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In the last two articles, we’ve been talking pretty much about creating hot water in a cold-climate, no-fossil fuel, NetZero multifamily project. In this article, we will start a conversation about distributing that hot water to users in the building. We’ll focus this article on flows and pressures in pipe, fittings and fixtures and the real numbers plumbing designers need to know.
Getting hot water to pour out of a fixture is pretty easy — just buy some pipe and fittings and hook them together. Getting that water to emerge at the right temperature quickly with sufficient pressure while minimizing waste of water, energy and money is more difficult — it requires designers to go back to basics, not just consulting outmoded guidance from yesteryear. We want to share with you some of what we’ve learned to make this job easier.
Pressure loss in pipes, fittings and fixtures
Forcing water through pipe, fittings and fixtures creates friction and thereby loss of pressure. While pressure loss can be minimized by choosing big-diameter pipe, it wastes water, energy and money and may increase the risk of bacterial contamination. So the question devolves to: How small can you make your pipe without causing problems? Of course, you need to know how much water the pipe will be required to carry. But, we’ll get to that a bit later.
While manufacturers and others provide tables for pressure loss, our work with real pipe and fittings has revealed some different values that can help you do the math.
Wimpy Showers at Solara
Shortly after the first building at Solara was started up, the contractor turned on the showers and found that the spray pattern was unacceptable. In the time-honored tradition, the flow restrictors in the base of the showerhead were removed.
Yes, the flow rate improved and the showers felt much less wimpy. But where was the problem? In the 3/8-inch home run twig tubing to the combination tub/shower valves? In the tub/shower valves? Or somewhere else?
Initial calculations narrowed the possibilities down to the pipe and the valves. One thing was obvious — a lot more hot water than we budgeted for was coming out of the showerheads.
I (Pete) contacted Gary for assistance. I learned that Gary had been measuring the pressure drop in modern pipe and fittings at a range of flow rates and velocities for a few years and was willing to mock-up the conditions at Solara and help figure out what was going on. I arranged to visit the pressure drop testing facility in Arcata, Calif., bringing with him the Peerless shower valve and piping representative of the installation.
Figure 1 shows the results of the testing of the Peerless combination tub/shower valve and the associated piping. The supply piping to the valve included a 3/4-inch PEX mini-manifold and 3/8-inch home run twig lines to the hot inlets of the valve. The home run twig line was about 30-feet long. The average water temperature during the test was 63.5 F +/- 0.6 F.
[Figure 1. Pressure Drop Through the Peerless valve and Piping Set up]
We separately measured the pressure drop through 3/8-inch twig tubing so we could subtract the pressure loss due to the tubing from the combined measurements.
Figure 2 compares the results for a gentle bend, a coil and using bend supports. The values are pounds per square inch gage (psig)/foot and are very similar for each of the three configurations.
[Figure 2. Pressure Drop Through Different 3/8 inch PEX Configurations]
The next step is to select the applicable flow rate, multiply these values by the feet of pipe and add in the losses for the fittings to get the pressure drop through the tubing. Assuming that the showerhead was flowing at 1.8 gallons/minute, the flow rate on either the hot or cold is less than this; the values depend on the difference in hot- and cold-water temperatures.
Assuming the hot side is 1.2 gpm and the cold side is 0.6 gpm, the pressure drop per foot is 0.11 and 0.3 psig, respectively. For 30 feet of pipe, the pressure drop would be between 0.9 and 3.3 psig.
What about the fittings? Each home run twig pipe has at least two fittings: one at the manifold and one at the valve. Separate testing of 3/8-inch engineered plastic (EP) expansion couplings found that the pressure drop through the fitting was virtually the same as the pressure drop through the pipe itself. In this case, we can use the numbers for the feet of pipe.
In summary, the pressure losses through the valve were two to eight times larger than the losses through the piping. However, this research told us that it is still important to keep the home run length of 3/8-inch tubing as short as possible and to use the minimum number of fittings on each path. At Solara, we established 30 feet as the approximate maximum length — especially to the shower.
Problem Pressure
In the simplest of terms, the problem is pressure. Specifically, the available pressure after accounting for all losses. Let’s take the case of a bathroom two floors above the level the water enters the building.
Table 1 contains example pressures. There is a column for you to input numbers for a case you are familiar with.
Regardless of the starting pressure, there are pressure losses built into the plumbing on the way to the shower. These include backflow preventers, elevation, water heaters, the plumbing itself and the shower valve. All have a certain amount of pressure drop at the flow rate of the shower.
The elevation above the street is 15 feet. The pipe friction is based on the losses from the backflow valve to the shower valve; this needs to be calculated for specific situations. A low value of 2 psig was assumed for this example. The tub shower valve with the largest measured pressure drop at 2 gpm was selected for this example. (See section on pressure drop through tub/shower valves.) For this example, the aggregate pressure losses are calculated to be 30 psig.
[Table 1 Pressure Available at the Shower head]
Column A starts with 70 psig and the losses bring the available pressure at the shower head down to 40 psig. Column B starts with 50 psig and the losses bring the available pressure at the shower head down to 20 psig.
Showerhead flow rates are determined at 80 psig (in accordance with the ASME A112.18.1-2018/CSA B125.1-18 standard). Actual pressures of 40 or 20 psig will result in lower flow rates. Just how low? This depends on whether there is a fixed or pressure-compensating orifice being used as the flow regulator in the showerhead.
Fixed orifice flow regulators follow the normal pressure versus flow rate curve. On this curve, if you reduce the pressure by half, you get 0.7 of the flow. Cut it in half again, you get 0.49 of the starting flow rate. So, if we specified a showerhead with a flow rate of 2 gpm as rated at 80 psig, we would get roughly 1.4 gpm at 40 psig and 1 gpm at 20 psig. (See the dashed purple lines in Figure 1.) No wonder people are so unhappy they remove the flow regulators!
Fixed Vs. Pressure-Compensating
Figure 3 is based on a showerhead flowing at 2 gpm when rated at 80 psig. The graph compares the flow rates on the normal pressure versus flow curve with the flow rates on two idealized curves for pressure-compensating flow regulators. For the pressure-compensating regulator with good performance, for all pressures above 40 psig, the flow rate is essentially the same.
For the pressure-compensating regulator with high performance, for all pressures above 20 psig, the flow rate is essentially the same. Due to the requirements of the test standard, these level flow rates will always be slightly less than the rated value. So, for a showerhead rated at 2 gpm, both the good- and high-performance flow regulators have much better performance than the fixed orifice type!
[Figure 3 Comparing Fixed and Pressure-Compensating Flow Regulators]
In short, you need to specify pressure-compensating flow regulators for the showerheads. The result will be more predictable performance over a wide range of operating pressures, increased customer satisfaction, and reduced risk of call backs due to low pressure at the shower.
This analysis is also applicable to faucets. While the numbers are a bit different, it still makes sense to specify pressure-compensating aerators. The nuances will be discussed in a future article.
A future article will discuss the complexities of selecting a showerhead with the right force and spread and the impact of droplet size on temperature drop. The next article will be focused on the ramifications on delivery times, sanitation, energy and water waste and installation cost containment.