Mechanicals-Electric Water Heaters

An electric hot water heater can be one of the most expensive electricity-using appliances in a home.  It’s not uncommon for a typical family of 4 to use 400 kWh per month heating water in my area with an expense of $40-$50.  There are some options to reduce these costs.  The topic of this week’s blog, electric hot water heaters.

Big box store display. Notice the recirculating pump and digital control panel.

Using electricity to heat water has both advantages and disadvantages. Let’s start with the advantages.

1. The cost of the water heater.  A standard electric water heater will typically cost a little less to purchase over a comparable natural vent gas water heater, but not by much.
2. Almost all homes have electricity, some do not have gas.  The cost to run gas lines, exhaust vents, and possibly a gas storage tank adds to the cost of a gas water heater.
3. Electric water heaters are safer.  No exhausting gases, no chance of back drafting carbon monoxide.

The disadvantages:

1. Cost to operate.  The current electricity costs in my area make them more expensive to operate.
2. Recovery rate.  Electric water heaters are slower to heat water.  Some families will run out of hot water during periods of high demand.
3. On demand options.  They are limited with electric water heaters.  An on-demand electric water heater needs a large electrical service dedicated to the water heater, often 200 amps.  If you are in an area with demand charges, definitely not a good idea.  I only recommend gas on-demand water heaters.

All storage water heaters have some sort of storage tank.  These tanks can vary in size from less than 5 gallons to 120 gallons or more with the average being 40 to 50 gallons.  The tanks can be metal or plastic with some metal tanks having an additional liner.  Glass, enamel or plastic coatings can be used.  The tank is located on the inside of the water heater’s shell and can only be seen by removing one of the element/thermostat access panels.  A layer of insulation is located between the tank and outer shell.  This insulation is often fiberglass, but foam is sometimes used.  An electric water heater will also have at least one heating element and thermostat, over temperature limit switch, fill tubes, a pressure relief valve, and an anode rod.

Thermal image of a standard electric water heater. The purple pipe is the cold incoming water and the yellow is hot outgoing water.

An electric water heater has two safety devices.  The first is a temperature over limit switch.  If the water temperature in the tank should become hotter than a set limit, around 180 degrees, the over-temp limit switch will turn off the power to the elements.  This switch must be manually reset.  The second safety device is the pressure relief, or T & P valve.  If water pressure should exceed a certain limit, around 150 psi, or the temperature with the tank should exceed 210 degrees, this valve will open to release the pressure.  Hot water will be discharged.  Plumbing code requires an extension of the valve so that hot water will not scald a person.

How does an electric water heater heat work?  Water is heated with a loop of metal called an element.  An element consists of a wire inside an outer metal shell. Electricity flows through the wire, which is resistive.  This resistance creates heat that heats up the element, which in turn heats the water.  Most electric water heaters have two elements, one in the middle and one at the bottom of the tank along with two thermostats and the over-temperature safety device.  The water heater starts by suppling electricity to the upper element.  Once the water at the upper level of the tank is heated, the power will switch to the lower element. It is typical that only one element will be heating at a time.

Installing an electric water heater.  Of course, the water heater will need to be attached to the plumbing system.  Most electric water heaters will require a 220-volt, 30 ampere circuit.  A 30 amp over current device, or breaker, requires a 10-2 or larger wire, modern #10 wire will be orange.  If the water heater is located “out of sight” of the electrical service panel, a disconnect, or a way to turn off the power to the water heater, is required and must be located near and with-in sight the heater.  There are variations, some electric water heaters only have one lower wattage element and are designed to be a cord and plug installations.  Always follow the manufacturers instruction and the National Electric Code requirements.

How water heater’s efficiencies are rated.  There are two recently reformed metrics for rating the efficiency of a residential water heater, Uniform Energy Factor (UEF) and first hour rating.  The first hour rating is how much hot water a water heater can deliver after an hour of use.  The Uniform Energy Factor is how efficient a water heater is based on it’s first hour rating and something called a BIN number.  Water heaters are categorized in four different BINs based on hot water usage.  The categories are Very Small-10 gallons, Low-38 gallons, Medium-55 gallons and High-84 gallons.  Assigning a water heater a BIN based on usage, then evaluating its performance based on usage and first hour rating determines its efficiency.  Clear as mud, right.  Just know that a water heater with a UEF of .95 will cost less to operate than a UEF of .80.

OK, so we know how electric water heaters work and how they are rated, how can I reduce the cost to operate them?  The amount of energy needed to heat water will vary depending on incoming water temperature, temperature setting of the water heater, how much hot water is used by the household and how quickly the water heater loses its heat.

We will start with the cheapest and easiest.  A water heater blanket will help reduce what is called “stand-by losses”.  Because most water heaters aren’t super insulated, some heat will be loss through the tank and case.  A blanket will help to reduce this heat loss.  Heat will also be loss through the plumbing connections.  The heat of the water will conduct through the water lines, especially if they are metal. Insulating the hot water pipes will help retain some of the heat.

Install low flow aerators and shower heads.  Most new faucets and shower heads will use lower amounts of water, but some older homes may not have been updated with water savings devices.  Some electricity providers may offer rebates, reduced cost sales or even free water savings devices.

Another option is to purchase a more efficient water heater, or one with a higher uniform energy factor.  Most efficiency upgrades will be accomplished by better insulated the tank and using materials that conduct less heat.

Many power suppliers, both rural electric cooperatives and public utility suppliers in my area offer reduced electric rates for off-peak or controlled water heating programs.  A good example is the rural electric coop I contract with.  The normal cost for electricity is around $.125 per kWh.  The rate can be reduced by using one of two programs, the first is called off peak water heating and has a rate of $.0495 per kWh.  This program requires at least 100 gallons of water heating capacity and the water is only heated overnight.  This program works well with an average family, but if the family is large or the home has a high demand for hot water, there is a good chance 100 gallons will not meet the household needs.  The second program is called interruptible water heating.  The water heater is “controlled” for up to 8 hours per day, a 50-gallon water heater can be used.  The rate for this program is $.0615 per kWh, typically reducing water heating costs by 50%.

105 Gallon Marathon water heater. This type and size of water heater is commonly used for utility controlled programs.

Hot water recirculating systems should be controlled.  A recirculating hot water system is when water is circulated throughout the home so that a faucet, tub or shower has instant hot water.  I’ve seen some older systems that recirculate continuously.  Some systems use a timer to only circulate at specific times of the day.  New systems might use an occupancy sensor in a bathroom to start the recirculating pump when someone enters a bathroom. Another option is to utilize voice control, I recently learned that you can use Alexa to control a hot water recirculating system by voice communication.  Not sure I would go there, but it is an option.  If you have a continuous recirculating hot water system, which could more than double your water heating costs, change it to either a timer or occupancy system.

Purchase an air source heat pump water heater (ASHP).  I was skeptical of this type of water heater for my climate when they first came out.  Moving heat from your home to your water didn’t make sense to me in a climate where you often need space heating 8 months out of the year.  Unless there is an area of the home with excessive waste heat, the movement of heat from the ASHP water heater would make the area around the water heater much cooler, which could result in comfort issues.  Most newer air source heat pump water heaters are duel fuel, meaning they can heat both with traditional electric elements or as an ASHP.  It’s possible to switch over to all electric mode during the winter months and use the heat pump mode the rest of the time.  An added bonus, ASHP water heaters also dehumidify.  Great if they are installed in a basement of other area where interior humidity is an issue.  Just remember they will need a way to discharge the moisture that is collected, a drain close by will be needed.  ASHP water heaters also require ambient temperatures to be over 40 degrees and most recommend installing in rooms greater than 1000 cubic feet.  What I like most about ASHP water heaters is their high UEF rating.  A good electric water heater might have a .90 UEF, an average air source heat pump water heater will have a 2.5 UEF.

Now for some new technologies.  I’ve already mentioned the use of voice control for hot water circulating systems.  How about an app to control your water heater?  Water heater manufacturers, both gas and electric are beginning to utilize smart home technologies to control a water heater remotely, great if you travel a lot or have a second or vacation property that isn’t always occupied.  The ability to turn on and off or the temperature up or down from a smart phone is a great option.  How about a water heater that learns your schedule and only heats water when needed?  Kind of creepy, right.  I think the “smart” water heater is going to be the “thing” in the near future.  I’ve recently learned of a water heater designed to be used with photovoltaic solar panels.  It anticipates the weather forecast and your hot water usage to know when to heat water.  Another coming technology is a water heater that communicates with a utility company so that it heats only when electricity demand is low, a technology that will be useful for power management and cost reductions.  These are just the technologies I’m aware of, I’m sure there are many more.

So, if I needed a new water heater today, what would I purchase?  I would seriously consider the air source heat pump.  With an energy factor of three while in heat pump mode, meaning for every dollar in energy spent, there is 3 dollars of heat moved to the water heater, will result in a short return on investment.  An added benefit is dehumidification, bonus!

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