Throughout my tutorials and articles on irrigation system design I recommend against the use of double check type backflow preventers on irrigation systems connected to potable water supplies (drinking water). This article is an explanation of why.
A double check backflow preventer is simply two spring-loaded check valves in a row, with a shut-off valve on either end and test cocks to allow the unit to be tested for proper operation. The double check backflow preventer is the only true backflow preventer which does not have a vent to allow air to enter the lines or to allow water to escape when backflow occurs. It relies entirely on the tight seal of the two check valves to prevent backflow. While it may come as a surprise, the experts who regulate backflow preventers consider the lack of a vent to be a major problem, and thus they consider double check backflow preventers as appropriate only for low-hazard situations. More on this below.
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So why can’t a double check backflow preventer be used with water containing hazardous substances? The answer is really pretty simple. Two check valves sounds like good protection, after all if one fails there is a backup, right? The problem is found in the cause of the check valve failure. They almost always fail because something gets stuck in them (sand, twigs, insects, clams) which prevents them from closing. Unfortunately, where there is one of those contaminants, there are often many! Thus while the chances of one check valve failing may be fairly low, the chance that the other check valve will fail at exactly the same time is very high. That is the reason why double checks can’t be used when hazardous substances may be present in the water.
Irrigation water is considered by many experts to be hazardous, this is especially true of the agencies that regulate health and safety laws. The reasoning is that dangerous chemicals (fertilizers, pesticides) as well as soil borne contaminates (animal excretions, decomposition) are found in irrigated areas and these substances may be drawn back into the irrigation water through the sprinklers or drip emitters when backflow occurs. This is especially likely if water ponds on the ground around a sprinkler head or emitter.
Pro Double Check Backflow Preventer Argument:
The double check backflow preventer is less susceptible to freezing damage because in most cases it can be installed below ground or inside a basement. This makes winterization of the irrigation system much easier. Supporters of double check backflow preventers also argue that irrigation water is not a hazard to health, it is merely objectionable (tastes bad, looks bad, or smells bad, but won’t make you sick). This point is essential, because pretty much all authorities agree that double check backflow preventers are not appropriate for use when health hazard substances are present in the water. The final argument often used in favor of double check backflow preventers is their lower cost to purchase.
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Anti Double Check Backflow Preventer Argument:
The con argument is really simple, they claim that irrigation water is hazardous to your health, and thus a double check backflow preventer is not suitable. The argument goes something like this– whenever an irrigation cycle is completed, gravity causes at least some of the water in the pipes to run out onto the ground through the lowest sprinkler head or drip emitter (even if they are only an inch lower). When the water runs out a vacuum is created in the pipe. This vacuum sucks stuff from the ground into the pipe through the higher sprinkler heads or emitters. That includes stagnant water, dirt, fertilizer, bacteria, animal waste, and anything else present at ground level. All of those can be hazardous to your health. Once in the pipe they can then flow back into the water supply. So irrigation water is a health hazard.
What the organizations involved in the backflow industry say about double checks:
The AWWA (American Water Works Association) is an organization consisting of people interested in issues related to water. Most members are people who work for public or private water companies or who work for companies that make water related products (like the backflow preventer manufacturers). The AWWA publishes consensus standards which are created by committees made up of various Association members who debate and attempt to reach a consensus on what should be in the standard. To put it another way, these standards are based on the collective opinions of the committee members rather than on research or other factual data. This is not bad, but it is important to recognize that these standards are the result of a political, rather than scientific, process. The position of the AWWA on double checks is stated in standard “C510-92 Double Check Valve Backflow Prevention Assembly”. This standard says that double checks may be used for sprinkler irrigation systems, provided no chemicals are injected into the sprinkler water.
The Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California creates a set of standards which it publishes as the “Manual of Cross-Connection Control”. The Foundation also tests various makes and models of backflow preventers for compliance with the USC standards and gives approvals to those that meet the standards. The foundation is an independent organization that bases it’s decisions and standards on research data. The backflow preventers that pass the Foundation’s minimum standards and tests are listed in the official “List of Approved Backflow Prevention Assemblies”. USC rates undesirable water into 3 classifications; Pollutants (non-health hazard), Contaminants (minor health hazard), and Lethal Hazard (sewage or radiation present). The position of the Foundation on double checks is that “The DC may be used to protect against a pollutant only”. So is irrigation water a pollutant or contaminant? Back to that same old question…
I do not recommend the use of double check backflow preventers on irrigation systems. I do not use double check backflow preventers on my projects except in very rare cases where I am either required to, or special conditions exist such as sprinklers mounted very high above the ground where contaminates can’t reach them. There is a very simple reason for this. I am a registered Landscape Architect, and as such I am required above all else to “protect the health, safety, and welfare of the public”. Thus I am held to a higher standard than a homeowner or other non-licensed person and my advise must reflect that. So if you write and ask me if I would recommend using a double check, even if you think you have a special circumstance that eliminates almost ALL of the risk, I am still likely to tell you no. Because that is what I have to tell you!
So, should YOU use a double check backflow preventer? Well, that is, of course, up to you. If you have a local regulating agency who says you MUST use a double check backflow preventer then you don’t have much choice. The good news is that I haven’t heard of any cases of anyone killed or disabled as a result of backflow from an irrigation system, as long as the irrigation system had some type of backflow preventer on it.
A final note:
I’ve taken a lot of heat from people over my position on Double Check Backflow Preventers. I’m rather tired of being flamed for it, it’s my opinion and that’s all. If you have a correction or addition to the arguments above I would love to hear it, and in most cases I will incorporate it into the arguments. If you simply disagree with my recommendation, that is fine, I don’t mind at all, but I don’t want to hear about it. I’ve tried to give my readers as fair as possible a view of both sides of the issue so that they can make a decision. I hope that I have accomplished that.