WATER HEATERS continued from page 24 MBH. Some commercial tankless units can fire at higher rates. The flow rate on tankless equipment is determined by the temperature of the incoming cold water, and the temperature required at the end use. This can reduce the flow to the fixture. It should not be a problem with showers operating at less than two gallons per minute, but multiple shower applications may be a different story. Whether traditional tank-type or tankless, there really should not be any difference in the approach to sizing . The end result is to supply all the hot w ater that is required by the customer.
TANKLESS FOR HOME USE Like any application, actual hot water requirements have to be considered when sizing tankless equipment. Just like tanktype heaters, tankless heaters are excellent products when applied correctly. In most residential applications a tankless heater will be able to provide the hot water required.
To properly size a residential tank-style water heater, an assessment of numerous factors will be necessary. Most home builders will install a heater based on assumptions of the average family size and the appliances in the home. Most heaters end up in the 40 to 50 gallon (U.S.) range. If a family is bigger than average there may be a requirement for a larger draw, and the heater might need to be upgraded.
Changes to the home also factor into possible alterations to DHW supply appliances. If there is an addition, like putting a new bathroom into a renovated basement, or if a standard tub is replaced with a soaker tub/whirlpool, or if body showers are added, the requirements for hot water increase. The tank may have to be upgraded. A tankless strategy may also Too large a unit can be inefficient. If too be investigated. large a tank-type water heater is installed,
Don’t oversize
there will be too much water in storage, and consequently it will take more energy to bring up the temperature after a draw.
To provide water at 140°F, similar to a tank-type heater, a tankless heater would provide a flow of around three gallons per minute (based on a 180 MBH firing rate). This is certainly enough to provide hot water to a shower and most fixtures in an average household. If the setting on the heater was lowered to 105°F (most showers operate at 100°F to 105°F), the flow would be around 4.5 gpm. This would certainly allow for use of more than one fixture at a time. Tankless heaters operate on demand, and require a minimum flow to fire. When they do fire, they fire in a set firing pattern, beginning around 15 MBH up to 180 MBH. There are other sizes available, but 180,000 BTUH is common. In some cases the flow, and therefore supplied temperature may not be enough to fill a large soaker tub in a timely fashion, since many tub spouts operate at seven to 11 gpm of flow. In these scenarios a second tankless may be required, or a storage-type heater may be more suitable.
B u s i n e s s
STORAGE
AND RECOVERY
One of the considerations when sizing for a tankless heater is the temperature of the incoming ground water. In Canada, ground water temperatures can be anywhere from 38°F to 60°F. In the southern Ontario region, 40°F is usually the lowest we’ll see.
M e c h a n i c a l
RESIDENTIAL USE
Also, as families age, toddlers become teenagers and the immediate hot water requirement becomes greater.
SIZING
26
SIZING FOR
0 8 . 1 1
All tank-type water heaters, regardless of manufacturer, are considerably more efficient today than in the past. The differential on gas valves now ranges from approximately seven to 15 degrees Fahrenheit. The tighter the differential, the more efficient the recovery. When water is drawn from the tank, cold water is provided to replenish the volume. Obviously this cold water, when mixed with the hot, will cool the tank down. Once the temperature has cooled down to the low point set on the valve, the valve will fire and warm the water up to the higher set point. This is the differential. In general, the tank only has to raise the temperature from seven to 15 degrees Fahrenheit. At the same time, all manufacturers make heaters that meet very stringent insulation guidelines, and most provide products greater than the minimum required. Stand-by temperature loss on most heaters is now less than one degree Fahrenheit per hour. This means the tank will not fire for large periods of time when not in use.