RPZ Valves, Check Valves and Cross Connections: the Mechanical Half of Backflow

A facilities manager rings: the backflow valve on the incoming main has been dripping from a port on its underside for a week, and someone has suggested capping it. Do not. That port is the valve doing its job.

The short answer

Where an air gap cannot be arranged, a mechanical backflow prevention device puts a physical barrier in the pipe. They wear, they can be fitted backwards, they need access and testing, and none of them reaches fluid category 5.

Read the device table as a ladder. One check valve is category 2. Two in series is 3. The RPZ with its relief valve is 4. And anything with an atmospheric vent is X for back pressure, like the tap gaps.

Verifiable means the device has test points so faults can be diagnosed in place: one on a type EA, two on a type EC, three on an RPZ.

The rules that apply to all of them

From the guidance to Schedule 2 paragraph 15, echoed by BS 8000-15 clause 3.7.6:

• Readily accessible for inspection, maintenance and renewal.
• Not outside the premises, except types HA and HUK1, which are made for outside taps.
• Not buried, and if vented, verifiable or fitted with a relief outlet, not in chambers below ground or anywhere liable to flooding. A flooded chamber puts contaminated water at the vent.
• For category 4 devices, a line strainer immediately upstream, with servicing valves upstream of the strainer and downstream of the device.
• For an RPZ or similar, a relief outlet ending in a type AA air gap not less than 300 mm above floor level.
• Check valves closing sharply set up hydraulic impulses; where water hammer is a problem, fit shock arrestors.

So a double check valve in a chamber under a path is the right device in the wrong place. It fails the Regulations before it fails the pipe.

Type DC: no moving parts, category 5 one way

A pressure flushing valve on a WC or urinal has no cistern, so no interposed air gap — and a urinal is category 5. The type DC pipe interrupter with permanent atmospheric vent is a brass fitting with no moving parts and air apertures that are permanently open. In normal flow water passes straight through; if back siphonage starts, air is drawn in through the apertures and the siphon breaks before any water climbs back.

Being open to air, it gives no back pressure protection. For back siphonage it is rated to category 5.

Schedule 2 paragraph 25(1)(c) requires the interrupter not less than 300 mm above the spill-over level of the pan or urinal. (Older guidance gave 150 mm above a urinal sparge outlet; if the question wants the Regulation figure, give 300.) The conditions follow from how it works: fitted vertically, discharging downwards; no tap, valve or flow restrictor downstream, because a closed valve after it would put pressure behind it; no reduction in pipe size downstream; and accessible for replacement.

The same paragraph limits the flushing valve itself: it may not be fitted in a house, and not anywhere that 1.2 litres per second cannot be achieved at the appliance, because a flushing valve needs that flow to clear the pan.

The RPZ valve, and what the drip means

Type BA is the highest rated mechanical device, protecting to category 4 both ways. Inside are two check valves, a differential relief valve between them, and three test points.

The first check valve is spring loaded to drop the pressure, so the middle chamber is a reduced pressure zone held below supply pressure, and supply pressure holds the relief valve shut. If supply pressure falls to within about 0.14 bar of the zone pressure, or a check valve leaks and the zone pressure rises, the relief valve opens and dumps the zone to drain — so contaminated water can never reach the supply side.

That is the facilities manager's drip. A steady discharge from the relief port means the first check valve is leaking. The valve needs servicing, not a cap. (A gradual loss of flow downstream over months is usually a blocked line strainer instead.)

An RPZ is made to BS EN 12729. It needs a line strainer immediately upstream and servicing valves either side. The relief port discharges through a tundish to a type AA air gap at least 300 mm above the floor, in vertical pipework. It is never below ground and never bypassed, even briefly. It is notifiable (Regulation 5, item 4(g)) and must be commissioned and then tested by a WRAS-approved tester; BS EN 806-5 Table A.1 has it inspected every 6 months and maintained once a year.

Its uses are whole site and zone protection, category 4 processes, and permanent connections to large heating systems. It is not acceptable for category 5: rainwater needs an air gap, however good the valve.

Single and double check valves

A shower runs hot for a second when the kitchen cold tap is opened. Hot water has crossed into the cold supply through a mixer, because nothing stopped it. The cure is the simplest device there is.

A single check valve is a spring-loaded one-way valve, rated 2 and 2. Type EA has a test nipple and is verifiable; type EB is identical without one. Made of DZR brass to BS 6282-1 or BS EN 13959, suitable to 90 °C. Its uses: both feeds to a single-outlet mixer tap or shower valve, where hot (category 2) could cross into cold; the check valve in the inlet group of an unvented cylinder; the outlet of each pump in a boosted set; rarely used branches; and the mains back-up branch to a rainwater cistern where a dead leg is unavoidable.

Two cautions: on a hot supply, expansion against a closed check valve can overpressure it — and BS EN 806-5 has EB valves replaced every 10 years.

A double check valve is two single check valves in series in one body, rated 3 and 3. Type EC has two test points, type ED none. It is the domestic workhorse, because most of what a house can send back is category 3: a new hose union bib tap with the valve inside the building; a filling loop to a sealed heating system, on the supply side only; submerged inlets to baths and basins; bath and shower mixers with hoses.

In one line: hot crossing into cold, or expansion pushing back, is category 2 and takes a single check valve. Anything a hose, a bath or a heating circuit could send back is category 3 and takes a double. A whole site or a category 4 process takes the RPZ.

Filling loops and outside taps

Schedule 2 paragraph 24 forbids a permanent heating connection without an approved backflow device. House heating water is category 3, so a temporary filling connection through a type EC or ED double check valve is accepted in a house; the withdrawn BS 6700 (clause 5.3.5.5) also allowed it on a non-domestic installation under 40 kW. The valve goes on the supply side of the loop, never the heating side, and the flexible loop is disconnected after use.

Where the connection is to stay, the word "permanent" changes the device:

• Temporary loop, house or under 40 kW — category 3 — EC/ED, removed after use.
• Permanent fill, category 3 system — type CA disconnector. A small RPZ without test points: two check valves with a disconnection chamber that opens to atmosphere when upstream pressure is lost, discharging via a tundish. Zone pressures differ by not more than 10%.
• Permanent fill, commercial system with inhibitor — category 4 — type BA RPZ valve.

Note why: heating primaries in a house are category 3; outside a house they are category 4, and a large system dosed with inhibitor is category 4 whoever owns it.

Outside taps. A new hose union bib tap in a house garden is category 3 and gets a double check valve inside the building, protected from frost. For an existing unprotected tap there are two devices — the only mechanical devices allowed outside:

• Type HA, the hose union backflow preventer, screws onto the tap outlet. Rated 2 and 3. Permitted only, fitted continuously, on an existing hose union tap in a house with no other protection.
• Type HUK1, a replacement tap with two single check valves built in and a test point. Rated 3 and 3. Permitted only as a replacement for an existing unprotected tap.

And a porous or permeable irrigation hose lying in soil is category 4, beyond a double check valve. It takes a type DB pipe interrupter with atmospheric vent and moving element, in addition to the double check valve on the tap.

Cross connections: the offence is the connection

A plumber runs a pipe from the supply to a rainwater tank, fits a gate valve, leaves it shut, and tells the client to open it only in a drought. Nothing has flowed. It is already an offence.

Schedule 2 paragraph 14(2): no supply pipe, distributing pipe or pump delivery pipe drawing from them shall convey, or be connected so that it can convey, non-wholesome water. Being connected is enough. The closed valve counts. So does a hose from an outside tap dropped into the tank for an afternoon.

A later amendment added "unless a device for preventing backflow is installed in accordance with paragraph 15" — which relaxes the ban only as far as paragraph 15 allows, and paragraph 15 says the device must suit the category. Rainwater and greywater are category 5, so the only qualifying devices are the type AA, AB and AD air gaps. No RPZ qualifies. No double check valve qualifies. And treatment does not change the fluid category — filtered, UV-treated rainwater is still category 5.

So the systems are designed apart. A rainwater or greywater system has its own collection pipework, storage, pump and dedicated distributing pipework, and nothing else. The mains back-up enters the cistern through a type AA or AB air gap at or upstream of the point where the two waters meet (BS 8515 clause 4.7.2). The water meets; the pipes never join.

Identification, and the test that proves it

Paragraph 14(1) requires non-wholesome fittings to be clearly identified so as to be easily distinguished from any supply or distributing pipe. The test is comparative: not "is this pipe labelled?" but "could someone standing here mistake it for the wholesome supply?" A green band on a green pipe fails that test.

Colour banding is to BS 1710: the basic colour for water is green, with code bands between the green saying which water — drinking water is green, auxiliary blue, green; reclaimed water is green, black, green. BS 8525-1 says greywater pipework, including buried back-up pipes, must not be blue, because blue is the potable colour underground. BS 8515 Annex C wants permanent labels at intervals of not more than 0.5 m, at least 100 mm long, green, with RAINWATER in black letters at least 5 mm high. Taps on non-wholesome water are labelled "Not drinking water" in all premises except a house.

And before handover, BS 8525-1 clause 5.5 requires an air test of the wastewater connections and a dye test of the greywater distribution pipework to prove no cross connection with any potable supply. The dye goes into the non-wholesome system before the final connection to the mains back-up, and every wholesome outlet in the building is run and checked clear. If any tap runs coloured, the job stops until it is found.

📝 10-Question Self-Test

Straight from the Level 3 course question bank. Click an option to see whether you got it right — the explanation appears instantly, and there is nothing to submit.

Your score: 0 / 10

Question 1 of 10

Look at the backflow prevention device pictured. In which situation is it most likely to be installed?

AOn the hot and cold supplies to a domestic bath and shower mixer BAt a garden tap on a private house COn the inlet to a WC flushing cistern DTo protect the supply when filling a commercial heating system

Question 2 of 10

A worn hose union bib tap is being replaced. Which mechanical backflow prevention device is the right one to use?

AEB. BHUK1. CLA. DAUK3.

Question 3 of 10

Where would you look for the performance specification of an RPZ valve?

ABS 1212 (float-operated valves) BThe Approved Document G tables CThe manufacturer's technical data, to BS EN 12729 DBS 1710 (identification of pipelines and services)

Question 4 of 10

Look at the image. Where would this backflow prevention device be used?

AOn the supply to a domestic washing machine BOn the inlet to a close-coupled WC flushing cistern in a house CAt an over-the-rim bidet DTo make a mains connection into a recycled water system

Question 5 of 10

Pipework carrying non-wholesome water must not be connected to pipework carrying wholesome water. In which one of the following cases might such a connection be permitted?

AA suitable backflow device has been fitted BThe pipework is clearly labelled CA servicing valve is fitted at the connection DA strainer is fitted upstream

Question 6 of 10

An RPZ valve will not reach the performance figures given in the manufacturer's installation instructions. What should be done?

AContact the manufacturer and discuss how the problem can be resolved. BFit a second RPZ valve in series with the first to make up the shortfall. CAdjust the relief valve spring until the quoted figures are reached. DReplace it with a double check valve of the same nominal size.

Question 7 of 10

Look at the image below. In which situation would this device be used for backflow prevention?

AOn the supply to a garden tap BAt the point of entry of the mains supply to a hospital building COn the outlet of a break cistern DTo prevent back pressure in an unvented hot water system

Question 8 of 10

Which tap type needs single check valves on its supplies because of the fluid category 2 cross-connection risk inside it?

ASingle-outlet mixer tap BBi-flow mixer tap CPillar tap DBib tap on an outside wall

Question 9 of 10

Supply pipes carrying wholesome water carry which colour banding?

ABlue -- Green - Blue BGreen -- Blue - Green CWhite -- Black - White DYellow -- Blue - Yellow

Question 10 of 10

What connections to a pond, pool or fountain are permitted?

AA permanent connection made through a double check valve assembly BA permanent connection made through an RPZ valve tested annually CAny connection at all, provided the pond or pool has been lined DNo permanent or direct connection; a temporary one must meet paragraph 15