The main thing to take away from this is that both backflow prevention device types perform the same function when they are operating properly, but only the RPZ is designed to protect the public water supply by disposing of any backwards-flowing water if any of the check valves or the relief valve fails.

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The Double-Check Assembly (Figure 1) was developed in the 1950s for the fire industry. Any time the pressure on the property (downstream) side exceeds the pressure on the city (public) side, the two redundant check valves close stopping the backwards water flow. There are two problems with the Double-Check backflow preventer. First, no remedy exists in the event of a malfunction of the valve closures or if debris in the water line causes the valves to not close completely. Second, and this is the big problem, there is no way to know when such a failure has occurred without conducting a full test by a qualified professional.  The Double-Check is a closed system. It has no method of revealing whether the internal check valves are functioning properly and no way to detect the presence of debris that is impeding full closure.

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The T&P valve is designed to open and release pressure inside the tank if the temperature or pressure inside gets too high. If your T&P valve is either always leaking or, if you test it and no hot water comes out, you should replace the valve immediately.

Consider a flow-stop situation, like one that might naturally occur at the end of the day (Figure 4). If you look closely, you can see that a small pebble has lodged in the #2 check valve. Now imagine some sort of back siphon event over night. Perhaps a nearby building fire leaches the water back with the draw of a fire hydrant; or a pump station fails; or a water main breaks. Because the #2 check valve is not closing, all the water that has been delivered to the building will continue to flow out the relief valve until the private lines are cleared. If this is a four story building, that’s a lot of water.

The Reduced Pressure Zone Valve Assembly (Figure 2) consists of two independently operating check valves just like the Double-Check as well as a hydraulically operated differential relief valve located below the first check valve.

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So, let’s make sure we check that T&P valve, shall we? Keep in mind that these instructions are designed to help the do-it-yourself homeowner take care of their own water heater. However, if you read the following instructions and feel like this might be a bigger job than you want to take on, call a plumber you trust to help.

Watch the video below for an example of RPZ flooding and to see just how much water is discharged when a small amount of debris becomes lodged in the device.

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When a flow stop occurs, both check valves close. At that moment, the relief valve opens and evacuates the water between the valves. (Figure 3) Some think that this event defines the limit of how much water can ever flow from the RPZ into a drain. This is not so.

Think about it this way: if the T&P valve is designed to release pressure in the water heater but doesn’t do its job, internal pressure will continue to build and build until the water heater actually explodes. While very rare, water heaters have been known to rocket through the roofs of houses, leaving 50 gallons of boiling hot water and steam in their wakes. What a mess!

This hydraulic valve and its placement makes the RPZ virtually fail-safe but it comes at a cost to the area around the device.

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Complete these 4 steps FIRST before performing ANY maintenance on your water heater to avoid the risk of water damage, scalding burns, electrocution, or explosion:

How do you determine hazard? The simple answer is that the purveyor determines it, but every jurisdiction is different. Every city has its own list of named examples for what constitutes the hazard threshold. Here is an example from Washington DC. They also stipulate that if the anticipated use, as you know it, is not named explicitly, then they reserve the right to make the decision during plans review. It’s also important to understand that you cannot, as a designer, over-protect the incoming water service. This means there is no penalty for providing the higher degree of protection.

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However, there are signs you can look for before your water heater turns into a bomb. In your home, the Pressure Relief Valve, or PRV, is designed to regulate internal pressure in your plumbing. When the T&P valve on your water heater isn’t working, that extra pressure is sent throughout the house. Therefore, all of the pipes and appliances in your house will expand and contract with the pressure, including the water heater tank. An expanding and contracting tank can crack the glass liners inside, which means the tank is now compromised, which can lead to failure. So, you can troubleshoot for a problematic T&P valve if you have leaking fixtures or experience a lot of pressure in your appliances connected to the plumbing system in your home. The good news is that when you turn on a faucet, this relieves system pressure; however, this is also why most water heater failures happen when people are out of town. No one is turning on faucets to relieve tank pressure.

You’ve heard these phrases and acronyms repeatedly. If you are a designer, you feel sure that you are expected to know what they mean. Here’s a quick rundown on what they are and how they differ.

A designer may specify one of 2 types of backflow prevention devices. First, the Double-Check Valve Assembly. This is often shortened to DC and is typically thought of as the appropriate solution for low hazard conditions. Second, the Reduced Pressure Zone Valve Assembly, often shortened to RP or RPZ, is thought of as the appropriate solution for high hazard conditions.

Mineral or salt buildups, rust, corrosion, or mechanical equipment failure can cause T&P valves to malfunction. A failed or malfunctioning T&P valve can lead to quite a mess and in some extreme cases, can lead to catastrophic damage.

Now consider a full failure of the #2 check valve, like one that might occur if the device is knocked out of round or has a mechanical failure. It’s essentially the same event with the exception that due to the larger valve opening, there is actually a higher flood rate. Now you have water flowing through the relief valve at full head pressure. Again, if this is a large or multi-story building, that’s a lot of water very fast.