Moen Tub/Shower + Diverter Valve Install: T vs Twin ell

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Hey folks – long time lurker and learner of all things plumbing install related, thank you for all of the advice and input on here over the years.

TL;DR - Quick question:
We’re building a Moen tub/shower combo with 2 showering devices (standard head plus hand shower) and the recommended install is to use a twin ell. My question is: instead of a twin ell, can I cap the shower port on the main valve, and provide a tee in the ½” pipe that goes down to the tub spout, with the tee feeding the transfer valve? Given that the transfer valve will have an off position, this seems like the best of both worlds – active shut-off to prevent stack leaks (if they do happen), and the lowest pressure drop method to get the showering devices the most flow possible.

I’ve checked a number of threads both here and at on the use of the twin ell. Most of them question the need for one, but stop short of a satisfactory explanation of the reasoning. Most reasons stated for using one are to prevent dripping at the shower head while the tub spout is in use (stack leak). But this doesn’t make sense: if installing a traditional tub/shower combo with tub spout diverter (no transfer valve), the recommended install doesn’t call for a twin ell and logically uses the top port of the control valve to connect to the shower. For the posi-temp control valve when the tub spout diverter is closed, the water is directed towards the tub port, where it takes a 180 into a small ¼” hole that flows up to the shower port, which is how the shower is fed. The only difference for the 2-shower device install is the addition of the transfer valve, which adds even more pressure drop to the flow path from the control valve to the shower head (if the traditional top-port shower install was used). Additionally, if using a twin ell while tub spout is diverted, the flow path instead goes down to the ell, has to do a 90 through a 1/4” hole on the supply side, then a 180 at the 3/4” side of the twin ell, then back up another ¼” 90 bend to head up to the transfer valve. The pressure drop of this configuration is likely almost identical to using the top port of the shower valve. In short: for a transfer valve + 2 shower device install, the reasoning to use a twin ell to prevent stack leak doesn’t make sense because the pressure drop would be higher compared to a traditional install, and the pressure drop of that configuration vs. a twin ell shouldn't be too differently (possible LOWER pressure drop throug the twin ell).

Well, I hope that makes sense and if you’re still with me you deserve a cookie. So I called Moen to gripe and they said that stack leak was NOT the reason they recommend twin ell. Instead, they said the twin ell is recommended because the traditional flow path to the top shower port of the control valve may be TOO restrictive to provide adequate pressure/flow for a transfer valve and multiple shower devices. This makes more sense, because the transfer valve itself adds significantly more restriction to the shower flow path compared to a traditional install. However, given the explanation on flow path through the twin ell in the previous paragraph, it doesn’t make sense that the twin ell provides the shower a lower pressure drop flow path. Hence, if what Moen is after is to give the transfer valve more pressure, why not just use a T in the pipe down to the tub spout?

From the Moen Shower Planning Guide our desired install is Configuration 12 - the posi-temp shower spa (pic below). The direct link to the PDF is here, or it can be accessed from here if the direct link doesn’t work for you. The exact part numbers we’ve bought are: Moen M-Pact Posi-Temp valve 2520 and Moen M-core U361 transfer valve.

Other twin-ell threads:

Hey, wait a minute.

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