Size Does Matter 

Match the Tankless Heater to the Job

  Tankless heaters offer an almost-endless supply of hot water on demand. But how do you size a tankless heater so that it’s the right fit to serve multiple fixtures, but not too large to reduce anticipated energy savings?

  Tankless manufacturers can provide the technical data required to help contractors match the ideal heater to the job. Technical literature include charts that take fixture numbers, gpm demand, and other factors into account to size units for optimum performance.

Here are the key factors to consider when sizing a tankless heater:

• Size of the structure
• Number of baths/showers
• Number of fixtures demanding hot water
• Type of fixtures/high-flow or low-flow
• GPM per fixture/total gpm all fixtures
• Incoming water temperature fluctuations

 Calculate Peak Hot Water Demand

  To correctly size a tankless water heater you have to know the anticipated demand at any given time. You need to factor in how water is used, how much hot water is used and when the peak demand for hot water occurs, then size the tankless heater to meet the daily peak demand, explained Tom Burrows, consumer service rep for Takagi USA in Irvine. Calif.

  “You have to size the heater based on how many fixtures you would have running at once. The peak time of day would usually be in the morning—shower time. In a typical household during that two-hour window you might have two showers running. If they were low-flow showerheads, that would be 2 gpm total flow each.”

  Tankless heaters should be sized to handle the average peak demand--such as between 6 and 8 a.m. for showers--not the potential use of every hot water-using appliance or fixture at the same time. Typically, all of these activities rarely overlap, Burrows said. “Homeowners soon find out that a tankless will run all day long and not run out of hot water. But if you do turn on everything in the house, you won’t get cold water; instead you’ll get less pressure and less flow out of each fixture.”

Incoming Water Temperature Critical

  The temperature of the water supply is another determinant, Burrows said. “In Southern California, for instance, a worst-case condition in the wintertime is 60-degree F water coming in. That means,  probably, between 1.7 and 1.8 gallons of that 2 gallons would be hot. The rest of it is cold water, mixed to give you the typical 106-degree shower temperature. So, you need to plan for your tankless water heater to supply at least that much and sometimes more if you plan to be also running the bathroom sink or kitchen dishwasher.”

  Colder incoming water will result in reduced flow at each fixture. Burrows explained why: “The temperature sensing device looks in its computer brain and says, ‘Well, you have me set for 120 degrees output, so this is what I have to do to make up that difference. A control valve slows down the flow of the water through the heater to allow the water to stay in there longer," he said. "Obviously, the longer it stays in the copper heat exchanger, the hotter it gets. It still comes out at 120 degrees, but it takes longer to get there from 40 degrees than 60. It reduces the output flow as the input temperature drops.”

  In colder regions, you have to upsize the heater to get the same number of showers at the same flow rate.

Undersizing Means Poor Performance

  If the heater selected is too small for the job, Burrows said, the unit can be overwhelmed, with only enough water for one shower and the dishwasher instead of two showers. “If the heater is undersized, it doesn’t run out of hot water, and the water doesn’t go cold. It still gives you 120 degrees, but you would be a little bit short of hot water in each fixture. You would compensate by turning the cold water down, so you’d have a lighter stream in each fixture.” The right size heater allows the user to maintain stronger water flow.

  Say you need 4 gpm to accommodate two simultaneous showers, Burrows posed. You might size for an additional 2 gpm to supply other fixtures. “It’s analogous to putting in a 60-gallon tank heater versus a 40-gallon heater. The difference is that the tank heater can give you the specified gpm all day long.”

In or Out?
Where should you mount that heater?

       Once a tankless heater is slated into new construction or to replace a tank heater in an existing home, the next question is: Where should the water heater be located, indoors or out? To answer that question,  consider: the size and layout of the structure, the location of hot water-drawing fixtures, air combustion and venting requirements, space available for the heating unit and climate.

Are You In or Are You Out?

   "The selection of a water heater to be installed outdoors should be determined based on the climate for the installaton, appropriate freeze protection devices in those areas susceptible to freezing temperatures and having the appropriate accessories to protect the water heater and its components when required," said Jim McGoldrick, national marketing manager at Bradford White in Ambler, Pa. "The required accessories are designed to minimize or eliminate the interruption of service due to the water heater's exposure to weather elements with an outdoor installation."

  Locating a tankless heater outdoors eliminates the need for venting the system, said Diane Ford, contractor program administrator for Bosch. “Generally, that’s an expense in installing a tankless water heater. What’s nice about the heater being outside is that it doesn’t take up any space inside your house at all. You also don’t have to put holes in your roof or wall for venting.”

  Outdoor installation of tankless units is not advised in areas subject to temperatures below freezing. “Our heaters are protected to 5 degrees F, but we really recommend that they don’t go anywhere (installed outdoors) where the outdoor temperature is going to be 32 degrees F or below consistently,” Ford said. “The heaters protected to 5 degrees F are designed for areas that freeze rarely, so in an emergency situation.”

Enough Space, Enough Air

  Most consumers have their tankless heaters installed indoors, Ford said. Indoors, there are a couple of considerations. “If air is required for combustion, the heater has to be installed where there is enough open space to provide combustion air. Our new larger appliance heaters, which will provide two showers at once, are sealed-combustion heaters. Still, in most cases you want to locate the heater close to an outside wall, so they vent to the outside.

   Fred Stiebel, president of Stiebel Eltron noted all electric tankless water heaters can be installed indoors because you don’t have the same venting issues you do with gas-fired units.

  Compact size allows tankless heater installation almost anywhere as long as combustion requirements are met. Units are being installed in an attached garage or in a utility room with other appliances, which is feasible if this doesn’t mean too lengthy a pipe run. Garage location also is discouraged in areas with high humidity and freezing temperatures. Heaters serving homes with sizeable master baths often have a unit located in an adjacent closet space.

  In Southern California, where interior space is precious, heaters often are installed with other appliances outside the home in a garage, Ford said. “The other place homeowners are locating tankless heaters is in a seasonal vacation homes and shore homes. This is a nice application for them. When you close up the house for the season, you drain the heater.”

Where’s the High Demand?

  Locating a tankless water heating system within the home requires some planning. Manufacturers agree that, ideally, the heating unit should be located near the demand for hot water. “The water heater needs to be installed as close as possible to the fixtures drawing the most hot water,” Stiebel said. “Heaters can be installed in can be in two locations if required—one near kitchen/laundry room and one near bathrooms.”

  The rule of thumb: The closer to the point of use, the more quickly the heater can respond to the demand. “The closer to the point of use, the quicker you get hot water to the appliance or fixture,” Ford agreed. Systems are typically placed as close as possible to the bath/showers which draw the most hot water. Sometimes a second unit is installed closer to the home’s utility room or kitchen, depending on where laundry equipment and dishwashers are located.

  If it’s a long, ranch-style home that’s easily plumbed, the contractor may plumb two sides of the house. “Look for heaters that you can “cascade” or gang together to get more output. In a multi-story home, a critical factor will be the home’s water pressure to ensure adequate flow to appliances,” Ford said.

Rheem Intermediate

Tankless Water Heater

Rheem Water Heating has simplified the process of sizing tankless heaters by adding an intermediate-size heater to its line. Designed to deliver 5.3 gpm to a single-family two-bath home, the RTG-53 Series is neatly positioned between the RTG2-43 for one-bath residences and the RTG-74 for three baths. The Gt-199 Heavy Duty Series handles more than three baths. The RTG-43 and above are offered in indoor direct vent, indoor power vent and outdoor versions. The new RTG-73 direct vent models are the perfect solution for households with high hot water demand and tight new construction. Features include Guardian sensing burner technology and overheat film wrap, two high-altitude options and GAMA certification.

Coming in the June issue of

Reeves Journal:

• A look at some of the cool products from K/BIS.
  
• Contractors reveal their "must-have" tools.
  
• The technology contractors say is shaping the industry.