Answers to the Questions You’ve Been Asking

Question: I’m confused about the backflow protection requirements for fire sprinkler systems. What type of device do I have to install?

Answer: You’re not the only one confused. If there is one area that plumbing codes, as well as building and fire codes disagree on, it is backflow protection for fire sprinkler systems. The required protection ranges from a reduced pressure principle backflow preventer to a double check valve assembly to a single check with a flow alarm. Unfortunately, the difference in the codes normally depends on the political strength of the different groups fighting over the code, not the technical reasoning.

It has been well documented that sprinkler systems piped in black steel pipe do not contain potable water. The entire concept of backflow protection is to protect the potable water supply against contamination from a non-potable supply. Hence, it would only seem logical to have a backflow preventer isolating the potable supply from the fire sprinkler system. I personally believe that a backflow preventer is necessary.

While a reduced pressure principle backflow preventer provides the highest level of backflow protection, it has one major drawback for a fire sprinkler system. When there is a pressure differential between the supply inlet and the downstream pressure that exceeds 2 psi, the valve opens the intermediate chamber and dumps water. Well, in a sprinkler system this can occur several times during the day. Since there is no flow in the sprinkler system, any time the inlet pressure lowers by 2 psi, the valve spits water. (The valve is doing its job.)

Because of this problem, double check valve assemblies were specifically developed for the fire sprinkler industry. The double check valve assembly is not as high a level of protection, however, it provides adequate protection against backflow since the fire sprinkler system is stagnant. If the double check valve assembly is inspected and tested annually, there should be no problem with the valve serving as the backflow preventer. The only exception (in my mind) is when chemicals are added to the sprinkler system. Then, an RP should be installed.

If you install a reduced pressure principle backflow preventer, then I would advise you take the appropriate steps to prevent the valve from constantly dumping water. What I do is install a single check valve ahead of the reduced pressure principle backflow preventer. The single check can help to fake out the backflow preventer to not realize that there has been a pressure drop in the supply piping. Many plumbers are shocked at how effective the installation of the single check is.

Every commercial sprinkler system is required by NFPA to have a check valve and flow alarm. However, these devices, in series, should never be considered as backflow protection. This installation is like playing Russian roulette with your water supply system. Like those who oppose backflow protection on sprinkler systems, you may get lucky and nothing will ever happen. Then again, you may be unlucky.

The bottom line is to check with the local jurisdiction regarding requirements for backflow protection to a fire sprinkler system. Many codes have changed in recent years adding requirements for backflow protection.

Question: If an existing fire sprinkler system only has a single check valve with a water flow alarm should I install a backflow preventer?

Answer: Not necessarily. I know how, sometimes, I can spout off about backflow in the first question and then renege on protection for existing systems. Sprinkler systems are calculated based on pressure loss. The systems have a history of being designed very tightly. Hence, any additional loss of pressure may cause the system to be out of specification.

Before any backflow preventer is installed on an existing system, an engineer must run the calculations to see if the valve will maintain the pressure requirements. If the valve results in the pressure being inadequate for a sprinkler system, then modifications to the system must be made before a backflow preventer is installed. You certainly don’t need a fire in the building with the sprinkler system failing because of a backflow preventer. Lawyers would have a field day with your company.

Question: I thought I knew how to calculate the pressure in a water piping system but every time I check the pressure against my calculations, I’m off by quite a few psi. What did I do wrong?

Answer: Probably nothing. If the pressure you measure is the same as the pressure you calculated, then you probably did something wrong. Every water pipe sizing method has had engineering input. Speaking as an engineer, all engineers are wimps. As soon as they figure out the right way to do something, they add a fudge factor. Of course, they call it a factor of safety. That sounds better than what it really is — a fudge factor.

The pipe is assigned a roughness coefficient. Every roughness coefficient is too conservative. The pipe is smoother, hence, there would be less pressure loss. The system is designed for peak demand. That means a huge amount of fixtures operating at the same time. That usually never occurs in the life of the system.

Many sizing methods have you deduct 0.5 psi for every foot you raise the water in height. For example, in a three–story building with the water piped 30 feet, you deduct 15 psi. In reality, the pressure loss is only 0.433, not 0.5. It is just easier to use one half instead of 0.433. Thus, for that 30 feet in height, the pressure loss would be 12.99 or 13 psi, not 15 psi.

Another difference is the flow rate. You may calculate a flow rate of 2.5 gpm out of the shower head. If there is a flow restrictor, the flow rate may only be 2 gpm. That changes the velocity, pressure loss, etc. So don’t get too excited if the pressure is not the same as what you calculated.

Question: Will I get thrown in jail if I install a 3.5 gpf water closet that I have in my shop?

Answer: No. The federal law only prevents manufacturers (and others) from making, or importing, water closets that flush more than 1.6 gpf. If you have a leftover, you are still permitted to use the fixture until you run out of them. However, many plumbing codes have long since required 1.6 gpf water closets. So, while you are not violating federal law, you may be violating the local plumbing code.

If you happen to see a 3.5 gpf water closet in a supply house or do-it-yourself place, I would check the date stamp very carefully. That water closet better have been made last year or earlier. If not, you are purchasing an illegal plumbing fixture, according to the feds. It is better not to mess around with black market water closets. I would recommend that you find a quality 1.6 flusher. There are many out there.

Question: Can a dishwasher connect directly into a garbage disposal unit? They all have the connection, but I’ve often been told that I can’t use it.

Answer: There is no technical reason for not connecting a dishwasher to a garbage disposer. That is how I hooked up the dishwasher in my house. The problem is that many plumbing codes prohibit this installation.

The codes that prohibit the dishwasher from connecting to the garbage disposer will give you all sorts of reasons for prohibiting the practice. None of the reasons are valid. If there were a problem with the installation, I can assure you that the manufacturers would not provide the knock out in the garbage disposer.

If your plumbing code prohibits the installation, get the code changed.

Question: I have one home where there is a tremendous bang when the sump pump turns on. The owner put in a super-sized sump pump but I can’t find anything wrong with the installation. What’s happening?

Answer: Water hammer! What everybody forgets is that water hammer (or hydraulic shock) occurs whenever there is an instantaneous change in velocity. The flow does not have to shut off completely for water hammer to occur.

If you check the installation, you probably will find a fitting that impedes the flow of water. With the super-sized pump, the water can be discharging at a rate of up to 10–12 feet per second. That’s pretty fast. When the flow hits a non-directional fitting, it slows the flow instantly to about 2–3 feet per second. The result is a hydraulic shock or water hammer.

Of course, you wouldn’t put a water hammer arrestor on a sump pump discharge line. But, there are ways to eliminate the hydraulic shock. Either slow the velocity of flow down, or prevent the flow velocity from being changed instantly. You can slow the velocity of flow by either increasing the size of the discharge pipe or by reducing the size of the sump pump. You can prevent instantaneous change in velocity by installing directional fittings at all intersections. A wye is better than a tee fitting for a sump discharge pipe.