Watts 9DM3 1/2 1/2 In Bronze Dual Check Valve Backflow Preventer with Intermediate Atmospheric Vent, Female Union Inlet and Outlet Connections

The 9DM3 1/2 is rated for non-health-hazard protection only, and it guards against both backpressure and backsiphonage. That covers typical residential and light commercial boiler makeup water where the fluid is a non-health hazard. If your local code or the fluid category calls it a health hazard (chemically treated boiler water, glycol, anything toxic), a dual check with atmospheric vent is the wrong device and you need a reduced pressure zone assembly instead. Check what hazard class your inspector assigns the connection before you buy.

Both ends are 1/2 inch (15 mm) NPT female threaded, so it drops into 1/2 inch threaded pipe. The inlet is a union female threaded connection and the outlet is a union joint, which means you can break the valve out of the line later without cutting pipe or unthreading the whole run. That union feature is the reason plumbers reach for the M3 version when servicing is expected.

No. The 9DM3 1/2 has a brass body that is not lead free, so it cannot go on potable water where lead-free legislation applies. Use it on non-potable duty such as boiler and heating system makeup, where lead-free requirements do not reach. For a potable connection, specify the lead free version of the Watts 9D family (the LF9D series) instead.

175 psi (12.1 bar) maximum working pressure, with a fluid temperature range of 33 to 250 degrees F. The 250 degree ceiling is what makes it usable on hydronic boiler makeup lines where the feed can see hot water backfeeding through it. It is also rated for continuous pressure operation, so it can sit under constant line pressure rather than needing intermittent duty.

Yes, plan on them. The 9DM3 1/2 ships with no inlet shutoff valve and no outlet shutoff valve, so you supply your own ball or stop valves on each side if you want to isolate the device for service or testing. The union connections at both ends do let you pull the body out without shutoffs, but you will be draining the line to do it.

That port is the intermediate atmospheric vent, and it is doing its job. The 9D design puts two independent rubber-seated checks in series with a vent between them; if either check fails to seal, the vent opens to atmosphere and discharges rather than letting contaminated water pass back into the supply. Continuous discharge means a check is fouled or worn, not that the valve is defective in principle. Pipe or drain the vent so the discharge is not damaging anything.

A strainer is included on the 9DM3 1/2, so you do not need a separate one ahead of it for basic debris protection. That matters on a boiler fill line, where scale and pipe cuttings are the usual cause of a check fouling open and weeping out the vent.

The 9DM3 1/2 is flagged Buy American Act compliant with USA origin, which is what a government, healthcare, or education procurement office needs on the submittal. Watts part number 61935 is the number to put on the purchase order.

A plain dual check has two checks and nothing between them; if both leak by, contaminated water passes and nobody knows. The 9D adds an intermediate atmospheric vent between the two checks, so a failed check dumps to atmosphere instead of back into the supply, and the leak tells you the device needs service. That is what makes the 9D acceptable for backsiphonage and backpressure protection on non-health-hazard connections like boiler makeup, where a bare dual check is often not accepted by code.

No. The 9DM3 1/2 is not built for hot and harsh water duty, and it has no polymer coating on the brass body for aggressive water conditions. It handles fluid up to 250 degrees F on ordinary service, but if your water is chemically aggressive or the application is classed as harsh, specify a device built for it rather than this one.

Neither. The 9DM3 1/2 is a plain inline device with no bypass line and no meter. If the connection has to stay in service during backflow device maintenance, you need to pipe a bypass yourself, and any metering of the makeup water has to be a separate meter in the run.