Water heater sizing guide

As homes get larger, and bathrooms more customized with larger and larger showers and tubs, the standard 40-gallon water heater isn't the one-size-fits-all appliance it once was.

Furthermore, as energy prices continue to rise along with the size of a home's hot water demand, the options in water heating technology have grown wide and varied. To help you select the correct water heater that both ensures you never run out, while at the same time, don't produce far more than you need, here are a few tips and techniques for selecting the right size appliance.

Since the goal is to never run out of hot water, always assume a worst-case scenario. This means delivering enough hot water based on the assumption that every tap and hot water-consuming appliance (i.e. laundry machines, dishwasher) will be used simultaneously.

With that in mind, calculate the gallons-per-minute (gpm) demand at every draw-point and fixture. To figure out the gpm-demand of any given fixture, get a one-gallon container (like a milk jug) and a watch with a minute-hand. See how long it takes to fill and the container and do the math (if the jug fills in 30-seconds, the flow-rate would be 2 gpm). For appliances like dishwashers and clothes washers, the water-consumption data will often be provided in the manufacturer's literature.

Once you've added the total gpm for all appliances, multiply by .85. Why? Your water heater will usually maintain a temperature of 120-degrees, however, you usually temper this down with a bit of cold water to about 100 degrees -- 100 degrees is about 85% of 120. Once you multiply the demand of all fixtures and appliances by .85, this is the total gpm that you need from your water heater.

For on-demand water heaters, figuring-out the size you need is pretty cut and dry. All manufacturers provide output-data (expressed in gpm) based on a specific temperature rise. The rise refers to the difference in temperature between what you'll heat the water to minus the temperature of the water as it enters the home. In many parts of the U.S., water enters the home at about 55 degrees and is heated to 120 degrees -- a rise of 75.

If the output of the on-demand water heater exceeds the demand of your home, you have a winner. In some cases, however, your demand will be larger than the output of the on-demand water heater (this is fairly common in larger homes). If this is the case, you can either use multiple on-demand water heaters or move to a large tank-type model. In most cases, it's more cost-effective to purchase a large-capacity tank versus multiple on-demand systems.

If you opt for a tank-type water heater, you'll want to refer to the appliance's "First-Hour Rating." This number basically equates to the number of gallons a water heater can provide per hour. Take this number, divide it by 60, and this the the total gpm the water heater can deliver. If the first-hour rating (divided by 60) is larger than the total gpm demand calculated for your home, the water heater should do the trick. But there's an exception.

The First-Hour Rating doesn't account for a potential "dump load." Here's what we mean. If you have a 40-gallon water heater, but have a bath tub that holds 50 gallons, there's a strong likelihod that the water heater will become depleted, unless it has a super-huge burner like those found in commercial water heaters (big bucks). So, if you have a large soaking tub, we recommend you purchase a water heater that has a capacity that's at least 1.5 times the size of the tub.

Why more? Two reasons.

First, just because the tank says 60 gallons, you don't actually get 60 gallons of hot water. As you fill the tub, emptying hot water from the tank, it gets replaced by cold water. Consequently, this cold water mixes-down the the temperature of the tanks hot water, thus diminishing the amount of total available hot water.

Second, again, we want to assume that other fixtures will be in use as you fill the big tub. By oversizing by a factor of 50 percent, we allow for extra demand.

Sizing water heaters is as much art as it is science. When in doubt, contact a qualified professional. Furthermore, by allowing an installation contractor to size the appliance for you, you're putting the liability on him (or her) instead of assuming the risk yourself.

Commercial Applications:

Commercial water heating applications can be much more varied, and sometimes requires a more sophisticated approach to calculating the correct size appliance(s). Moreover, commercial equipment can be very expensive. At the same time, water heating can represent a lrge percentage of a business' energy overhead. As a result, it is important to get it right -- the first time.

Many factors come into play in commercial water heating applications -- overall demand, time of use, energy costs, storage needs versus btu-output, etc. If you have a commercial water heating need, we highly recommend you either: 1) hire Enhanced Living to design your system, or 2) hire a mechanical engineer with specific expertise in this area.

If you have fairly generic needs and/or a small demand, use the above guidelines for residential applications, or use one of the following calculators:
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