Double Check
Valve Assembly.

Construction

A Double Check Valve Assembly is a Reduced Pressure assembly with the relief vent omitted. Two independent spring-loaded check valves sit in series along a single bronze or brass body, each with its own test cock immediately upstream. Inlet and outlet shutoffs frame the assembly. Three test cocks (TC-1, TC-2, TC-3) are accessed during the annual test. The two checks are independent and identical — neither relies on the other for sealing. Because there is no vented intermediate zone, a DC is sealed and may be installed in a vault or below grade where local code allows, though Milwaukee Water Works prefers above-grade installation for serviceability.¹

When a DC is right

Wisconsin's plumbing code allows a DC only where the protected cross-connection has been classified moderate hazard — a nuisance, an aesthetic problem, or a non-toxic chemistry. The protected systems we see most often on DCs in Milwaukee:

• Sealed hydronic heating loop (no glycol)

  A sealed boiler-and-radiator system where the only makeup water is plain potable. Most Milwaukee mid-rise office buildings and brick four-flat hot-water systems.

• Domestic-water booster pump

  A premises with a booster pump where the city's PSI runs low — common in West Allis blocks east of 70th Street. The booster is the moderate hazard.

• Tankless water heater feeds

  Sealed instantaneous heaters with no chemical treatment.

• Fire service without antifreeze, wet pipe

  A sprinkler system with no glycol, no foam, no chemicals — stagnant potable water under pressure. Use a DCDA (detector variant) for the meter requirement.

• Cooling-tower makeup with no chem injection

  Rare but it happens. Most cooling towers are dosed with biocides and are high-hazard; if the dosing is downstream of the assembly and the assembly is on city-supplied makeup, a DC is allowed.

When a DC is wrong

We see four scenarios more than any others where a DC was specified but an RP is required:

1. Boiler dosed with glycol. A DC on a glycol-treated boiler-loop makeup is non-compliant per SPS 382.41(11) table 41 since 2002, but we still find dozens of them on mid-century commercial blocks where the loop chemistry was added after the install.
2. Restaurant carbonator. Soda-fountain carbonators are high-hazard back-pressure scenarios — CO₂ presses citric acid back through the copper supply line. Always RP. We replace 20+ of these per year on restaurant accounts where the prior assembly was a DC.
3. Irrigation with fertigation. Any irrigation system with an injection pump (fertilizer, pesticide, surfactant) is high-hazard. Pre-2005 commercial irrigation in Milwaukee often used a DC; the 2005 SPS revision tightened the rule.
4. Carwash reclaim isolation. Reclaim loops handle hydrocarbon-laden water. Always RP.

Test sequence (ASSE 5110, §5.5)

Step	Action	Read	Acceptance
1	Close SOV-2. Hose gauge to TC#2 / TC#3.	p2 − p3 (CK-2 differential)	≥ 1.0 psid, tight
2	Bleed gauge to TC#1 / TC#2.	p1 − p2 (CK-1 differential)	≥ 1.0 psid, tight
3	Reopen SOV-2. Verify no leak.	visual	no continuous discharge

Failure modes

From our 2025 books, the seven most common failure modes on DCs (n = 884 tests):

• CK-2 weak · 31%

  Downstream check fatigues first; it sees the most thermal cycling.

• CK-1 weak · 23%

  Upstream check; usually a contemporary failure with CK-2 in old installs.

• Both checks weak · 12%

  Time for a rebuild on both ends. We do both even when only one fails.

• TC plugged · 11%

  Calcium or solder flake.

• SOV worn · 9%

  Won't fully close; the assembly cannot be tested.

• Body crack from freeze · 8%

  Replacement.

• Wrong assembly for hazard · 6%

  DC found where RP required.

USC FCCCHR-approved DC models we stock

Mfr / Model	Sizes	USC list #
Watts 007 / 009 / 919	1/2″ – 2″	11-046
Watts 709 / 757	2.5″ – 10″	11-048
Wilkins 350 / 350A	3/4″ – 10″	11-035
Apollo 4A-100	3/4″ – 2″	11-058
Febco 850 / 805Y	3/4″ – 2″	11-061
Conbraco 40-100	1/2″ – 2″	11-022
Hersey FDC	2.5″ – 10″	11-014

Install rules per SPS 382.41(11)

DC-specific items, in addition to the general install rules in the SPS chapter:

• Above-grade preferred but not required.

  Pit-installs are allowed for DC where space and freeze protection are otherwise impossible. Milwaukee Water Works prefers above-grade for ease of annual testing.

• Identification tag with hazard class.

  Tag must say "moderate hazard" on a DC. If the assembly were ever to protect a high-hazard cross-connection, the tag becomes a citation.

• Test clearance.

  6″ around the body. We need access to all three test cocks at once.

References