System Size Needs To Be Calculated to Ensure Proper Function

When it comes to installing a new system, measurements will factor into what is best

System Size Needs To Be Calculated to Ensure Proper Function

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This month I’m going to shake up my column a bit to discuss the topic of retrofitting a plumbing system with a water softener and/or tankless water heater.

I have no doubt that most of you will consider this to be a fairly straightforward job to do from an installation standpoint. I question, however, if the consequences of undersizing the water distribution system are taken into consideration by the plumber.


Let’s start by referring back to our plumbing codes. Simply stated, most plumbing codes require that the water distribution system (inside the building) be sized in such a manner that it shall supply the plumbing fixtures, appliances and appurtenances with water in sufficient volume and at pressures adequate to enable them to function properly and efficiently at all times and without undue noise under normal conditions of use. Plumbing systems shall also be designed and adjusted to use the minimum quantity of water consistent with proper performance and cleaning.

We can also glean from most codes that the maximum velocities allowed by the code and the applicable standard are not to be exceeded. In addition, the system shall be adequately sized to provide for pressure losses due to water filters, water softeners, backflow prevention devices, tankless water heaters or similar devices installed in the water supply lines.

If we take the code one step further before we get started, it also states that no water filter, water softener, tankless heater, backflow prevention device or similar device shall be installed in a potable water supply piping where the installation of such a device produces an excessive pressure drop in the water supply piping.

The last phrase is what I want you to focus your attention on: “Where the installation of such a device produces an excessive pressure drop,” it shall not be installed.

So, my question to you is twofold: What is an excessive pressure drop, and what are you doing to compensate for it?


Let’s take a look at a simple example for a single-family residence where a softener is to be added to the existing system. The total water supply fixture units, or wsfu, is 17. The existing system has an available pressure of 53 psi, and the pipe sizes are properly sized for a maximum of 17 wsfu on ¾ inch Type M copper and 5 wsfu on ½ inch. (This is based on the table method of the Uniform Plumbing Code, Chapter 6).

After taking a water sample and determining the size of the water softener needed for this project, the next step before installing the unit should be to determine the pressure loss that will occur through it. To do this, you must convert your water supply fixture units to gallons per minute. This can be done through a step called interpolation. This method, while fairly simple to do, has a somewhat lengthy explanation and could be a completely separate topic for us to discuss and practice in the future. To save time and space, let’s use 17 wsfu as the total load that will flow through the softener — 17 wsfu converts to 12.5 gpm (reference UPC, Appendix A).

We would then take 12.5 gpm and cross-reference it with the pressure loss graph or table from the water softener manufacturer we intend to use. In this case, the manufacture stated I could expect to lose 10.25 psi downstream of the unit.

If you recall, we had an available pressure of 53 psi with the existing system. With the softener installed, we must subtract 10.25 psi: 53.0 - 10.25 = 42.75 psi. This means 42.75 is our new available pressure. Again, using the table method of sizing from the UPC, the new pipe sizing required would look like this:

While this it isn’t a huge change, you can see it definitely is a change and one that cannot be ignored because to do so could lead to excessive velocities that could over time wear out the pipe and might also lead to excessive noise in the system (aka water hammer).


Next, let’s look at retrofitting this home with a tankless water heater. For this home, there is a potential for a total of 9 wsfu to flow through the water heater at one time. This is important information when installing a tankless water heater, as it will help you determine the pressure loss that will occur through the heater. I selected the smallest-size heater that would work based on fixtures and manufacturer recommendations. The heater is 190,000 Btu natural gas.

Again, you need to interpolate your water supply fixture units to gallons per minute — 9 wsfu converts to 7.5 gpm. Using the manufacturer’s tankless heater pressure loss table based on the heater I selected, here is what I found: 7.5 gpm = 34 psi loss.

For most practical purposes, this would be an excessive amount of pressure loss, and as the codes state, it would not be allowed to be installed. Now you could work around this by installing a booster pump, a larger heater with less pressure loss, or even two smaller heaters or repiping the home. The point I’m trying to make is that it’s not as simple as just pulling out a tank-type heater and swapping it for a tankless one.

I would venture to say that most of you may not have considered this and might be looking at this a little differently after this brief discussion on retrofitting. I will also say that if you are, then my point has been made: Size matters.  


Randy Lorge is a third-generation plumber and the director of workforce training and development for the International Association of Plumbing and Mechanical Officials (IAPMO). Lorge is also a member of the planning team for the International Water, Sanitation and Hygiene Foundation (IWSH). This 501(c)(3) foundation has completed water and sanitation projects for those less fortunate in India, South Africa, Indonesia and, more recently, the United States. He enjoys time with his family and spending as much time as possible in his deer stand. To contact Lorge, email


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