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I am interested in getting a hot water recirculation pump installed so that I can get hot water faster to each of my faucets.

From looking at the picture and researching I believe I have PEX in a "home run" configuration:

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When doing my walk through with my warranty guy I mentioned I was interested in getting a recirculation pump, but he told me he wasn't sure how that works with a manifold and that I should investigate first.

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up vote 2 down vote accepted

I'm aware of two types of recirculation pumps.

The traditional system runs a connection from the most distant hot water tap (often the kitchen or master bath) back to the hot water heater. From there, you would have a check valve and a pump that moves the hot water from the tank, to that distant fixture, back through the return line that was added, and into the hot water tank via the cold inlet. With a home run PEX system, you don't have a most distant fixture.

A second type avoids the need to run the return line to the hot water tank by connecting the distant fixture by using the cold water line for the return. This means your cold line would end up being filled with water that had gone through the hot water tank, which is a significant drawback to this system (you really don't want to drink the water that's been through a hot water tank with all the sediment).

Since PEX is typically a home run install, this means you'll need to install a device on each fixture you want to have recirculated. If, for example, you only hook up the bath tub, you won't see any benefit at the sink if they are each a separate home run. Each of these connections would be independently controlled, either with a thermostat, timer, or some other on demand switch, so that your cold water lines don't fill with hot water. You will also need to have a power source at each fixture to run the pump.

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On a side point just out of interest as I'm a plumber apart from the individual flow control what is the advantage of this home run manifold over the traditional main run with tees off it? (which would use heaps less materials) – UNECS Sep 20 '12 at 2:38
@UNECS It seems a bit odd to me as well, but then I'm used to working with copper. With PEX, much of the cost is in the connections, so it might be cheaper. You also avoid the pressure changes from someone flushing a toilet. With any issues or repairs, you can shutoff the line running to a single fixture. And hopefully there are fewer issues since there aren't any joints hidden inside of the walls that could fail. – BMitch Sep 20 '12 at 11:38
Yes, the biggest advantage to a PEX home run is in the combined material and labour savings. All you need to do is layout where the fixtures are going to be installed and where the header is to be located and then no more measuring is required. Just drill holes and pull the PEX. – pdd Dec 27 '12 at 17:20

There are two issues to the time needed for hot water; one: the home run from the manifold to the faucet and two: the feed line from the hot water heater through the manifold. The key is to know how much cold water is sitting in the lines and the flow rate of your faucet. You can do a search for "PEX specifications" to get the volume of water in the lines. The volume of water is based on inside diameter and length so is applicable to any pipe that is measured by inside diameter, not just PEX. You can time how long it takes to fill a container, like a 2 quart measuring cup to calculate your flow rate.

1/2" PEX holds .92 gallons per 100'. On a relatively short 25' run, that's .23 gallons, on a relatively long 50' run, that's .46 gallons. With a 1.5 gal. per minute faucet, thats almost 10 seconds for the 25' run and 20 seconds for the 50' run. But wait, there's more! Look at that 12' of 3/4" line from the water heater to the manifold. And also consider the more that 1' run through the hot side of the manifold which can be as large as 1 1/4" inside diameter. Because of the larger diameters, that's another .25 gallons. You've added another 10 seconds. Now you have 20 seconds for the 25' run and 30 seconds for the 50' run.

The most effective and expensive option is to place the line that you most want fast hot water to at the far end of the manifold and recirculate only that one. It now has almost instant hot water and everywhere else has it 10 seconds sooner because the water in the manifold is now hot.

A less effective but probably much cheaper alternative is to move the last hot line off the manifold to a tee above the manifold like the other one shown there. That frees up the end port to use for recirculating. No sink has instant hot water but this cuts 10 seconds off wait times at all locations and is simple to plumb and insulate.

The absolute cheapest would be if the plumber had set the water heater a couple feet over and used the code minumum 18" connector between the water heater and the manifold. That would have cut out 10 1/2' of 3/4" pipe and would have reduced the wait time at all locations by about 7 seconds. It could still be done but, since the manifold would have to be turned over so that the feeds are on the bottom, which also reverses which side the hot lines are on, it would be time comsuming.

Note that this example used a typical 1.5 gal/min lavatory faucet. A 2.0 gal/min kitchen faucet would take less time. If you have a 1.0 or 1.2 gal/min low flow faucet, the wait is longer. If you have an ultra low flow 0.5 gal/min faucet, make a sandwich or read a book while you wait.

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