than burning fossil fuels onsite, factoring in total energy use, renewable energy production, occupant health and safety, and carbon emissions. Despite the downsides of heat pump water heaters, they are usually my first choice for new homes and renovations.
The split-unit water heater is a twist on the heat pump water heater, currently supplied in the United States only by Sanden, a Japanese company. Like the mini-split heat pumps for space heating, Sanden’s split system consists of an indoor tank and an outdoor condenser/compressor. The split system means that the tank can be tucked pretty much anywhere it fits, even in small spaces, and any noise occurs outdoors. The efficiency of these units is impressive, up to five times the efficiency of electric resistance heaters, though three times is more typical during operation. The efficiency is thanks perhaps to Sanden’s most appealing virtue: It uses carbon dioxide as a refrigerant, which is several orders of magnitude less potent a greenhouse gas than the refrigerant used in most heat pumps and HPWHs. There are downsides, though. Because water travels from inside the house to the outdoor unit, it needs to be protected from freezing, so installation is more complicated than with more conventional systems. A split-unit water heater is also expensive. You will eventually see a payback, but the return on investment is not a high point.
How hot?
Water heaters can usually be set from 120°F to 140°F. The lower end is safe from Legionnaire bacterium, but will require more water at the tap. (Legionnaire’s disease is life-threatening, caused by a bacterium that, according to the CDC, thrives in water 77°F to 113°F.) The higher-end temperatures can be dangerous—able to scald sensitive skin—and standby losses are greater because of the larger difference in temperature with the air in the mechanical room, but less water will be required.
Choosing a water heater
Which type should you choose? With my focus on environmental and health impacts, I prefer all-electric homes, when possible, which can be run on renewable energy generated on- or offsite. My go-to for most homes is a conventional heat pump water heater. For off-grid homes, every kilowatt is valuable, and the primary heat source is often wood, so I recommend sealed-combustion gas- or propane-fired units. (Or old-school solar-thermal). For homes that have high-efficiency boilers, an indirect system can make sense. For a vacation home that will be used only occasionally, or for a household of just one or two people, tankless units are often the best option. For the most energy- and environmentally conscious, the Sanden split unit can be a good choice.
This article appeared in the Fall/Winter 2021 edition of Green & Healthy Maine HOMES. Subscribe today!