Reduced Pressure
Principle Assembly.

Construction

A Reduced Pressure Principle Assembly contains, in series along its main flow path: an inlet shutoff valve (SOV-1), a first check valve (CK-1), a differential pressure relief valve (RV) opening to atmosphere, a second check valve (CK-2), and an outlet shutoff valve (SOV-2). Four test cocks (TC-1 through TC-4) are drilled into the body at locations that let a tester read the pressure at the inlet, at the upstream side of CK-2, at the relief vent, and at the downstream side of CK-2. The intermediate zone between CK-1 and CK-2 is mechanically biased to sit at least 2.0 psi below the inlet pressure; when supply pressure drops or downstream pressure rises, the relief vent opens and dumps the contents of that zone to atmosphere through an air-gap drain. The assembly is the only device the state allows on a high-hazard cross-connection because it is the only family that cannot be defeated by a single check failure — the relief vent is the third line of defense, and it is loud when it works.¹

Test sequence (ASSE 5110, §5.4.3)

The ASSE 5110 standard prescribes a specific five-valve sequence for annual testing of an RP. We perform it in this order, on every test, on every assembly, with a Mid-West 845-5 differential gauge bearing a NIST-traceable calibration sticker dated within the last twelve months. The full sequence appears below; the public-facing version is in the test report we file the same day.

Step	Action	Read	Acceptance
1	Close SOV-2. Bleed gauge to TC#2, TC#3.	p2 − p3 (relief vent Δp)	≥ 2.0 psid
2	Open TC#4. Read CK-2 differential.	p2 − p4	≥ 1.0 psid, no leak
3	Close TC#4. Open TC#3, read.	p3 (relief opens)	opens at ≥ 2.0 psid
4	Verify CK-1 holds against simulated back-pressure.	p1 − p2	≥ 1.0 psid, tight
5	Reopen SOV-2. Verify no continuous discharge.	visual	no leak after stabilization

Failure modes

The seven failure modes we record, in order of how often we see them on a typical Milwaukee account (data from our 2025 books, n = 2,950 tests):

• CK-2 fouled · 28%

  Sediment, biofilm, or a flake of solder fouls the second check seat. Common on small irrigation or kitchen RPs that sit without flow for weeks. Rebuild kit usually resolves.

• Relief vent continuous discharge · 21%

  The vent opens and stays open. Diaphragm pinhole or seat damage. Customer notices a puddle under the assembly. Rebuild diaphragm and seat.

• CK-1 weak · 18%

  First check holds less than 1.0 psid. Spring fatigue, scaling, or thermal stress. Rebuild typically fixes; replacement if the body itself shows pitting.

• SOV-2 won't fully close · 11%

  A ball valve seat is worn, usually from being used as a throttle. Doesn't fail the test directly but prevents step 1.

• TC#3 plugged · 9%

  Calcium deposit in the third test cock. Clean, lubricate, occasionally replace.

• Body crack at brass-to-bronze joint · 7%

  Freeze damage. The whole assembly is condemned. Almost always a non-heated mechanical room.

• Wrong assembly for hazard · 6%

  A DC found protecting a high-hazard cross-connection. Cited and replaced with an RP per SPS 382.41(11).

USC FCCCHR-approved models we stock

Mfr / Model	Sizes	USC list #	Rebuild kit in cabinet
Watts 909	1/2″ – 2″	09-115	Yes — CK-1, CK-2, RV diaphragm
Watts 957	2.5″ – 10″	09-118	Yes — large-body kit
Wilkins 375	3/4″ – 2″	09-090	Yes
Wilkins 375A	2.5″ – 10″	09-091	Partial — special-order body kits
Apollo 4ALF-500	1/2″ – 2″	09-128	Yes
Conbraco 40-200	3/4″ – 2″	09-076	Yes
Febco 825Y / 825YD	3/4″ – 2″	09-103	Yes
Febco 860	2.5″ – 10″	09-104	Partial — special-order body kits

Install rules per SPS 382.41(11)

Wisconsin's plumbing code sets installation rules that override manufacturer recommendations when they conflict. The ones we cite most often:

• Above grade

  RPs may not be installed in pits or vaults. Their relief discharge must be visible and drain to a floor sink, hub drain, or air-gap drain — not to a sealed line.

• Minimum 12 inches above the flood-rim

  The bottom of the assembly must be at least 12″ above the highest expected flood plane in the room.

• Clearance for testing

  12″ minimum clearance below; 6″ on each side. We will not test an assembly that's been buried behind shelving — we will move the shelving or charge to.

• Freeze protection

  In an unconditioned mechanical room, a heat tape and insulation jacket are required. In Milwaukee, "unconditioned" includes most parking-garage utility rooms.

• Identification tag

  A metal or laminated tag with the assembly's serial number, install date, last test date, and protected hazard. We supply ours engraved on aluminum on installs and replacements.

Typical Milwaukee premises we install RPs on

The list below is the top ten premises types where we install or maintain an RP, in 2025 order. Numbers in parentheses are the share of our 2025 books:

1. Restaurants and bars with beverage carbonators (24%)
2. Dental clinics with vacuum (12%)
3. K-12 chemistry classrooms (8%)
4. Medical / urgent-care offices with autoclave (8%)
5. Light industrial cooling-loop with glycol (7%)
6. Veterinary clinics (5%)
7. Commercial irrigation with fertigation (5%)
8. Car washes (4%)
9. Breweries (4%, including Lakefront's mash-tun fill line)
10. Boiler feed at high-rise multifamily with glycol (3%)

Rebuild intervals

USC FCCCHR doesn't prescribe a calendar — they prescribe condition. Our internal calendar, refined from 11 years of Milwaukee water chemistry:

Service	Typical rebuild interval	Notes
Indoor mechanical room, treated MWW water	7–10 years	Most kitchen and dental installs
Foundry cooling-loop with glycol	3–5 years	Glycol attacks EPDM diaphragms
Outdoor irrigation, freeze-cycled	2–4 years	Thermal cycling fatigues springs
Carwash reclaim-system isolation	2–3 years	Particulate-heavy
Dental vacuum isolation	4–6 years	Amalgam particulate, mercury alloy

References