feature

by David Shevick

As colder weather approaches, it is time to start thinking about upgrading your hot water systems for better efficiency and cost savings. The demand for natural gas in the state has risen, and soon we will start importing liquefied natural gas from other countries, making it more expensive than ever. Also, because natural gas has the lowest carbon emissions of the fossil fuels, global warming legislation could increase the demand on fuel and drive prices even higher.

Most landlords do not take a systematic evaluation of their energy bills and hot water systems, only paying attention to the problem when the tank leaks, and then installing the same short-life, lower-efficiency equipment. Most central domestic hot water systems use approximately 60-therms per tenant per year, making it the primary landlord-paid gas usage in most apartment buildings. An older design concept–the direct-fired hot water storage tank–can be applied with new, high-efficiency equipment to give significant savings on the order of 25% to 30%, and increase the hot water system’s lifespan two to four times.

Conventional Commercial Tank Water Heaters
The fast-recovery 100-gallon commercial tank water heater is the mainstay of apartment water heating. Recent design changes mandated by Title 24 and other energy codes, such as automatic flue dampers and electronic ignition, have improved its efficiency, but it is still an inferior technology in value and longevity.

The recovery efficiency (amount of latent heat in the natural gas transferred into the water) is 80% on the modern models. Standby losses are 10% to 15% for this type of unit, making the overall efficiency of this technology around 68%. The inherent drawback of this technology is that the firing surface where the burner’s flame contacts the tank is at the bottom, where the sediment collects, limiting the heater’s life to an average of 6 to 12 years. The later years also suffer from a decline in efficiency due to the sediment’s insulating effect against the burner flame. Although most units allow for access to the sediment, so that it can be cleaned out after draining the heater, that access is rarely used.

The Alternative
An alternative to this conventional heater is the direct-fired tank heater. This technology, although in a much less efficient form, has been around for 75-plus years and actually preceded the tank water heater. Many older apartment systems still have these heaters, installed 50 to 60 years ago. The tanks were often unlined steel with asbestos insulation, and the boilers had efficiencies around 70%.

New direct-fired systems have condensing boilers with efficiencies around 90%. A condensing boiler is one in which so much heat is extracted from the exhaust gas that the water vapor in the combusted gas starts to condense. This condensate is slightly acidic, requiring the boiler heat exchanger to be stainless steel. The condensate must be routed to the drain system of the building. Modern 120-gallon glass-lined tanks with non-CFC insulation have extremely low standby losses, around half-a-degree Fahrenheit per hour.

In this design, an aquastat (thermostat) is located near the bottom of the tank. When the water at the bottom of the tank falls below the set point (usually 125 degrees Fahrenheit for apartments), the pump is turned on and the boiler fires. Sediment does collect at the bottom of these direct-fired systems, but it does not cause the catastrophic failure seen with the commercial tank water heater.

Choosing a Design
There are many different choices and configurations in the direct-fired tank design. One can use a condensing-efficiency tankless or high-efficiency boiler. One or two boilers can be used and one or two tanks can be used. Valving can be installed for redundancy. These systems are flexible and modular. Boilers can be increased in size, or a second boiler can be added for very large buildings. Generally, it is better to use 120-gallon storage tanks in quantities of one, two or three for larger buildings.

The first step in determining the storage and boiler BTU capacity of the building is to use sizing software and/or charts. Raypak makes a downloadable program called SureSpec (www.raypak.com/surespec3.htm), which is very detailed (wintertime water temperature, number of fixtures, flow rate, etc). There is a tradeoff between required storage capacity and boiler BTU capacity. A.O. Smith Water Heaters also has sizing charts on its website (www.hotwater.com).
Attention should be paid as to whether or not fixtures are low flow. If they are not, upgrading them to low flow can have good paybacks. Showers should be 2 gallons per minute and bathroom sinks around 0.7 gallons per minute.

Let’s look at the economics of upgrading to one of these systems, assuming the water heater is 5 years old with a 10-year lifetime. The payback in this case, assuming a reasonable 10% fuel rate escalator for natural gas, is 5 to 6 years. It is often better to replace equipment as a preventive measure before it breaks so there is time to implement the superior option.

There are two significant sources comprising the revenue stream: saved natural gas, which is likely to become more expensive in the future, and equipment savings based on lifetimes 2 to 4 times longer than conventional tanks. This technology is a way to get out of the “tank trap,” greatly improving longevity and efficiency. Clearly, direct-fired storage tank water heaters are a very sound investment.

The opinions expressed in this article are those of the author and do not necessarily reflect the viewpoint of SFAA or SF Apartment Magazine. David Shevick is a registered mechanical engineer and licensed plumbing contractor in the state of California, specializing in premium efficiency hot water systems. He has extensive experience in residential, multifamily, commercial and industrial settings. He can be reached at 415-794-2084 or via his website, www.tanknot.com. Copyright © 2007 by SF Apartment Magazine. All rights reserved.