Do you worry about going to the bathroom and not getting the hot water you need?
Without a proper hot water pipe recirculation system, you would only see cold water running from the tap.
And it can be very frustrating waiting for hot water to come out of the shower or sink. You wouldn't want to do this every day before leaving for work, would you?
During winter, you may even leave the bathroom without washing your hands, simply because you don't have time to wait for hot water. In turn, it can lead to several health problems.
And that's not the only thing to worry about.
When you wait a few minutes, think about all the wasted water; We're sure you wouldn't like it.
This is not why plumbing codes and engineering office standards in Chicago require some type of domestic hot water piping recirculation system. The basic idea of a recirculating piping system is to supply hot water to whatever water supply device a person is using. In the absence of an insulation system, hot water can lose heat as it passes through the pipes.
Furthermore, without regular demand or flow, the temperature of the water in the pipes can drop to room or house temperature. Because of this, the cold water has to go down the drain as waste water before the hot water starts to come out.
With domestic hot water pipe recirculation systems, the temperature drop can be controlled reasonably. Now the question is: how much flow is needed in Chicago homes?
How much is the BTUH loss?
The total heat loss from the pipe depends on the insulation rating, the temperature difference between the ambient and supplied water temperatures, and the length of the pipe. According to ASHRAE standards, water supplies with temperatures of 140°F and below must have a conductivity rating ranging between 0.22 and 0.28 BTU-IN/(h-ft2-°F).
Heat loss from pipes is explained in a publication on insulation from the American Society of Plumbing Engineers (ASPE). With or without a safety factor, the actual difference in safety factors can range from 1 to 2 GPM. Therefore, this estimate can provide the approximation. Let's assume a loss of 10 BTUH/foot. For a pipe smaller than 2″ and for pipe sizes between 2″-4″, we will assume 20 BTUH of loss/foot. For pipe sizes larger than 4″, consider the BTUH/foot loss as five times the pipe size, which can give you the approximate value.
For example, let's take the example of a 4-story domestic residential building with 4" pipes in the basement and 200 feet in length. Assume there are four 2-inch hot water pipes that rise 85 feet, with each riser having 100 feet of less than 1-inch outlet pipe travel. What will be the BTUH loss in this case?
Electrical network: 200 X 20 = 4000 BTUH
Floor: 4 X4 X100 X10 = 16000 BTUH
Risers: 4 X 85 X 10 = 3400 BTUH
In this example, the total heat loss is 23,400 BTUH. As the temperature at the last water installation needs to be at least 10°F colder than the supply temperature, the BTUH formula will be:
GPM = BTUH/(∆TX 500) so GPM = 23,400/(10 X 500) =4.68 GPM
The heat loss would be 22,700 BTUH.
The reason only the supply pipe is used is to get the right water to the last water supply device. The return piping to the water heater will have some temperature drop. This may be necessary if you want to keep the minimum temperature above a certain value.
Minimum flow rate for balanced return
For certain systems, it is feasible to have a very low flow rate on each balanced return. The example we are using has 16 returns and the total flow rate is 5 GPM. In this example, the average flow per balance valve will be five divided by 1/3 GPM per valve. An engineer can calculate the exact flow required for each balance valve. In this case, two things will happen: flow rates will be less than 1/10 of a GPM and the time required will cause the engineer to close the deal.
Problems with low flow rates
Low flow has two problems: the valve's ability to be accurate at such a low flow and the problem of calcium buildup. Considering the fact that the contractor can set such a small flow rate, the valve's small opening can become clogged with debris.
No matter whether you choose water heaters or under sink water heaters, the above factors are the ones you cannot ignore.
What is a pleasant temperature drop in domestic hot water pipe recirculation project?
According to the instructions above, an eight GPM pump would be required. And because the system's BTUH loss will remain at 25,200 BTUH, at the end of the last water installation the temperature drop will be 6.3°F instead of 10°F. This means that the first person to use the tap will receive warm water. The drop in temperature is not surprising.
The engineer would need to maintain the lowest system temperature in the main and secondary piping above 124°F for legal reasons. The engineer can do the math and calculate the ΔT required for the length of the pipe and modify the GPM accordingly. Although the commonly accepted ΔT is ten, it can be changed if a different feed temperature is desired.
Also, remember that your engineer needs to use approved temperature usage control codes in the water installation to avoid the problem of scalding due to the extremely hot temperature of the water supplied.
With the right precautions, you don't have to worry about anything.
