Tankless Water Heaters have become very popular in recent years. While there are some applications where these types of units are a good choice for providing Domestic Hot Water, they are easily bested in systems where there is already a boiler providing hot water for heating. Boilers in general, have a long, reliable, and highly efficient service life. When combined with an Indirect Water Heater, are an EXCELLENT source of hot water for household use. Below is fine article that explains the issue clearly.

Reprinted from the June/July 2007 issue of Plumbing & Hydronic Contractor News Magazine

Hydronic kiss leads to mechanical matrimony


contributing writers

It’s now official. The relationship between the boiler and his chosen mate is recognized by the hydronic community as one worthy of long-term marriage. But the honeymoon’s over, and it’s now off to the races.

An indirect-fired water heater gives us all the power of a NASCAR racer for supplying virtually limitless hot water for bathing and washing, yet with the fuel economy of Mini Cooper!

Modern bathrooms are equipped with waterfall tub fills and multi-nozzle body washers instead of the old 2-gpm showerheads we once used to carefully moisten ourselves with. These new fixtures demand huge quantities of hot water. The sudden loads require that we store plenty of reserve, but also that we put it there and keep it there without breaking the bank. If that sounds to you like the perfect job description for an indirect water heater, you’re right.

Modern hydronics now provide what consumers want most from their home’s most essential mechanical systems -- maximum heating comfort, and endless hot water at a lower operating cost than ever before.

Indirect water heaters might be the most significant technology enhancement to enter the residential hot water industry in the last 30 years. Two key needs have driven demand for these incredibly useful and practical heating systems here in the United States: fuel efficiency, and an ever-increasing need for more hot water.

Hot tubs, large baths, multi-head showers and bigger homes with more bathrooms -- all create a voracious appetite for hot water and, fortunately, these heaters can shovel it faster than anything else.

Until indirects came into play, boiler-integrated tankless coils ruled alongside direct-fired tanks. The internal tankless coils, in either a gas or oil boiler, are inherently inefficient, but inexpensive. As early as 1935, advertisements promised “free hot water” during the winter, but there certainly was a cost penalty for firing the boiler all summer long: laboratory tests in the 50s showed that tankless heaters with large-volume boilers were about 18% efficient during the non-heating season!

Of course, even during the heating season, hot water coming out of the tankless coil requires fuel. Today’s tankless units, like earlier models, rely on high internal boiler convection and very high boiler temperatures. Because their gasketed attaching plates are metals dissimilar to the boilers’, over time they all leak at least a little, usually onto sensitive components. They’re also notoriously troublesome because, as hard or otherwise untreated water passes through them, the superheated coils collect mineral deposits that quickly diminish coil efficiency and then require costly acid bath cleanings.

Electric water heaters are almost 100% efficient, but utility companies don’t generally share well and operating costs can be comparatively high, especially when loaded with enough kilowatts to fully satisfy modern demands.

Direct fossil fuel-fired tank water heaters are all moving toward greater and greater operational efficiencies, but some of them also have a problem with the temperature difference between the combustion passages and the large tank of relatively cool water either above them or around them. That difference puts a lot of stress on metals and glass linings, causing fatigue, thus shortening lifespans. This is especially true when they are harnessed for use as a heat source, primary or auxiliary.

Indirects, on the other hand, use the high efficiency and the inherent sturdiness of modern hydronic boilers by becoming an attached “zone.” This arrangement now mechanically connects and, with use of a circulator, directs the boiler’s full power to heating potable water in a highly insulated tank without the stress, and with the efficiency that’s so hard to combine in direct-fired tanks.

As an indication of their efficiency and ability to produce hot water, many indirect-fired water heaters provide two to four times the recovery rate of gas-fired water heaters. They also offer two to three times the peak flow of a tankless coil, and three to six times peak output for comparably sized electric water heaters. (Though when volume and efficiency are not key concerns, these other units generally meet the need quite well.)

With indirect water heaters, the relatively simpler tank designs and lower temperature difference between the heating medium and heated potable water also allows for the use of potentially longer lasting tank materials like plastics, cupronickel, stainless steel, as well as high R-value insulations for optimal storage efficiency. Glass or stone-lined steel tanks -- to protect the steel from electrolysis and natural oxidation -- now have a better chance of long life as well. Most indirects also do not require the use of a mixing valve, allowing better flow rates and fewer mechanical issues in that flow.

Indirects are almost always the single largest zone attached to the heating plant and the most noticed if underpowered. It takes a tremendous amount of energy to heat water enough to raise its temperature 90 or 100 degrees at a flow rate sufficient to satisfy the modern family.

If we’ve seen one common problem with indirect jobs over the years, it’s improper supply of boiler energy to the tank, no matter the brand. It starts with the boiler size. Most 30- to 40-gal. tanks, with a 15- or 20-sq.-ft. edr (equivalent direct radiation) coil, can absorb a boiler output of 120,000 to 130,000 Btus, generally more than the average home’s heat load. Clearly, under-sizing the boiler to match the heating load could adversely effect the indirect’s performance.

What might not be quite as obvious is properly sizing the pump and piping, along with prioritizing with good piping and controls. Even an oversized boiler doesn’t have a chance if the piping or pumping between it and the indirect is too small or not oriented for priority. Flow rates and temperatures presented to the unit make it perform or not perform. It’s really that simple.

Indirects, like any popular product, are available in a wide variety of configurations and sizes from a number of manufacturers, European and domestic, each claiming superior something or other. There are similarities, and vast differences, among them. Most use a heat exchanger coil submersed in the potable water to be heated.

There is a diversity of coils, and materials used to manufacture them. Some use copper alloys such as cupronickel. These coils are often finned to gain surface area for more efficient heat transfer in a shorter length of tube. There are those in the industry that believe tube-finning leads to premature accumulation of precipitates, so they choose to use a smooth-surfaced tube. To boost efficiency and output, that requires a longer tube, replicating the same effective area for heat transfer.

Though there are as many designs as there are suppliers, most commonly potable water is piped to the inside tank, while boiler water fills the outer tank, surrounding and heating the inside tank. No coils to think about, but when the inner tank’s thermostat is satisfied, the large mass of 180- to 200-degree boiler water is left surrounding it. Heat migration (hot moves to cold) then raises the inner tank temperature over the stat set point. This style then recommends a mixing valve to overcome this issue … and we thought we left mix valves back in the tankless era!

All of the leading manufacturers of indirect units insulate their tanks very well. Most claim losses of two degrees or less per hour during standby; some measure heat loss at less than 1/2F per hour!

The physics of combustion efficiency involve the struggle between reducing mass in the boiler and the medium to gain speed, while maintaining good steady-state efficiency. It means cold-starting for greater delta T, or temperature difference, and lower stack temperatures while avoiding condensation. It also means improving surface area within the combustion chamber and flue passages so that refractory is minimized and hot gasses give up more energy without increasing the boiler mass or creating “cold flame syndrome.” Optimizing combustion so that the gasses give up energy to the iron in a smaller area is also part of the equation.

Why the mention of all this? An amazing evolution took place not long after introduction of the indirect water heater into to our hydronic marketplace. Boiler design engineers soon directed their interests and ingenuity into development of vessels that no longer needed high internal convection, or a large cavity and port to accommodate an internal tankless coil.

The market no longer required these features to support sales for that old design style. Manufacturers were soon focusing on the development of lower mass, higher-pressure, lighter weight, and lower temperature equipment. The Europeans had been doing this successfully for years, and suddenly our market was ready.

Quickly, we were introduced to new features and a new vocabulary. Boilers were “pressure fired,” “cast over” (no tankless port), “low mass,” “fully-modulating,” “condensing” and offered “encapsulated combustion,” and were referred to as “radiant friendly” or “radiant-ready.”

Ah, yes. Low temperature radiant floor heating. With these slick, new enhancements to the venerable boiler -- especially considering the newest generation of energy-miser condensing boiler systems -- there’s now an ideal relationship between the boiler, indirect-fired water heater, and radiant panels. Connect all three partners with good primary/secondary piping and you have a system straight from the hydronic gods.

At the same time, consumer demand has pushed the development and sale of unprecedented numbers of multiple-head showers and waterfall baths. Informed consumers also want to step out of that “body wash” onto a warm tile floor! And, why not? Your customer should have nothing but the best from you and their hydronic system. With hydronic radiant heating, they can have just that. No more drafty, cold floors. No more “satisfied thermostat and unsatisfied customer.” Radiant heat, warming the room from the floor up, keeps the part of your body that’s hardest to heat (your feet) toasty warm, while your head stays as cool as Paul Newman’s. Most people find that with radiant heat, a lower thermostat set point feels more comfortable than a higher one with convective heating.

Imagine: all of this comfort from a heating system that operates about 30% more efficiently than a convective system! Therein lies the kinship between indirects and radiant heat, getting the very best, while paying the very least to get it.

With a boiler operating efficiently and happily, a well-insulated water heater making water hot and keeping it that way, warm floors bringing you the heat energy you want, where you want it... you’ve got a system that’s optimized for maximum comfort, efficiency and economy. There is no greater, greener way to operate a fossil fuel system without spending a king’s ransom.

Fortunately, there’s a diversity of manufacturers who offer some outstanding equipment. Among the many suppliers of this technology, tank sizes range from 20 to 120 gallons and more, with prices that vary greatly, with a retail price range at the 40-gallon size from $500 to $2,000, and from $1,000 to $4,500 for 120-gallon units. The wide variance in pricing stems from quality of craftsmanship, accommodation for faster heat recovery, ease of installation and maintenance, resistance to deterioration and mineral build-up, integration with boiler controls, efficiency, standby losses and expected service life.

As examples of this technology, two leading suppliers, Bradford White and Laars Heating Systems Company, are among several U.S. firms that offer indirect water heaters.

Bradford White Corp. offers double- and single-wall PowerCor indirect water heaters that provide standby heat loss at less than 1/2F per hour, Vitraglas lining, dielectric waterway fittings, protective magnesium anode rod, T&P relief valve opening on top of unit, brass drain valve, 2" non-cfc foam insulation and a 10-year limited warranty on tank and heat exchanger. Bradford White’s PowerCor and PowerStor were engineered to provide the lowest pressure drop, the highest hot water output and the highest amount of effective actual heat transfer area of any indirect fired water heater in the industry.

And, at this year’s AHR Expo, Bradford White introduced its Combi2 combination water and space heating system: a direct-fired, quick-recovery water heater that -- unique to the industry -- also “acts” like an indirect system because of its internal, 11/2" o.d. glass-coated heat exchanger coil that ends with 3/4" npt connections, welded to the tank. It comes in 50 and 75 gal. sizes in power-vented and atmospheric-vented models and, in addition to meeting domestic water heating needs, these units serve as an ideal heat source for a variety of radiant heat applications. The system’s closed-loop, double-wall carbon steel coil provides a positive leak path. The coil’s large diameter assures low-pressure drop (8 gpm flow gives 2.3 ft. of head loss) and super-fast recovery.

And offered by Laars Heating Systems Company, the Laars-Stor single- and double-wall indirect water heaters offer a super-efficient, 11/2" glass coated steel heat exchanger, sediment reduction system, three protective anode rods, a glass-lined tank, fully automatic Honeywell controls, a brass drain valve, T&P relief valve, supply and return connections and a five-year limited warranty of the steel tank and heat exchanger.

Chesapeake, Va.-based Rich McNally has been eastern regional sales manager for Springfield, Mo.-based Watts Radiant for four years. Watts Radiant is the largest American-owned radiant heat manufacturer. His 25 years in HVAC distribution include sales of five different brands of indirects, giving him ample experience in selling, installing and troubleshooting water heating equipment. He spent two years as a rep for hydronics manufacturers.

John Vastyan is president of Common Ground, Uncommon Communications, based in Manheim, Pa. He has 20 years of marketing communications, writing and PR involvement in the hydronics, radiant heat, HVAC and broad home construction industries. Email: cground@ptd.net.