View Full Version : pressure balance valves, flow rate and pipe diameters

09-24-2010, 02:03 PM
Need some opinions from expert plumbers. Our small, older house (1949) was completely re-plumbed a few years ago by professional plumbers. We have a 1" copper line from the meter to the house and then mostly 3/4" throughout the house. Only one bathroom. The flow rate in our bathroom increased tremendously once the nearly clogged galvanized pipe was gone. We enjoyed our showers like never before. Now, just last month, we remodeled the bathroom completely and installed a Grohe single lever shower/tub control to replace the three "dumb" valves (hot/cold/diverter) we had before. The Grohe PBV has a listed flow rate of 5.8GPM @ 80PSI. Our house has no where near 80PSI- I believe it's more like 35PSI. Our shower is now much less enjoyable (with same shower head) and the tub takes longer to fill. So given all this my questions are:

1) the valve has 3/4" fittings on it. The tub spout and shower pipes are 1/2". Would simply changing the pipe from 1/2" to 3/4" help to increase the "pressure" and flow from the valve?

2) If not, what would be the proper solution to this? Remove the PBV and install a thermostatic valve?

BTW, Terry, we are loving the elongated, sanagloss Drake! Thanks bro. And thanks all for any input.


09-24-2010, 02:48 PM
It sounds like it is a 1/2" valve since the 3/4" version flows over twice that. Have you actually measured the flow - a bucket and a watch or stopwatch would tell you.

Most of the Grohe valves have a wire mesh prefilter on the inlet. If your flow is quite low, you may need to clean those. Grohe often specifies a twin-el when using both he tub and shower even though it may have two outlets, you are supposed to have one of them pluged, and rely on the twin-el to provide the path to the shower. The twin-el is connected to the larger port. If the waterpressure is quite low, that would affect any valve.

Under normal circumstances, the pressure balance valve has no effect on the outlet volume...only whe the pressure is lower on one side verses the other does it do something.

09-24-2010, 03:34 PM
Jim, my mistake. The valve has 1/2" NPT fittings. So I guess the pipe diameter is a moot point.

Piping is all new and has been flushed for two years prior to this remodel so I would hope the prefilters are completely clean. Supply pressure from the city is low in our neck of the woods here in Seattle up high on a 500' hill. It's always been such. But after the repipe the flow rate from our shower head was fantastic. Now it's not and they only thing that has changed is the valve. I found out only after it was installed that the flow rate of that valve is 5.8GPM @ 80PSI. Since we don't have even half that pressure value I suppose we're not even getting 3GPM from the valve. What I'm starting to realize now is that the old, simple valves we had before had no such flow restriction. It's nice to turn only one handle instead of three but I miss the flow. That's why I wonder if the thermostatic is what I should replace it with.

09-24-2010, 07:01 PM
Measure your flow. It is also easy if you put too much teflon tape on things to have some of it clog things up. Take the showerhead off and see how much flow is there, too.

09-24-2010, 07:03 PM
With 35 psi, 3/4" would do nothing, because 1/2" will give the maximum capacity at that pressure. Another factor is that the PB unit will reduce EVERYTHING to the lowest pressure, which would normally be the hot water since it usually has a longer path to the faucet.