BY GEORGE LANTHIER
for the beauty and simplicity of multi-speed pumps and I ’ m sure you thought that radiant loop was definitely going to be a reason for a different pump , right ? Finally , the temperatures in our system and the easy part is to first establish what the temperature requirements of the loops are and establish the Primary
Figure 4 loop temperature based on that .
The DHW loop requires a temperature over 140 ° F to ensure that all bacteria are killed off . The proper outlet temperature , below 111 ° F , can be achieved through the use of an automatic mixing valve , Figure 4 .
Secondary loops Z1 , Z2 and Z3 are all copper fin-tube loops , Figure 5 and Figure 6 , and will work , as designed , at 175 ° F from using Figure 7 .
Figure 5
Average Temperature of Water in Radiators
KP-3 / 4-50-2 210 200 190 180 170 160
Figure 7
780 720 660 590 530 460 Figure 6
Figure 8
Z4 is a fan coil heater , Figure 8 . The fan-coil heater was designed to operate with 165 ° F incoming water . Z5 is a radiant grid , Figure 9 , under a tile floor and only needs 100 ° F for proper
Figure 9 operation , so now what ? Well , now comes the part you ’ ve probably been dreading . The math . But as you ’ ll see I ’ m going to make this really easy .
My winter high-limit only needs to be 175 ° F and the summer ‘ low-start temperature ’ only needs to be 140 ° F or as determined by the indirect if you ’ re still using a cold-start system . The problem is the radiant circuit ( loop ) of Z5 .
I have to confess and tell you that I jumped way ahead of you a while back and cheated by how I took the Secondary loops off the Primary . See , I knew this was going to happen and took my loops off based on gpm and heat requirements . It ’ s called ‘ pre-planning ’.
The normal operating design temperature in the winter of my Primary loop is 180 ° F and so I ’ ll set my high-limit control for 190 ° F . The first Secondary loop off my Primary loop was my water
heater . I ’ m doing this for two reasons . First to get the hottest water I can to the indirect to get great recovery and second to dump temperature and Btus before I get to the radiant loop . Next were my three baseboard loops ( zones ), and if you want to be extra careful , pull them off by size with the biggest , Z3 coming off first followed by Z1 and finally Z2 . Then was my fan coil loop , Z4 , followed by the radiant panel loop Z5 .
The next thing to do is to look at some basic math and figure out just what temperature that water is going to be when it reaches Z5 and to protect that radiant circuit . All of the calculations we are going to do are based on the following formula and that all of the zones are calling , it ’ s what they call a ‘ design day ’. We ’ ll also round up or down to keep the math even simpler . In addition , the math and calculations must follow some basic rules and I ’ ll explain them .
First of all , we are going to take a twenty-degree differential across each Secondary loop except Z4 . That is a fan coil and for that unit we will take a 30 ° F drop . Just a reminder , 20 ° F is the number normally used across residential equipment , what the engineers call Delta T ( �T ).
The numbers in ( parentheses ) are the two numbers we ’ re multiplying and the number in [ brackets ] is the quotient or answer . Here ’ s the formula and the legend to go with it . ( Fa x Ta ) + ( Fb x Tb ) = Sc ÷ Pf = Pt Fa = the flow coming into the Secondary loop tee . Ta = the temperature coming into the Secondary loop tee . Fb = the flow coming out of the Secondary loop tee . Tb = the temperature coming out of the Secondary loop tee . Sc = total flow / temperature of Secondary loop . Pf = the Primary loop flow . Pt = the temperature after Secondary loop tee . So , let ’ s do the water heater loop first and I ’ m going to round up my Primary loop from 15.9 gpm to 16 gpm . To determine Fa , you must subtract the gpm of the secondary loop from the Primary loop gpm . In my first calculation that ’ s : 16 gpm – 7.5 gpm = 8.5 gpm ( Fa )
So , here ’ s the calculation for the DHW loop . ( 8.5 gpm x 190 ° F )[ 3040 ] + ( 7.5 gpm x 170 ° F )[ 1275 ] = 2990 ÷ 16 = 180.6 ° F Okay , so the temperature now moving through the primary loop to Z1 is 181 ° F . Let ’ s do Z1 now .
( 13.8 gpm x 181 ° F )[ 2498 ] + ( 2.2 gpm x 161 ° F )[ 354 ] = 2852 ÷ 16 = 178.2 ° F
Now the temperature entering Z2 is 178F . ( 14.1 gpm x 178 ° F )[ 2510 ] + ( 1.9 gpm x 158 ° F )[ 300 ] = 2810 ÷ 16 = 175.6 ° F
Entering Z3 the temperature is 176 ° F . ( 13.2 gpm x 176 ° F )[ 2323 ] + ( 2.8 gpm x 156 ° F )[ 437 ] = 2760 ÷ 16 = 172 ° F The temperature entering Z4 is 172 ° F . Remember , across the fan coil we ’ re going to take a 30 ° F �T .
( 15 gpm x 172 ° F )[ 2580 ] + ( 1 gpm x142 ° F )[ 142 ] = 2722 ÷ 16 = 170 ° F
28 OCTOBER 2023 | FUEL OIL NEWS | www . fueloilnews . com