Recovery is the amount of water per hour the heater can raise the temperature by 100 degrees. The reason these numbers are important is that many people believe that all 30 gallon water heaters produce the same amount of hot water; however there is a HUGE difference in the amount they actually produce. When sizing indirect water heaters, it’s important to make sure that you have the right circulator to enable the the tank to supply the desired demands of hot water. The chart for the SSU-45, which is a 45 gallon indirect tank, shows that the required minimum flow rate through the coil in the indirect tank is 10 gallons per minute. See Figure 9. With that flow rate, the pressure drop through the coil alone is 7.9 foot of head. Add the associated piping to and from the tank and the boiler to this number to get the proper circulator size. If there is 30’ feet of 1” copper to the tank, the pressure drop for that would be: 30 x 1.5 = 45 40 National Oilheat Research Alliance Pipe Sizing Model Dimensions Capacity Heat Exch. Surface Recomm. Flow Rate Pressure Drop (Feet) 180° Boiler Water First Hour Ratings* (in Gallons) 200° Boiler Water First Hour Ratings* (in Gallons) Ht. Dia. (Sq. Ft.) 115°F 140°F 115°F 140°F SSU-20 27" 19¼" 20 15 8 6.0 121 168 136 185 SSU-30 39½" 19¼" 30 15 8 6.0 154 212 172 234 SSU-30LB 29½" 23¼" 30 15 8 6.0 169 234 189. 257 SSU-45 52½" 19¼" 45 20 10 7.9 212 292 237 322 SSU-60 52½" 23¼" 60 20 10 7.9 266 370 298 40 SSU- 80 72" 23¼" 80 34 12 9.1 9.1 330 440 370 503 SSU-119 73½" 27" 119 34 14 11.3 423 564 474 645 DOE test method based on 90°F temperature rise, 50°/140°W/ boiler water at 180°F Recommended flow rate: 10 GPM Coil head loss 7.9' Figure 9 This was discussed in Chapter 3 on page 27, under section 'Head" explaining Head Pressure Drop. Thus, 45 x .04 =1.8 foot of head loss. Add that to 7.9 for the coil and you have 9.7 foot of head. So your circulator demands just for the indirect tank would be 10 gallons per minute with 9.7 foot of head loss. Use the performance curve to select the correct circulator. Figure 10. Unfortunately, none of these circulators are able to meet the required performance. So, you will need to use a multispeed circulator. An improperly sized circulator will prevent the water heater from delivering the gallons described in its specification sheet. See Figure 11 on following page. This circulator, set on high speed, does meet the requirements. In additIon to sizing the circulator correctly, to ensure the correct amount of hot water is delivered, the boiler Figure 10
NORA Advanced Hydronics
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