By Chas Kerin, Bosch Thermotechnology Technical Trainer
As green building increases in popularity and more people become conscious of their carbon footprint, building owners aiming to reduce energy costs and attract eco-minded buyers and renters are turning to gas-fired tankless water heaters as an alternative way to heat water in a residential unit.
A tankless water heater has four main benefits that attract both buyers and renters to a condo or apartment unit—and help building owners profit in more ways than just filling up units.
Better use of space: A tankless heater’s footprint is more compact than that of a tank heater, and are generally small and wall mounted—some are not much bigger than a briefcase. They fit in a closet or utility area, consuming far less space than a 40- or 50- gallon tank heater, which requires up to 9 square feet—so whether a unit is a studio or a penthouse, the prospective buyer or renter is able to reclaim living space.
Optimal hot water production: Even though the tankless water heater is saving energy by not keeping water constantly hot, it heats water to the pre-set temperature on demand—yet still can provide hot water for an entire unit, as long as the building owner has purchased the appropriately sized heater for his or her needs. A 40-gallon tank will deliver about 64 gallons of hot water in one hour, while a small-capacity tankless will deliver three gallons per minute, or 180 gallons per hour.
Long-term savings: A tankless water heater usually requires a greater initial investment than a conventional storage water heater, but they typically last longer, with a life expectancy of more than 20 years if maintained properly. That’s compared with 10 to 15 years for tank heaters. Most, manufacturers incorporate easily replaceable parts, which can extend their life by many more years.
For building owners managing rental units, this means lower long-term costs, because the appliances will last longer before needing to be replaced.
Over the life expectancy of the tankless heater the building owner’s investment is almost always offset by lower operating and energy costs. Water heating accounts for about 18 percent of a utility bill after heating and cooling, according to Energy.gov, but going tankless can cut water heating costs by up to 50 percent. Building owners also can recoup some of the initial cost via tax credits, incentives and rebates.
Energy savings: A tankless heater means a lower carbon footprint. A tank heater uses energy to heat water 24 hours a day, 7 days a week—even when no one is actually using hot water. All this unnecessary energy use adds up, creating excess greenhouse gas emissions. A tankless heater sized for a small space can reduce carbon dioxide emissions by 3.6 tons over the heater’s 20-year lifespan.
Sizing a heater
To size a gas-fired tankless water heater, a building owner needs to determine the gallon-per-minute demand, or flow rate, at each fixture and add up the flow demands for those that could occur at the same time—such as two showers at once, or a kitchen sink and shower. Being realistic about demand will keep a building owner from either over- and under-sizing a water heater.
Another factor in correct sizing is determining the maximum temperature rise needed to run the highest water temperature applications. Temperature rise can be determined by subtracting the incoming groundwater temperature from the desired output temperature (for most uses, a homeowner would want water heated up to 120°F). Use the coldest possible groundwater temperature for the immediate area to ensure the water flow is adequate even during even the most frigid winters.
However, the longer the distance a hot water pipe runs between the water heater and the fixture, the more likely it will result in reduced hot water temperatures at the fixture, especially if the pipes are not insulated. For any installation best results, and energy efficiency, are achieved when the hot water pipes are insulated. If it is not practical to insulate the piping then be sure to factor in any remaining temperature drop from the length of the pipes when calculating the required temperature rise.
Size is determined based on BTUs and the efficiency of the tankless heater. For example, in most of the United States, a condensing gas tankless heater with 199,000 BTUs per hour can handle two to three major applications (showers, dishwashers, washing machines) at once without issue.
Energy Star has a general estimate for how to size a tankless heater, but also notes that buyers should consult with a plumber to estimate their hot water demands.
It is possible to install enough tankless units for all hot water use in a multi-unit building, but building owners also can combine tankless and storage tanks to offset the number of tankless units needed rather than try to size strictly on the number of tankless units required for peak demand.
For example, in a large apartment community, peak demand could be a two-hour time slot in the morning when residents are showering before going to work. The cost to install enough tankless units to meet that once-a-day peak demand could be prohibitive to the owner, especially because the number of units required to provide enough hot water during peak demand may be overkill for the rest of the day.
In this case, the contractor could use a tank-loading application, which combines well insulated storage and tankless units. The storage units buffer the supply of hot water during peak demand, and the tankless units help recover the storage tank more quickly than a typical direct-fired storage tank would on its own. This application also offers higher efficiency because, in a properly designed system, the tankless units are still condensing during the heating cycle.
Gas tankless water heaters offer an energy-efficient option for hot water supply for residential buildings, as well help building owners save money over the long term and reduce their carbon footprint—making a tankless heater a green choice in every way.
Chas Kerin has been working exclusively with tankless water heaters for over 10 years; primarily with the products manufactured by Bosch Thermotechnology but, has additional direct experience with two other manufacturers of tankless water heating products. To learn more about tankless water heaters, visit Boschheatingandcooling.com.