BY GEORGE LANTHIER

Okay , so now we ’ re going to look at our example system again . We ’ re also going to add an indirect for domestic hot water ( DHW ) and build a new system that will be a Primary-Secondary design . In Figure 1 we have a schematic of the piping for that system .

Figure 1

Our Primary loop must be resized to carry the heating and DHW load which is now a total of 159400 Btus or 15.9 gpm , 8.4 gpm which is for heat and 7.5 gpm that is for our DHW demand for a 40-gallon indirect . This would now make our Primary loop require a 1-¼ ” steel pipe size and our circulator would now be sized , using Figure 2 , to use the same pump , but with the speed now set to the highest . I bet you thought we were going to need a different pump , right ? That ’ s the true beauty of Primary- Secondary systems .

Figure 2

The indirect loop needs a closer look , too . To supply the indirect heater that has a 1-inch pipe connection , the piping for this Secondary loop should be all 1-inch pipe or tubing from the Primary loop fitting ( double tee fitting ) and back to it . The Btu requirement is 75000 or 7.5 gpm , and the foot of head is 4.6 feet . That foot of head number is based on a ten-foot pipe run ( 10 ’ x . 06 =. 6 ) and a pressure drop across the coil of the indirect of 4 feet of head . That will allow me to use my Figure 2 pump again . You must know the restriction of any coil and the OEM can tell you what it is .

Our new requirements are the following :

• Primary loop = 15.9 gpm at 1 foot of head , 1-1 / 4 ” pipe size .

• DHW Secondary loop = 7.5 gpm at 4.6 feet of head , 1 ’ pipe size .

• The first Secondary loop now becomes Zone 1 ( Z1 ) and delivers 22000 Btus or 2.2 gpm that needs a ¾ ” pipe .

• The second Secondary loop now becomes Zone 2 ( Z2 ) and delivers 19200 Btus or 1.92 gpm that needs a ¾ ” pipe .

• The third Secondary loop now becomes Zone 3 ( Z3 ) and delivers 28800 Btus or 2.88 gpm that needs a ¾ ” pipe .

• The fourth Secondary loop becomes Zone 4 ( Z4 ) and delivers 9600 Btus or . 96 gpm that needs a ½ ” pipe .

• The fifth Secondary loop becomes Zone 5 ( Z5 ) and delivers 4800 Btus or . 48 gpm that needs a ½ ” pipe . We need to look at how the temperature can be affected in the

Primary loop and all of the Secondary loops and select our pumps again based on foot of head of each Secondary loop . Let ’ s do the pumps first .

Our pump requirements would be based on the following pipe runs and resulting foot of head requirements .

Z1 contains 40 ’ of baseboard and has a total lineal length of 80 ’. Using our formula , we find that 80 ’ x . 06 = a pump head of 4.8 ( 5 ’) is required for Z1 .

Z2 contains 36 ’ of baseboard and has a total lineal length of 70 ’. Using our formula , we find that 70 ’ x . 06 = a pump head of 4.2 ( 4 ’) is required for Z2 .

Z3 contains 52 ’ of baseboard and has a total lineal length of 70 ’. Again , using the formula we find that 70 ’ x . 06 = a pump head of 4.2 ( 4 ’) is required for Z3 .

Z4 contains 30 ’ of pipe and the heater . Using our formula , we find that 30 ’ x . 06 = a pump head of 1.8 ( 2 ’) is required for Z4 . But the specifications for the heater are for 4 ’ of head through the heater so our actual foot of head will be 6 ’.

Z5 is our radiant loop . It contains only 5 ’ of pipe that connects to the radiant panel that consists of 165 ’ of radiant tubing . Using our formula , we find that 170 ’ x . 06 = requires a pump head of 10.2

( 10 ’) for Z5 .

The settings for the pumps are as follows and are all based on using the pump data shown in Figure 2 , the Taco VR-1816 , Figure 3 , for all 7 circulator applications . The only thing that will change is the motor speed . The Primary loop should be set at Speed 3 , the DHW Secondary loop at Speed 2 and all of the other pumps

Figure 3 at Speed 1 and you now know the reason

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