The values in the code are minimum, not recommended.

Based on experience and guidance from manufacturers, we require 40 psig at low flow showers (1.5 gpm) to get "good performance" from them.

You need to get a copy of the 2015 IPC. Flow rate values in Table 604.3 have changed, but pressures have not.

It is a good idea to ALWAYS have a copy of the latest building codes on hand and keep up to date on when AHJ's adapt newer versions. Some AHJ's are slow in adapting the latest versions, while some adapt the latest within a couple of months of release. Also be aware that in the USA, some states (such as New York, California and Florida) have their own building codes and do not use the IBC, while others may use the Universal Building Codes.

Thanks for the replies!.

I've been trying to find the pressure requirements for the Pfister and for the Symmons shower/mixing valves but I've had no luck.

PEDARRIN2, have you had any luck finding that info online?. Do you know of a manufacturer that has that info available online.

Again, thanks for the guidance!

daaguirre,

I have not found much of anything in manufacturer's cut sheets, but all meet the code requirements.

Current code dictates maximum flow rate of showers is 2.5 gpm at 80 psi. But there will likely never be 80 psi at the shower.

In speaking with local representatives and from experience, when using low flow showers, I try to have 40 psi at the fixture. Don't always get it, but the intention is there.

It is mostly to meet a client experience criteria. To give them a similar experience, raise the pressure to overcome the lower flow.

The mixing valve is not where you need the pressure and they are not designed to provide a specific GPM.
In the case of a shower, it is the actual head that does the real water metering.
As for required pressure, several Symmons shower head/mixing valves refer to ANSI A112.18.1M or more currently ASME A112.18.1 PLUMBING SUPPLY FITTINGS. This standard specifies

Quote (ASME A112.18.1)

5.12.2 Flow rate
5.12.2.1 Maximum
The maximum flow rate for high-efficiency shower heads and hand-held showers shall be
(a) specified by the manufacturer but in no case shall be more than 7.6 L/min (2.0 gpm) at each test
pressure;
(b) verified through testing at flowing pressures of 140, 310, and 550 ± 7 kPa (20, 45, and 80 ± 1 psi)

An issue is these codes set a maximum flow, which is only a concern for water conservation. It may be good to get the LEED plaque or other notoriety, but the users/clients typically do not care about that when they are taking a shower.

Other than maximum flow, they don't address what happens when the shower only has 20-30 psig available

And how often would 80 psig be available at the shower, unless it is on the lowest floor and the static pressure is 80 psig (max per IPC).

The problem lies when you are designing for a dormitory or hotel and there are multiple floors. Try asking your daughter/wife/SO how she washes her hair with a low flow shower with low water pressure?

My experience has been to design it for 40 psig at the shower. Sometimes this means a booster pump, which must be discussed with the owner since it is an additional cost and not necessarily required by code. This pressure gives a "good experience" for the user and still meets the maximum flow at the prescribed pressure. And don't forget that most of the water at the shower is hot, so the calcs need to take into consideration a master thermostatic mixing valve, if used.

If you look at the flow limiters
Originally they were just orifices size for the specified flow rate at 80 psi
made for a terrible shower at anything less than 60 psi.
Newer flow limiters are flexible disk with a central orifice followed by a flat plate downstream of the disk with larger orifices at the outer edge, so if the supply pressure is higher the disk squeezes tighter to the downstream plate creating a smaller orifice by the bending of the first disk. but at lower pressures the first disk does not bend as much and allows more water to flow while still keeping the flow below the limit. So for most cases with variable pressure and new better designed shower heads this should not be a problem anymore. There is also better design in the spray pattern that gives the feel of more water but those are only available in the high priced models.

Hydrae