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How to Choose a Bathroom Water Heater

November 12th, 2009

bathroom-water-heater.jpgHere’s a bathroom water heater scenario that I’ve heard about one too many times. A homeowner decides to add a giant whirlpool tub to his or her bathroom, and imagines coming home from work and soaking away all the stresses and strains of the day. The hot water comes churning out of the faucet at first, but before the tub is even 4 inches full, it’s colder than a glass of iced tea in August.

Although you can get away with a limited amount of hot water in the kitchen (after all, just how many gallons does it take to fill a sink), the same is not true in the bathroom. Fill up the tub, or take a long shower, or try to have an entire family get ready for the day at the same time, and the hot water could run out. Guaranteeing a steady supply of hot water when you want it is a major factor in a bathroom’s success, and here’s how I think you should go about it.

Everyone is familiar with the conventional water heater in their basement or garage, that round cylinder that looks something like a gigantic roll of paper towels. Though getting a bathroom water heater large enough to supply hot water to your tub is an essential requirement, determining the right size requires some mind-bending calculations, as well as an understanding of how water heaters work.

First, let’s start with the mechanics. In the tanks, the heating element lies at the base, whether the source of heat is electricity or a boiler fueled by gas or oil. Take yourself back to sixth grade science class, and review the theory that hot air, as well as hot water, rises and gathers toward the top of the water heater. If you don’t use any hot water for an hour or so, the entire tank then contains uniformly hot water. But the moment you turn on the hot water faucet somewhere in the house, cold water is drawn into the bottom of the tank. Although the burner or heating element also kicks on at the same time, it does not heat this water instantaneously. This means that the more hot water you draw from the tank, the more cold water is pulled into it. Although this does not affect the temperature of the water at first, the cold water eventually dilutes the hot water and reduces the temperature of water flowing from the tap. At first, the water might spill out at a steamy 120 degrees, but then drop to 90 degrees as the dilution effect kicks in.

This principle is important to consider when choosing the appropriate size of bathroom water heater for your needs, and it is something that people almost always underestimate. If you have a 75-gallon whirlpool tub, which is perhaps the average two-person tub, you might think that you’d need to fill it with 70 percent hot water and 30 percent cold water, which would require about 50 gallons of hot water. A 50-gallon tank should be sufficient, right? Wrong. The problem is that with a 50-gallon water heater, you’re only going to get about 25 gallons of truly hot water at 120 degrees. By the time you get to that 26th gallon, you’ll be lucky to get water at 100 degrees, and the temperature will continue to slide from there. So in order to fill a tub this size, you would need to have a 75-gallon water tank, unless you like the idea of luxuriating in tepid water.

Until now, the solution to this ever-increasing need for hot water was to add larger and larger water heaters. This is sort of the same thinking that led to the creation of the SUV, however, and with the same costly side effects. To put it briefly, water heaters cost a great deal of money to operate. It’s one thing to have the burner kick on when you’re actually using hot water. These tanks, however, are designed to create and store hot water without giving any thought to whether anyone will actually be using it. That means if you have a new 75-gallon tank in your basement to handle that once-a-week whirlpool bath, you’re paying to keep 75 gallons of hot water on hand every second of every day, whether you’re asleep or at work. The water heater doesn’t know you only need this amount of water one evening a week, and could probably get by with only about 10 gallons or even 5 gallons the rest of the week. It has one job: to keep that tank hot, and it does this with budget-breaking efficiency. The average water heater burns up about $300 worth of energy a year, which makes it more costly to operate than most refrigerators.

Until recently, there hasn’t been a solution to this water-heating problem, but I’ve found something that works very well. It’s called the tankless water heater. This device operates just like a conventional water heater in terms of heating water, except for one important distinction: it does not store hot water at all. When you turn on the hot water tap, the burner kicks on and hot water is produced only as you use it. If you’re washing your hands, the water heater quickly produces the pint of hot water you’d use. If you’re filling the big soaking tub, it will keep pouring out hot water until the job is done. And then when you’re done, it shuts off until you need more hot water again.

In terms of logic and energy efficiency, nothing comes close to these tankless water heaters. They come in several sizes as well, from those that produce about 3 gallons of hot water per minute (which is more than adequate to feed a low-flow shower head) to those that can produce about 6 gallons of hot water per minute. This is the size you would need to fill a big tub.

Although popular in Europe where they originated, these tankless water heaters have been slow to catch on here, which has been puzzling to me. Some of the problem, obviously, has to do with their cost. While a standard water heater might cost $100 or $200 for one with a 40 gallon capacity, a tankless heater might run between $500 for the small ones that produce about 3 gallons of hot water per minute, and $1,000 for the large ones that produce 6 gallons per minute. But this is a misleading number because the cost of running the tankless heaters is far less, and probably only about $125 a year on average for one with a 3 gallon per minute capacity. Since a 40 gallon standard water heater might cost about $300 a year to operate, this represents an annual savings in operating costs of about $175. At this rate, the tankless units would pay for themselves in energy savings in about three years.

This article on how to choose a bathroom water heater was supplied to us by “Plato” from the DigitalPoint forums under a private label rights license.

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