INDIRECT-FIRED Water Heaters - Common Ground

1 indirect Water Heaters may be the most important technologyenhancement to enter the residential hot Water industry in thepast 30 years. Two key needs have driven demand for these use-ful and practical heating units: fuel efficiency and an everincreasing desire for more hot Water . Hot tubs, large baths, multi-head showers, and bigger homes with more bathrooms haveincreased America s appetite for hot Water , and indirect waterheaters can satisfy the need faster than any other Heating ChoicesBefore the advent of indirects, the options for heating domesticpotable Water included conventional direct- fired units or boiler-integrated tankless coils.

2 Direct- fired Water Heaters (electric-, gas-, or oil- fired ) are by farthe most Common technology, primarily because they have theMAYJLC2002 indirect -FIREDW ater HeatersFor homesheated witha boiler,adding anindirect tankwill provideefficient, reliable hot waterby John Vastyan and Rich McNallyINDIRECT-FIREDW ater HeatersMAYJLC2002lowest front-end cost. They have their disad-vantages, however. While electric Water Heaters are almost 100%efficient, operating costs can be high, espe-cially given the demands of a large, moderncustom tank Water Heaters are typicallyinefficient, unless they re specifically designedfor high efficiency, which means a substan-tially higher expense on the front end.

3 Also,the difference in temperature between thecombustion passages and the large tank of rela-tively cool Water above or around them,depending on design, puts a lot of stress onmetals and glass linings. That causes fatigueand shortens life span. This is especially truewhen they are harnessed for use as a heatsource, because they really aren t designed forthat much tank-type Water Heaters collect precipi-tates. The incoming Water slows suddenly,heats up, and gives up its mineral particulates,which fall to the bottom, insulating the tankfrom the heat source over time, adding to ther-mal stress, and decreasing they re inexpen-sive, internal tankless coils, in either a gas or anoil boiler, are inefficient.

4 As early as 1935,advertisements promised free hot Water dur-ing the winter, but there was a cost penalty forhaving to fire the boiler all summer long toproduce domestic hot Water . Laboratory tests inthe 1950s showed that tankless Heaters withlarge-volume boilers were only about 18% effi-cient during the non-heating season. Today s tankless units, like earlier models,rely on internal boiler convection and veryhigh boiler temperatures. Because their gas-keted attaching plates are made of metals dis-similar to the boiler s, over time they all leak atleast a little, usually onto sensitive compo-nents.

5 They re also notoriously troublesomebecause, as hard or otherwise untreated waterpasses through them, the super-heated coilscollect mineral deposits that quickly diminishcoil efficiency and then require costly acidbath units,on the other hand, takeadvantage of the high efficiency and the inher-ent sturdiness of modern hydronic boilers bybecoming an attached zone. This arrangementdirects the boiler s power to heating potablewater in a well-insulated tank but without thestress on the tank Common in direct- fired units.

6 As an indication of their efficiency and theirability to produce hot Water , many INDIRECT-FIRED Water Heaters provide two to four timesthe recovery rate of gas- fired Water also offer two to three times the peakflow of a tankless coil, and three to six timesthe peak output of comparably sized electricwater Heaters . With indirects, the simpler tank designs andlower temperature difference between the heat-ing medium and heated potable Water alsoallow for the use of potentially longer-lastingtank materials like plastics, cupronickel, andstainless steel, as well as high R-value insula-tions for optimal storage efficiency.

7 Some indi-rect tanks have a glass or stone lining to protectthe steel from corrosive electrolysis and naturaloxidation. Most indirects do not require theuse of a mixing valve, allowing better flow ratesand fewer mechanical issues in the an INDIRECT-FIRED TankThe most Common problem with INDIRECT-FIRED units is undersizing. The indirect tank istypically the single largest zone attached to theheating plant and the most noticed if under-powered. It takes a tremendous amount ofenergy to heat Water enough to raise its tem-perature 90 or 100 degrees at a flow rate suffi-cient to satisfy the modern family.

8 Proper sizing starts with the boiler. Most 30-to 40-gallon tanks with a 15- or 20-square-footEDR (equivalent direct radiation) coil canabsorb a boiler output of 120,000 to 130,000 Btu, generally lots more than the averagehome s heat load. Thus, sizing the boiler tomatch the heating load rather than hot waterneeds can obviously adversely affect the indi-rect s performance. What may not be quite as obvious is howcritical it is to properly size the circulator pumpand piping, and to use controls that give prior-ity to domestic hot Water when necessary.

9 If thepipe and pump are too small and there s nopriority control, there won t be enough hotwater, no matter how big the boiler is. Shopping for an indirect Water HeaterIndirects, like many other popular products,are available in a wide variety of configurationsand sizes from a number of European anddomestic manufacturers, each claiming superi-ority in some way or other. There are similari-ties and vast differences among them. Most use a heat exchanger coil submersed inthe potable Water that s to be heated.

10 There are adiversity of coil profiles and materials. Amongthe best and most expensive is cupronickel,a copper alloy with good heat-transfer ability aswell as good resistance to metal coils are often finned to add surface areafor more efficient heat transfer in a shorterlength of tube. Some manufacturers claim thatthe fins lead to premature accumulation of pre-cipitates, so they use a smooth-surfaced boost efficiency and output, these makersmust use a longer tube, replicating the effectivearea for heat transfer.