Choices in PEX Manifold Systems

June 14, 2018
Summer 2018
A version of this article appears in the Summer 2018 issue of Home Energy Magazine.
Click here to read more articles about Hot Water

There are a number of ways to combine PEX tubing with manifolds to get better-than-normal hot-water delivery in a home. In the old days, manifolds were sometimes used with soft copper tubing, but as copper isn’t cheap these days, it should be used only in a tiny home that has nonaggressive water—and by that I mean water that isn’t too hard on the pipes. Aggressive water can be acidic or have compounds in it that corrode copper.

There are two basic plumbing systems that use manifolds. The first is the standard home run system. The second is the pumped system, which consists of distributed manifolds combined with recirculation. Depending on the home’s layout, the home run system works for small and medium-sized homes, while the pumped system works for bigger homes.

An old idea that is getting some discussion now is the concept of central core plumbing. In central core plumbing, the wet rooms and the water heater are located as close together as possible. This keeps piping short and small, so that as little water as possible is wasted while waiting for hot water (see Figure 1). This is a perfect place to use a manifold, as long as it is installed in such a way as to get hot water quickly.

Figure 1. This page from an old Audels Guide shows the concept of central core plumbing, with small copper tubing feeding fixtures that are generally not far apart. Another part of the same book discusses using ¹⁄4-inch copper tubing, as long as it supplies a sink and is not more than a 20-foot run. With low-flow fixtures and adequate pressure, we could do even better now. For example, ¹/8-inch tube will give just under ¹⁄2 gallons per minute (gpm) at a flow rate of 4 feet per second. Modern public lavatories use ¹⁄2 gpm maximum.

The holy grail of hot-water distribution is to keep the volume of water in the system as low as possible. Doing this conserves both energy and water and improves performance by cutting wait time at the tap. It’s a poorly understood phenomenon, but roughly double the amount of water in the pipe must be run out in order to get hot water. If there is ½ gallon in the line, 1 gallon must be run out before hot water arrives. Combine that with the requirements for low-flow fixtures and bigger pipe (to accommodate more fixtures) and you can easily see why it often takes longer to get hot water in newer homes than in older ones (see Figure 2). It would be nice if the energy code people actually talked to the plumbing code people and vice versa! One more reason for keeping pipe size small is that it speeds up flow rates in the lines. This helps to scrub the lines of bacteria (like Legionella) and biofilms. Any line that sees flows of less than 1 foot per second is more likely to have entertaining things growing in it.

A 1-inch copper manifold placed directly on top of a water heater and connected to ³/8-inch PEX tubing. When insulated, the manifold stays full of hot water, making hot water arrive at the taps faster. (Larry Weingarten)

Home Run Systems

With traditional home run systems, it’s normal to have the manifold some distance from the water heater. Often a 1-inch pipe is run between the heater and the manifold. Unfortunately, this adds even more volume that has to be flushed out to get hot water. Manifolds are made of either metal or plastic. Plastics cannot be too close to the water heater or they might melt. Metals don’t have that limitation, so it is possible to close-couple the manifold to the heater so it remains full of hot water. This makes for quick hot-water delivery. Delivery is even quicker if the metal manifold is well insulated. One other thing plumbers seem to like doing is to bundle the hot tubes together and the cold tubes together. This is about the worst thing they could do, because it promotes heat loss to the other lines, which certainly aren’t all full of hot water. Best practice is to insulate tubes individually. That goes for cold lines as well, to prevent condensation in humid climates and also to deliver colder water than you would get with bare pipe.

One of the disadvantages of traditional home run systems is that running hot water at one tap does little to speed up delivery at other taps. This is not a problem in trunk-and-branch systems, where the first draw takes time for hot water to arrive, but all subsequent draws get hot water faster once the main line has been filled with hot. One place where this disadvantage can be fixed is in the bathroom. When people take a shower they aren’t normally using the bathroom sink. So it would be easy to run a single hot line to the bathroom and T off to both the sink and the shower. And even if someone snuck in to use the sink, there would be no problem, as long as the shower had a pressure and/or temperature balanced valve. All’s still OK; no need for shower dancing—which is the quick maneuver we do when the shower water changes temperature too fast.

Other ways to fix the problem of slow hot-water delivery with home run systems are to keep the manifold as close to the water heater as possible; to place the heater as centrally as can be; and to keep line size as small as you can get away with. All of these measures help to reduce waste and cut wait times. Fellow hot-water nerd Gary Klein often walks through new homes under construction to look at the installed hot-water systems before the walls and ceilings get closed up. Routinely he finds about twice as much length of pipe in the house as is needed. Most plumbers just aren’t thinking about how to reduce the volume of the system.

Weingarten-fig2 copy
Figure 2. We face a magnitude of problems when energy and plumbing codes conflict.

Plumbers prefer straight and square plumbing. Perhaps it’s a code of honor thing. PEX doesn’t need that way of thinking, as normal 90s add a lot of unnecessary flow restriction. With PEX, it’s better to use bend supports to make your bends. Better yet is just to let the pipe make long curves. This way the friction loss remains low, and the possibility of using smaller pipe just might be workable.

Pumped Systems

Pumped systems use distributed manifolds with recirculation. This allows architects and owners to place wet rooms wherever they feel like it. The concept is simple: Install a main line that always runs very close to the wet rooms. After the last wet room, run a recirculation line about as big as the main line. (This will depend on the fixtures. For example, a tub at the end of the main line might let you downsize the recirc line a little, since it need never carry the full flow to the tub.) Install a mini-manifold at each wet room, so the run of pipe to each fixture remains short, preferably 10 feet or less. Then add a good-sized demand-controlled pump, so the system runs only when asked to. Like any recirculation pump, a demand-controlled pump primes the main line with hot water, so you get hot water quickly when you open a tap. The advantages of demand pumping are much lower heat loss from the pipes and lower electrical use from the pump. Demand pumping does mean learning to push a button when you want (or just before you want) hot water, but that’s easily learned.

PEX done wrong. No need for the flow restricting 90s, or the brass-to-steel connections, which will clog up with rust eventually. (Larry Weingarten)

Once the system is activated, it will quickly move water through the main line, priming it with hot water. Now when the user wants hot water, only the water in the short branch line needs to be flushed out, usually delivering hot water in five seconds or less. Another plus to this system is that the pump may run only five minutes a day, so very little heat is lost from the pipes. A rule of thumb we hot-water nerds have is that 24-hour recirculation triples your water-heating bill. Even a well-set timer will run around eight hours. Demand pumping also won’t erode copper pipes or brass fittings the way 24-hour pumping does.

Manifolds come in a range of port numbers and sizes. They may or may not have shutoff valves. (Distributed manifolds often get buried in the framing, so there would be no use for shutoff valves unless the manifold is intentionally exposed.) They come in copper or plastic. They can be flow-through or sealed-end, and they can be made to fit any type of PEX. Some of the main manufacturers are Uponor, Viega, Watts, Sioux Chief, and Zurn.

learn more

Find out more about the Viega PureFlow.

Anderson, Edwin, Audels Home Appliance Service Guide. 1954. Theo. Audel & Co. New York.

One thing I haven’t mentioned yet is water pressure. For any system to function well, it’s important that the plumber and/or designer understand what the incoming pressure is and how it might vary. At a minimum, get a 0–200-psi gauge that can be screwed onto a hose bibb and has a pointer that tells you what the highest pressure has been. With this, you can check pressure periodically and understand just what it’s doing. Check it during periods of high use in your neighborhood to see what your lowest pressure will be. Pressure has everything to do with the size and length of pipe you can use. Viega has a tech sheet on its website called “PureFlow PEX Tubing Tech Data.” It’s a useful tool for figuring out the smallest tube size that will work for you. It takes into account length of run and working pressure. One thing to consider is that with copper, you don’t want flows in excess of 4 feet per second (FPS). With PEX, that goes up to 10 FPS! This gives you a wider range of possibilities if you have adequate pressure to play with.

I hope this introduction to manifolds and their use makes it easier to install plumbing in your next project that exceeds everyone’s expectations while saving resources.

Larry Weingarten  got his General Contractors license in 1982. He has written articles on water heating and energy for various trade journals and has taught classes on these topics for PG&E, Affordable Comfort (now Home Performance Coalition), and others. He helped create DVDs on these and related topics. He finished building an off-grid home in 2006. Designed to be very efficient, comfortable, and inexpensive, it was the 13th home to meet the Thousand Home Challenge. For over 35 years, Larry has been involved in rehabbing rental properties. He runs the web site

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