3. Storage tank size selection: NOTE: The draw efficiency of a gas or electric water heater storage tank is considered to be 70%.

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1 INTRODUCTION Residential water heaters are prduced in a large variety f tank sizes and heat inputs t permit the selectin f the ne best suited t d the jb. Ideally this heater wuld have a cmbinatin f strage and heat input equal t the usage. In additin t the design factrs and the sizing examples which fllw, a glssary sectin prvides detailed explanatins f selected terminlgy. This is dne t avid expanding the cntent f the sizing prcedure. DESIGN FACTORS Theses design factrs are the result f cmbining engineering test data and practical experience t frm a usable guide fr the selectin f minimum water heater tank sizes and heat inputs. As stated previusly, the factrs may be adjusted t suit individual needs. 1. Tw hur peak usage perid. Residential peak usage, based n accepted practice, is the tw hur perid during the day when the heaviest draw f ht water will ccur. Fr example, frm 7:00 t 9:00 A.M. 2. Gallns f 140 F ht water required: 20 gallns per persn fr the first tw persns. 5 gallns per persn fr each persn ver the first tw. 10 gallns fr each full bath ver the first bath. 10 gallns fr an autmatic dishwasher 20 gallns fr an autmatic clthes washer. 3. Strage tank size selectin: NOTE: The draw efficiency f a gas r electric water heater strage tank is cnsidered t be 70%. 30 galln size (21 galln draw) fr ne bath residence. 40 galln size (28 galln draw) fr tw bath residence -r- ne bath with an autmatic clthes washer. 50 galln size (35 galln draw) fr three bath residence -r- tw baths with an autmatic clthes washer. When a whirlpl tub is part f the hme equipment, it is suggested that the heater strage tank capacity, r the sum ttal f an additinal auxiliary strage tank and heater, be sized in accrdance with the fllwing table. This methd f tank sizing, will in mst cases, cancel all previus statements as nted abve cncerning tank sizing.

2 4. Heat input VS recvery capacity. Gas water heater recvery table (calculated at 75% recvery efficiency) GALLONS Tub Capacity T Overflw Outlet (@ 140ºF Water) Min. Stred Water Capacity (@ 160ºF Water*) Min. Stred Water Capacity *A mixing valve is recmmended t be installed in heater r auxiliary tank ht water utlet piping. Based n tub water temperature f 105º Gas Water Heater Recvery Table (Calculated at 75% Recvery Efficiency) Input Rating GPH Recvery At Indicated Temperture Rise Btuh 60º 70º 80º 90º 100º 30, , , , , , , , , , , Electric Water Heater Recvery Table (Calculated at 100% Recvery Efficiency) Heating GPH Recvery At Indicated Temperture Rise Element Wattage 60º 70º 80º 90º 100º

3 Ntes n element peratin: a. Tw element water heaters, simultaneus element peratin; figure the upper element recvery at 1/3* the GPH shwn fr wattage, figure lwer element at the GPH shwn. *The bttm element cntributes t the heat at the tp f the tank. This tends t shut ff the tp element. Metered tests indicate the upper element perates abut 1/3 f the time. b. Tw element water heaters, nn-simultaneus (interlcking) element peratin; figure the largest wattage element recvery nly - at the GPH shwn. c. Single element water heaters; figure the recvery at the GPH shwn. 8. Strage VS Input. Water heater selectin is best made n the basis f ht water usage. Hwever, calculatins may lead t a cmbinatin f tank size and heat input which desn't exist. In this case, the tank size and/r heat input must be balanced t achieve the desired result. Therefre, it is necessary t understand that heat input prvides ht water, at the

4 hurly recvery rate, hur after hur. The strage tank represents instant ht water at greater-than-heater recvery. The supply f ht water in the strage tank cannt be replenished until the peak usage perid has ended and heater recvery is available fr this purpse. Having enugh strage tank capacity is imprtant when large quantities f ht water are required in a shrt perid f time. If the peak usage perid is fr an extended perid f time (mre than tw hurs), the heater recvery capacity assumes majr imprtance. DESIGN EXAMPLES Given: Family f fur persns Tw full baths Autmatic dishwasher Autmatic clthes washer Ht Water Required: Tw 20 gallns/persn Tw 5 gallns/persn Secnd full bath Autmatic dishwasher Autmatic clther washer Ttal tw hur peak ht water usage 40 Gallns 10 Gallns 10 Gallns 10 Gallns 20 Gallns 90 Gallns This mean 45 gallns f water per hur, fr tw hurs, must be prvided by the water heater thrugh strage and heat input. Strage Tank Size: Accrding t design factr 3, the strage tank is 50 gallns. The draw efficiency f the strage tank is cnsidered t be 70%. Therefre, 35 gallns f "usable" ht water is available frm the tank. Strage VS Input: 90 gals. tw hur peak ht water usage

5 -35 gals. f ht water frm strage tank 55 gals. f ht water t be prduced by heat input during 2 hr. peak. This means 27.5 gallns f ht water per hur must be prvided by heat input at the accepted temperature rise used in the lcale. GAS WATER HEATER SELECTION Frm the gas water heater recvery table it is fund that, at 90 temperature rise, 27,225 Btuh will prduce 27.5 GPH. A gas water heater with at least a 50 galln strage tank and at least 27,225 Btuh input is required t meet the peak usage requirements. Cnsult water heater specificatin sheets t determine mdel needed. ELECTRIC WATER HEATER SELECTION NOTE : In the fllwing example the fuel used is electricity instead f gas. This des nt change the amunt f ht water required by the family f fur. It may mean a change in the rati f tank strage versus heat input t reflect the availability r amunt f electricity which is btainable during the peak usage perid. Tw element, nn-simultaneus peratin: Upper element watts Lwer element watts 90 gals. tw hur peak ht water usage -35 gals. f ht water frm strage tank 55 gals. f ht water t be prduced by heat input during 2 hr. peak. Figure recvery f ne element the ne with largest wattage. In this example bth elements are f same wattage GPH recvery x 2 hurs = 41.0 gallns available frm element recvery. This is less than the amunt f recvery needed gallns f ht water needed frm heat input (using 50 galln strage tank) frm tw hur recvery f (1) 4500 watt element *11.0 gallns f ht water "shrt" tw hur peak It is necessary t increase the size f the strage tank and/r element wattages t satisfy the calculated peak usage. Check yur lcal utility fr maximum

6 strage tank size requirement. Cnsult water heater specificatin sheets t determine mdel needed. *T allw fr draw efficiency, divide the "shrtage" by.7 when increasing tank size. GLOSSARY The fllwing prvides detailed explanatins f selected terminlgy used in the sizing prcedure. This is t prmte a greater understanding f water heating terms, frmula and thery. BTU abbreviatin fr the British thermal unit, which is the amunt f heat required t raise the temperature f ne pund f water ne degree Fahrenheit. Stated anther way, 8.25 Btu will raise the temperature f ne galln f water ne degree. A Btu may be sensed and visualized as abut the amunt f heat prduced by burning ne wden match. One watt-hur f electricity prduces Btu. This is the frmula fr determining the Btu required t heat a given quantity f water a certain number f degrees: Gallns x 8.25 x 1.0 x temp. rise = Btu Where gallns = Ttal gallns f ht water required 8.25 = Weight f ne galln f water 1.0 = Specific heat f water (See Specific heat) Temp. Rise = Difference in degrees between lwest incming water temperature and desired ht water temperature. Btu = Gas water heaters; divide answer by.75 (recvery efficiency) t btain equivalent gas input in Btu. Electric water heaters; multiply by t btain element wattage equivalent. In actual practice a cmbinatin f strage and input is used t assure the availability f ht water. Draw efficiency is cnsidered t be 70% in this reprt. When using strage type heaters it is cmmn practice t assume 70% f the strage

7 cld water lwers the ht water temperature belw an acceptable level under nrmal draw cnditins. Fr example, a 40 galln strage tank wuld deliver abut 28 gallns f usable ht water. Input rating The amunt f fuel in British thermal units (Btu) cnsumed by a gas r il water heater in an hur. In an electric water heater, input is usually expressed in watts r kilwatts. Cnsuming ne watt-hur f electricity prduces Btu. Interlcking (See Nn-Simultaneus) Nn-Simultaneus (Interlcking) element peratin is where bth f the heating elements in an electric water heater are nt permitted t perate at the same time. The electrical circuit is interlcked thrugh the upper thermstat t prevent simultaneus peratin. Recvery (capacity)...the amunt f water in gallns per hur, raised at a given recvery efficiency and Btuh input. Refer t Recvery table. This is the frmula fr determining recvery capacity: Input x efficiency 8.25 x temp. rise = Recvery in GPH (See Btu) Efficiency =.75 fr gas-fired water heaters 1.0 fr electric water heaters (als see Recvery efficiency) Recvery efficiency The rati f the heat in the water delivered at the heater utlet t the heat input f the heating unit. Als see Btu. Gas-fired residential water heaters are generally cnsidered t have a 75% recvery efficiency. This means 75% f the ttal heat prduced by the burner is absrbed int the water in the tank. The remaining 25% f the heat is used t mve the prducts f cmbustin thrugh the flue t the utdrs. Electric residential water heaters are generally cnsidered t have a 100% recvery efficiency. This is because immersin style elements place all the heat int the water and there is n flue. Simultaneus element peratin is where bth f the heating elements in an electric water heater are permitted t perate at the same time if necessary. The actual peratin f each element is individually cntrlled by its wn thermstat.

8 Specific heat...the amunt f heat required t raise the temperature f a given weight f a substance ne degree as cmpared with the amunt f heat required t raise the temperature f the same weight f water 1 at sme specified temperature. Strage tank, used fr string ht water in advance f needs. Prperly sized, the tank permits large vlumes f ht water t be drawn frm the system at flw rates exceeding the recvery capacity f the heater. Als see Draw efficiency. Temperature rise, the amunt f temperature difference (between incming and utging water) in degrees Fahrenheit. Draw efficiency, the amunt f water that can be drawn frm a strage tank, at a 3 gpm flw rate, befre the temperature drps 30 F. Heater utlet water temperatures belw 110 F is generally nt cnsidered as satisfactry r usable.