Water Pressure Regulators for an RV
David E. Damouth <damouth@attbi.com?
18 March 2002

Every RV should have a water pressure regulator, to protect the RV's internal plumbing from damage by excessive pressure in a campground water system.  But some regulators have an unfortunate side effect - they can limit the maximum flow rate, which can create wimpy flow from the shower nozzle, and annoyingly long times to fill a pot with water in the kitchen.   The worst culprits are the small inexpensive generic brass or plastic regulators that are widely available anywhere RV parts are sold.   They may do a good job of regulating the pressure, but their very small internal diameter limits the flow to unacceptably low rates.   There are at least two solutions to this problem:

a:  Use two or more of these cheap regulators in parallel.    If you already have a couple of these cheap regulators, this may be the most economical solution.  Or you can often find them very inexpensively in RV-oriented flea markets.  (Escapees RV Parks often have a table of donated RV gadgets, sometimes with a "pay what you think they are worth" policy.)

This, as shown in flow rate measurements below, can be a very effective solution.    A manifold which connects two or more regulators in parallel can be assembled from copper or plastic inexpensive hardware-store plumbing fittings, in several different ways.  Several different versions of this solution have been described in RV-Talk, and may be available in the RV-Talk Archives. 

b:  Buy a bigger and better regulator.  When purchasing the small cylindrical regulators, look inside.  Some have a noticeably larger diameter water flow path than others, and larger is better.  The older Marshall Brass products are larger than most of the new regulators I've seen. 

For higher flow rates, you can go to almost any big hardware or plumbing or building supply store and buy several different sizes of regulators intended for permanent installation in a house, and these offer far greater inlet size and flow rate, as well as allowing the pressure to be adjusted to your own preference.  I got the smallest home-style regulator, and added adapters from pipe threads to garden hose threads (also a hardware store item).  The shower flow with this regulator is quite satisfying, and doesn't change much if another faucet in the RV is turned on while showering.   I paid $29.99 for the regulator.   This regulator has an inlet and outlet with female 3/4" pipe threads, so I also had to buy adapters from pipe thread to garden hose threads.  The store, of course, was out of one of the adapters I needed and I was impatient, so I improvised.  I bought a pair of adapters with male 3/4" pipe thread on one end and a 5/8" hose nipple on the other end.  I then went to the garden supply section and bought one of those little "hose savers" (standard hose fitting on each end and a stiff steel spring around 6" of 5/8" dia. hose).  I cut it in half, threw the spring away, and clamped the cut hose ends on the nipples, so my regulator now has a hose fitting on both inlet and outlet - an ugly but rugged and functional system.  (Be careful not to hook it up backwards. There's an arrow on the side of the regulator showing flow direction). 

I should note that when you buy an adjustable regulator, you must also buy some sort of pressure gauge for use while initially adjusting it.   A pressure gauge is also handy for diagnosing problems - it will quickly tell you whether a pressure problem is due to your equipment (typically, clogged filters) or to low pressure or low flow rate in the campground's pipes.  

Here's a photo of a typical set-up I'd use when checking the output pressure of my regulator.   Rather than buy a separate gauge and adapters to hook it up, I use the gauge on  the little "IAPMO" regulator (with the regulator disabled by setting its pressure to something higher than I'll ever use). The outlet from this "test gauge" is capped (in the photo, the cap is a little hose shut-off valve, but simple hose-end caps are also available.  The parts in this photo are, starting at the right:  garden hose from campground water supply; Wilkins pressure regulator with improvised garden hose adapters on each end;  a 45º elbow (to reduce stress on the hose); a  "Y" fitting with shutoff valves on each output leg, to allow hooking up an outside hose as well as the hose to the RV inlet;  on one leg of the Y is the hose to the RV inlet; on the other Y leg is the pressure gauge, used while initially setting the pressure on this adjustable regulator. Generally, for routine use instead of testing, I'll put the "Y" on the upstream end, before the regulator, so as to have higher pressure water available on the "outdoor" hose for car washing, etc.  Note the flow direction arrows cast into the brass of the regulator.   If you hook it up backwards, it won't work.

If you are starting from scratch, buying a big regulator is simpler and probably cheaper than buying the parts for the manifold (and buying two or more of the small regulators).  It also has the advantage of being adjustable to set the pressure where you want it.   If you already own a couple of the small cheap regulators, making one of the several different manifold designs that have been previously described on RV-Talk is the most economical solution for getting more shower flow.  

I got curious and measured the flow rates with the old and new regulators, both at the shower head, and directly from the regulator outlet outside the trailer.   I tested the new home-style regulator and three different small RV regulators:  

1:  The common cylindrical brass RV regulator, originally by Marshall Brass but now imitated by others.  The one I have is quite old, probably a real Marshall Brass product, although not labeled. Non-adjustable.  These can be bought with a pressure gauge on the outlet side - this one doesn't happen to have a gauge.  Probably pre-set at about 45 psi output pressure.

2:  Another externally similar cylindrical regulator, newer, and with a noticeably smaller interior diameter.  This one carries a "Made for Coast Distribution System" label, but no other manufacturer indication.  It has a gauge, which shows 49 psi.

 3:  A small adjustable regulator which has a cylindrical body mounted perpendicular to inlet and outlet axis.  It has a manufacturer's logo  - the word "IAPMO" surrounded by a shield.  It has a gauge on the outlet, and I had it set for 58 psi.  These are most commonly available in the garden stores, among the accessories for drip irrigation systems.  

4:  The new 3/4" home style regulator from Home Depot.  The word "Wilkins" is cast into the brass housing.

Here's the results of the flow tests:

I found these results somewhat surprising.   The big home-style regulator is only about 30% better than the "standard" Marshall cylindrical regulator.  The smaller inside diameter of the newer and cheaper "Coast-to-Coast" regulator causes only about a 12% flow reduction compared to the older one.  The small adjustable regulator has a very poor flow rate.

At the shower head, the flow rate improvement (from the best "Marshall" RV regulator to the home-type regulator) is  29%.  The subjective forcefulness of the spray seems like a considerably bigger difference.   The trick of running the internal water pump to increase the shower flow no longer works with the new regulator.  With the shower running, the pressure in the pipes inside the RV remains high enough so that the water pump won't run.  Even turning on another faucet in addition to the shower doesn't drop the pressure enough to turn on the pump.  

Note that these measurements were made in an RV park with a very good water system - large pipes and 80 psi pressure. In an old RV park with small or clogged pipes, the flow rates would be less and the differences among the various regulators would probably disappear.   

In these tests, I used 25' of  5/8" hose between the campground water outlet and the regulator.  

It appears that building a manifold to allow using two of the "Marshall Brass" regulators in parallel (doubling the flow rate) would actually perform considerably *better* than a single 3/4" home-style regulator.  If you already have the old regulators (and they can often be found in RV flea markets for almost nothing), then buying a few pipe fittings and building a manifold  is a sensible thing to do, both from a cost and a performance standpoint.  

all rights reserved; permission granted to copy, only for personal non-commercial use, the complete unmodified document including this notice;    copyright: 2002 by David E. Damouth <damouth@attbi.com>