Copyright Goodheart-Willcox Co., Inc. 260 Hydronic Heating: Systems and Applications Example: A room measures 200 ft2 in area and has a heat load of 25 Btu/ft2. The tubing is spaced 9″ on center. Using Figure 12-40, the minimum flow for this area will be 0.75 GPM. Feet of head loss: The final determination when selecting a circulating pump is to calculate the total pressure loss in feet of head. To do this, the following information must be known: ■ ■■ Water flow per loop in gallons per minute (GPM) ■■ Total loop length ■■ Tubing size ■■ Type of tubing used ■ Supply water temperature The table in Figure 12-41 can be used to determine the head loss per foot of pipe using 3/8″ tubing and 100% water (versus a water and glycol mix). If glycol is required for freeze protection, a different table should be referenced. These tables can be found on the Internet or by consulting the PEX fitting manufacturer. Example: The flow has been calculated through a particular zone as 0.45 GPM. The total loop length is 160′ and the tubing being used is 3/8″ PEX with 100% water (no glycol). The supply water temperature is 110°F. Using the table in Figure 12-41, the feet of head per foot of tubing is 0.03692. When this number is multiplied by the total loop length of 160′, the total feet of head for this loop is equal to 5.9. TECH TIP Adjusting Loop Lengths and Tubing Size If the feet of head loss is higher than desired once all pressure loss calculations are complete, the loop lengths may need to decrease or the tubing size may need to increase. Once these factors have been changed, recalculate the pressure drops using the new loop length and tubing size. Goodheart-Willcox Publisher Figure 12-41. Use this chart for calculating the feet of head loss for circulating pump sizing. Head Loss for 3/8″ Hydronic Tubing, 100% Water (ft head loss per ft of pipe) Velocity (ft./sec.) GPM Water Temperature (°F) 80 90 100 110 120 130 140 150 160 170 180 190 200 1.0 0.30 0.01912 0.01845 0.01785 0.01732 0.01685 0.01642 0.01603 0.01568 0.01535 0.01505 0.01478 0.01453 0.01430 1.1 0.33 0.02286 0.02207 0.02136 0.02074 0.02017 0.01967 0.01921 0.01879 0.01841 0.01805 0.01773 0.01744 0.01716 1.2 0.36 0.02688 0.02595 0.02513 0.02441 0.02375 0.02316 0.02263 0.02214 0.02170 0.02128 0.02091 0.02056 0.02024 1.3 0.39 0.03117 0.03011 0.02917 0.02833 0.02758 0.02690 0.02629 0.02573 0.02522 0.02474 0.02431 0.02392 0.02355 1.4 0.42 0.03572 0.03452 0.03345 0.03251 0.03165 0.03088 0.03019 0.02955 0.02897 0.02843 0.02794 0.02749 0.02706 1.5 0.45 0.04054 0.03919 0.03799 0.03692 0.03596 0.03510 0.03431 0.03359 0.03294 0.03233 0.03178 0.03127 0.03079 1.6 0.48 0.04562 0.04411 0.04277 0.04158 0.04051 0.03954 0.03867 0.03786 0.03713 0.03645 0.03583 0.03526 0.03473 1.7 0.51 0.05095 0.04928 0.04780 0.04648 0.04529 0.04421 0.04324 0.04235 0.04154 0.04078 0.04010 0.03947 0.03888 1.8 0.54 0.05653 0.05469 0.05306 0.05161 0.05027 0.04911 0.04804 0.04706 0.04616 0.04533 0.04457 0.04387 0.04322 1.9 0.57 0.06237 0.06035 0.05856 0.05697 0.05553 0.05423 0.05306 0.05198 0.05100 0.05008 0.04925 0.04848 0.04777 2.0 0.60 0.06844 0.06624 0.06427 0.06255 0.06098 0.05957 0.05829 0.05711 0.05604 0.05504 0.05413 0.05330 0.05252