One of the biggest heating and cooling equipment trends of the green movement over the past 10 years has been the growing popularity of tankless—also known as on-demand—water heaters in new construction and retrofit projects across Canada and the United States. A standard in Europe and Asia for more than 40 years, this space-saving, long-lasting systems provide a constant, on-demand supply of clean, hot water for all appliances and water fixtures simultaneously in high-frequency commercial applications when properly sized. These systems consume less energy than traditional tank water heaters, resulting in lower utility bills. However, to ensure optimal performance, engineering, architecture, and construction professionals must have a good understanding of tankless water heaters—how they function and how to right-size a unit for a particular application. Knowing these fundamentals will help guide clients toward the best water heater choice for a non-residential project.
Condensing and non-condensing
Tankless systems heat water directly from the building’s cold water supply whenever there is a demand for hot water; as their name suggests, they do not store heated water. This sets them apart from traditional tank-style heaters, which typically hold at least 150 L (40 gal) of water at a constant temperature, resulting in standby losses from the dissipated heat of up to 20 percent. With tankless water heaters, standby losses are limited or even eliminated. Tankless units generally range in capacity from less than 15 L (4 gal) to more than 42 L (11 gal) of hot water per minute, and vary in efficiency from about 80 to 98 per cent.2 In contrast, many traditional tank water heaters have an efficiency rating of 60 percent or less.
There are two types of tankless systems—non-condensing and condensing. The defining difference between the two is the former has only a primary heat-exchanger, while the latter also has a secondary one. With this dual heat-exchanger format, condensing units use a different exhaust-gas venting strategy than non-condensing units, allowing them to operate more efficiently. Condensing tankless water heaters, therefore, earn a higher Energy Factor (EF) rating (up to 0.96) than their non-condensing counterparts (up to 0.82). The foremost unit of measurement for water heater energy efficiency, EF is the ratio of useful energy output from the heater to the total amount of energy input to it. To guide project teams toward the most energy-efficient tankless water heater for their application, Natural Resources Canada (NRCan) provides a searchable database of popular models, listed with their EF ratings online.
American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, is currently considering a revision to provide guidance for service water applications that require water-heating systems with an output of more than 1,000,000 Btu/h to use condensing water heaters. Replacing a traditional tank-style water heater with a non-condensing tankless water heater will still result in significant energy savings. It is up to facility owners, however, to decide whether they want to invest more money in a condensing tankless water heater for even higher energy efficiency. It all depends on the size of the facility and its water-heating needs. Most often, the larger the condensing tankless water heating system, the larger the return on investment (ROI), justifying a higher upfront cost.
In the case of projects pursuing certification under the Canada Green Building Council’s (CaGBC’s) Leadership in Energy and Environmental Design (LEED Canada) program, project teams also sometimes opt for a condensing tankless water heater instead of a non-condensing unit for the extra energy efficiency points, as it can be a tradeoff for more expensive energy efficiency improvements in other areas of the building.
How do they work?
With tankless systems, whenever a faucet or appliance is set to request hot water, the demand is sensed by a flow sensor, setting the heating process in motion. A temperature sensor inside the tankless water heater then measures the temperature of the incoming cold water to calculate how much heat must be generated to deliver the desired temperature. It communicates this information to the unit’s gas burner, which adjusts its flame to match the demand.
One common myth about tankless water heaters—sometimes incorrectly referred to as ‘instantaneous’—is they deliver hot water immediately to the faucet. Merely replacing a tank-style heater with a tankless one does not reduce the time it takes for hot water to travel to fixtures unless the project’s engineer or architect takes advantage of the flexibility allowed by the unit’s small size and places it closer to where hot water is most in demand. The shorter the distance between the tankless unit(s) and the outlet requesting hot water, the shorter the wait time.
Benefits of tankless water heaters
From their small size to their long-lasting, efficient performance, tankless water heaters have much to offer project teams, as well as end users.
Lower utility costs
Since tankless water heaters are highly energy-efficient, they help project teams meet green building standard goals, and assist owners and operators in reaping significant savings in monthly utility bills. The water stored in traditional heating tanks must be kept hot over long periods, instead of being heated strictly on-demand, as with tankless units. This consumes more energy and creates higher utility bills.
Space savings
Tankless water heaters are small in comparison to their tank-style counterparts, taking up considerably less room—an important consideration in construction environments where space is confined or comes at a high premium. These units are generally the size of a carry-on suitcase, but range from the dimensions of a breadbasket to a medicine cabinet—they can easily be delivered through small doorways. As they are hung on a wall instead of being floor-mounted, they are esthetically better than a traditional tank-style water heater and also have smaller, less conspicuous vent pipe runs.