Principles of tankless
In a tankless setup, an open hot-water fixture creates a pressure differential in the plumbing system, causing water to flow in its direction. The water-heater detects this pressure differential and a flow sensor measures the gallons per minute (gpm). The internal computer then automatically ignites the burner, modulating British thermal unit (Btu) usage to overcome the temperature differential between the incoming and set-point temperatures. Finally, the primary heat exchanger brings the water up to the set-point temperature.
In the following water-heating equation, the heat input (i.e. Btu/hour) is represented by Q. The variable ‘F’ represents the flow rate, and the temperature differential between incoming and set-point water temperatures is represented by (T2−T1):
Efficiency x Q = 500 x F x (T2−T1)
Btu input has a direct relationship to the flow rate and temperature differential. As a result, energy will increase with larger temperature differentials.
Condensing tankless technology
Condensing water-heaters will utilize the high-temperature exhaust in standard-efficiency units to preheat the incoming water temperature 8 to 11 C (15 to 20 F) higher through a secondary heat-exchanger. As a result, when the preheated water enters the primary heat-exchanger, less heat is needed to raise the water temperature to the set point. In fact, a condensing unit can have a thermal efficiency up to 10 percentage points higher than a conventional tankless unit and 20 points higher than a standard storage tank heater.
The extraction of heat from the exhaust gases to the incoming water provides an additional benefit—it cools the exhaust gases below the dewpoint. For example, the gas temperature at the top of the flue for conventional tankless units is roughly 200 C (400 F). In condensing units, the secondary heat exchanger will lower this temperature to around 50 C (120 F)—a major difference—without sacrificing hot-water output or water pressure. The cooler exhaust temperatures allow the use of venting materials that are less expensive, such as polyvinyl chloride (PVC).
A natural byproduct of the cooler gases escaping out the flue is condensate, which should be drained per local code requirements. Some codes may specify a neutralizer to lower the pH of the condensate entering the plumbing system.
Both condensing and noncondensing types of water-heater are significantly more efficient than a standard storage tank system. These efficiencies can be multiplied when banking heaters in larger commercial applications.
Benefits of banking and modulation
Some applications, such as hotels or large office buildings, can require significant quantities of hot water at any given time. Specifiers may be hesitant to install tankless systems in such situations, fearing either the demand will be too large or too many units will need to be installed.
To clarify, no hot-water demand is too large. In a multi-unit commercial system, the tankless heaters will communicate with one another to generate enough output to meet demand and to ‘smoothen’ that load among the various operating units in a way that extends system life. The typical firing sequence is as follows:
1. The first tankless water-heater activates as soon as there is a demand for hot water within the system—for example, when someone turns on a faucet or a shower.
2. That first unit continues to provide all the needed hot water until demand exceeds 80 per cent of the unit’s capacity.
3. Once demand surpasses the first unit’s preset capacity, it is equalized among two or more units while still meeting high-volume demand.
Modulating burner technology can track and meet any hot-water demand with pinpoint accuracy, matching energy consumption to present requirements. If energy from only one water-heater is needed, then only one heater activates. In contrast, a conventional heater or boiler without a modulating burner will immediately ramp up its full capacity to deliver hot water to that one hand sink or shower; a traditional system consumes far more energy than needed.
Some manufacturers offer prefabricated rack systems of up to 24 units controlled by one remote thermostat. However, for even larger applications, multiple systems can be linked together to meet demand. The ratio of the maximum to minimum Btu output of a system is called the turndown ratio. The higher the ratio, the greater the system’s flexibility in efficiently meeting hot-water demand. For example, a bank of 24 heaters with a Btu range of 11,000 to 6 million would have a turndown ratio of 545:1 to meet fluctuating hot-water demand.
