Why is Domestic Hot Water Piping Recirculation Important?

Topics: steam domestic hot water generation, hot water piping recirculation

Michael Tobias
Author : Michael Tobias on September 27, 2018

Do you worry about going to the bathroom and not getting the hot water you need?

Without a proper hot water piping recirculation system, you would just see cold water running out from the tap.

And it can be very frustrating to wait for hot water to come out of a shower or basin. You wouldn't like to do it each day before heading out to work, would you?

During winters, you may even leave the bathroom without washing hands, simply because you don't have the time to wait for hot water. In turn, it can lead to several health issues. 

And that's not all there is to worry about.

When you're waiting for a few minutes, think of all the water wasted; we are sure you wouldn't like it.

It't why plumbing codes and engineering office standards in Chicago require some kind of domestic hot water piping recirculating system. The basic idea of a recirculating piping system is to provide hot water at whichever water fixture a person is using. In the absence of an insulation system, hot water can lose its heat while moving through the pipes.

Other than that, without a regular demand or flow, the water temperature in the pipes can drop to the room or house temperature. Due to this, the cold water has to go down the drain as waste water before hot water starts coming out.

With domestic hot water piping recirculation systems, the temperature drop can be controlled to a reasonable extent. Now the question is, how much flow rate is needed in Chicago homes?

How Much Is The BTUH Loss?

The total heat loss of the pipe depends on insulation rating, the temperature difference between the surrounding temperature and supply water, the pipe length. As per ASHRAE standards, the water supply of temperature 140°F and less must have a conductivity rating ranging between 0.22 and 0.28 BTU-IN/(h-ft2-°F).

Heat loss from pipes is explained in an insulation publication of the American Society of Plumbing Engineers (ASPE). With or without a safety factor the actual difference in safety factors can range from 1-2 GPM. Therefore, this estimate can get you the approximation. Let's assume 10 BTUH loss/ft. For a pipe smaller than 2" and for pipe sizes between 2"-4" let's assume 20 BTUH loss/ft. For pipe sizes more than 4" consider the BTUH loss/ ft to be five times the pipe size which can get you the almost approximate value.

For example, let's take the example of a 4 stories domestic residential building with 4" pipes in the basement that are 200 feet long. Assume that there are four 2-inch hot water pipe rises at 85 feet with every riser has 100 feet of run out pipe of less than 1-inch. What will be the BTUH loss in this case?

Mains: 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. Since the temperature at the last water fixture needs to be no less than 10°F cooler than the supply temperature so this way the BTUH formula will be:

GPM = BTUH/(∆T X 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 fixture. The return piping to the water heater will have some amount of temperature drop. This can 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 in each balanced return. The example we are using has 16 returns, and the total flow rate for it is 5 GPM. In this example, the average flow rate per balance valve will be five divided by 1/3 of a GPM per valve. An engineer can calculate the exact flow rate needed for each balance valve. In this case, two things will happen, the flow rates will be less than 1/10 of a GPM and the time required will make the engineer go out of business.

Problems With Low Flow Rates

The low flow rate has two problems, the capacity of the valve to be precise at such a low flow rate and the problem of calcium build-up. Considering the fact that the contractor can able to setup such a small flow rate, the small valve opening can get clogged with debris.

It doesn't matter whether you choose over-the-water heaters or under-the-sink water heater, the above factors are ones that you cannot ignore.

What is an Agreeable Temperature Drop in Domestic Hot Water Piping Recirculation Design?

As per the instructions above, an eight GPM pump would be needed. And since the BTUH loss of the system will remain at 25,200 BTUH, at the end of the last water fixture the temperature drop will be 6.3°F instead of 10°F. This means the first person using the water faucet will get warm water. The temperature drop is nothing surprising.

The engineer would need to keep the lowest system temperature in the main and branch piping above 124°F for legal reasons. The engineer can do the math and calculate the required ΔT by the length of the pipe and modify the GPM accordingly. Although the commonly accepted ΔT is ten it could be changed if a different supply temperature is desired.

Also, remember that your engineer needs to use the approved codes of temperature use control at the water fixture to avoid the problem of scalding due to the extremely hot temperature of the water being supplied.

With the right precautions, you have nothing to worry about.

 

 

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