To Twin Ell Or Not To Twin Ell...

[thatjohn]

We're building a new bathroom and installing a 36" x 72" tub in an alcove and are trying to find a cost effective way of pulling ~80 gal. for the tub in a reasonable amount of time, while also being able to divert to a shower head.

The lowest cost thermostatic mixing valve with volume control valve we could find having an acceptable flow rate (around 13 GPM @ 60psi) is the 3/4" American Standard R540. (We are able to pick it up for under $200, trim kits are available for under $130. Other manufacturers, after including the trim kit, seem to be pushing over $600. We understand the R540 has a weak link with some of its plastic parts, so this lower cost option does have some caveats. )

We're trying to decide between a twin ell with diverter spout or a transfer valve to select between shower or tub spout. The twin ell/diverter spout seems the lowest cost and least complicated option, but our plumber says that he hasn't installed a twin ell "since the 80's" (a bit of a hyperbole to be sure) and that transfer valves are far more common today. Problem is, American standard doesn't seem to have a 3/4" transfer valve (so it may reduce the flow rate?) and we're not sure the extra cost is worth it.

What are the downsides of using a twin ell with diverter spout, and do we need to stick with a 3/4" diverter spout in order to keep the 13GPM flow rate?

Also, are there other cost effective thermostatic valve bodies with relatively high flow rates you would recommend over the R540?

Thanks!

 

[hj]

A "twin ell" WILL require a 3/4" diverter spout, because that is the way they are made.

 

[Sponsor]

 

[thatjohn]

A "twin ell" WILL require a 3/4" diverter spout, because that is the way they are made.

Thanks for the reply.

Our plumber is telling us that we could use a 3/4" to 1/2" bushing and then use a 1/2" spout. He points to the Kohler twin ell installation as an example. American standard offered no real guidance, saying "our twin ell has been discontinued, but any other twin ell should work fine with the R540". American standard's twin ell seemed to be a different beast than others, with 3/4 NPT feed from the valve, and 1/2 NPT for the shower and tub -- where most others are 1/2 NPT from valve and to shower with 3/4 NPT to tub spout.

I've noticed other posts by yourself and others saying that a twin ell needs a 3/4 spout -- so I'm a bit confused. The selection of 3/4 tub spouts is rather limited, so 1/2 would give us more choice, but then I don't want to select a tub spout that'll slow the flow and/or send water shooting out the shower head when trying to fill the tub.

 

[Jadnashua]

The copper tubing institute's recommendation on flow rate through pipe calls for a maximum of 5fps, and on 1/2" when using hot, that's equivalent to about 4gpm. 3/4" is almost exactly double that. You can exceed that flow rate temporarily, but things start to get noisier and you can (long term) erode the pipe. If you throttle down the tub spout and are using a twin el, the flow restriction will force some water up to the shower head regardless...the tub spout must be at least as large as the twin el's tub opening to prevent that.

 

[hj]

You CAN reduce the twin ell's outlet to 1/2" but you do it at your own risk.

 

[Jadnashua]

Can vs should...water will take the path of least resistance. Having a larger outlet to the tub spout pretty much ensures all of the water will take that path. If you reduce it, it could create enough backpressure to cause some to come out the showerhead. Certainly, you CAN do it, but you MIGHT not like the results!