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Water Heaters – Expansion Tanks

Hot Water Heater Expansion Tanks

Virtually, every modern plumbing code requires the installation of an expansion tank on hot water heater installations. The reason is simple. Water expands when heated. This extra volume of water needs to go somewhere. Before the widespread usage of backflow preventers, check valves and pressure reducing valves, this expanded water simply pushed the cold water back into the city water main. If your house has one of the above mentioned devices, you could have problems. These devices prohibit the flow of water from your house back into the public water system.

Without an expansion tank, the expanding water can cause your hot water heater to possibly fail because of the increased pressure. This pressure can cause serious life threatening problems as well, if you heat your water with natural gas or propane. The water heater tank can collapse around the internal flue and cause carbon monoxide to enter your home. It is serious business.

Expansion tanks are really simple devices. They contain compressed air and a special rubber bladder. When your hot water heater turns on, the water within your piping system begins to expand. This expanding water enters the expansion tank. Eventually, hot water is drawn from the system thru a faucet and the expansion tank releases the extra water into the piping system.

Expansion tanks come in various sizes. The size you need depends upon two very important variables. You need to know the capacity in gallons of your hot water heater and the water pressure of your house piping system. The capacity of your hot water heater is stamped on a label or a plate on the side of your hot water heater.

Water pressures within municipal water systems vary widely. Here in Cincinnati, water main pressures vary from 50 pounds per square inch (PSI) to over 200 PSI within a distance of a mile! This same thing may be true in your city. People with cisterns or wells control their own system pressure thru the use of electric pumps.

It is easy to determine your incoming water pressure. Many plumbing supply houses sell a little gauge that attaches to any faucet which has garden hose threads.  Or if you like, you can call your local water department. They will possibly send a technician to your house. This person has very accurate gauges which will do the same thing.

Once you have this information, visit a local plumbing supply house that sells these tanks. They will be able to provide you with the proper sized tank to suit your needs.

Be sure to follow the directions that are packaged with the expansion tank. It only takes a few moments to read them. This will insure that your tank will function properly.

If you install a pressure reducing valve to control water hammer, be sure to buy one with a bypass feature. Without this, your water heater will begin to malfunction. You will see water dribble out of the pressure/temperature safety valve without a doubt.

The reason lies in the fact that heated water expands. Without a pressure reducing valve, this expanded water can easily go right back outside to the water main. Low quality or malfunctioning pressure reducing valves block this backwards flow of expanding water.

An inexpensive expansion tank installed on the cold water side of your hot water heater will solve this problem. The tank absorbs the expanding water and then releases it once hot water is drawn from the hot water heater. They are simple yet effective devices.

Installing an Expansion Tank

The first thing you need to do is to size the expansion tank according to the size of your hot water heater. The tank manufacturers make this easy. All you have to do is to determine the capacity of your hot water heater. This is always stamped on the side label of the hot water heater. You may have a 50 gallon or 80 gallon or whatever size hot water heater. Take this information to your plumbing supply house to get the right sized expansion tank.

A Simple Tee Fitting

The expansion tank installation requires you to install a simple tee fitting in your cold water supply line. I like to install these on a horizontal run of pipe, not a vertical piece. A vertical piece of pipe can cause undo stress on the connection point between the tank and the supply piping. I always hang my water-heater expansion tank above the horizontal pipe and recommend NEVER to hang it below. However, ALWAYS read the instructions provided by the manufacturer.

The tanks simply have a threaded connection. You will install the opposite type threaded adapter to your tee fitting. Apply Teflon tape or pipe dope to the male threads and screw the tank onto the fitting. To get a tight fit, you may have to use a pipe wrench or a leather belt around the tank. Most tanks come with a place to attach a tightening wrench. Follow the instructions and this can be done in 1/2 hour or less!

Column B192

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17 Responses to Water Heaters – Expansion Tanks

  1. You state one needs to know the capacity of a water heater ( in my case 75 gallons) and the water pressure (in my case 55 pounds per square inch) to calculate the size of pressure tank (2.00 gallons in my case) and the air pressure induced at the top of the pressure tank, I assume. So what air pressure am I to apply to a 2.00 gallon pressure tank to be safe and not sorry, given the parameters shown above?

  2. I have been getting pulsating water periodically. I am thinking my expansion tank may have failed. I have a pressure regulator on the cold side of the line near where the water enters the house. My expansion tank is on the outflow side of the hot water heater and hung under the line - two things I believe were ill advised per the guidance in this blog. Do these tanks fail? I am wondering if mine has failed due to the placement and way it was installed perhaps made it wear out prematurely. Thoughts? Thanks - Ian

  3. Had a new hot water tank installed last summer and since I blow all of the water out of my system for the winter in Vermont, I was suprised that the expansion tank ruptured on the bottom. Kind of a big mess when I turned on the water supply this spring and flooded the downstairs below the ruputured expansion tank. This has never happened before ( more than 25 years of draining my system). I suspect that it was a faulty expansion tank because I was getting water from the pressure relief valve during the weeks following installation.
    Now I'm thinking my old expansion tank was mounted Input down meaning it looked upside down but the new installation is opening up like a bottle. This might be why water remained in the tank and made it rupture ? Finally my question - Should I re- orient the expansion tank with the inlet down or is that not a good idea ? I just paid $55 for this new expansion tank and now I am not getting water from the pressure relief valve as I did with the old bad ruptured tank. Thanks in advance for any help.

      • That's what I figured. I guess the plumber did not realize that I drain my system for the winter and if the house was heated all year there probably would not have been a problem. I will have to flip it over this fall before I drain the system. Thanks for the reply

  4. I need to know the truth about expansion tanks; I recently contacted two plumbers and got two different answers. The problem I am having is I have little black deposits coming from my hot water faucets. I called my insurance company to see if the tank was covered, they in turn sent out a plumber to check out the problem, which he said it was the expansion tank improperly mounted or should I say incorrectly placed. He informed me that the tank should have been placed on the right (cold valve) instead of the left (hot valve). I contacted the builder and he in turn gave me the person who installed the expansion tank. After contacting him and explaining to him the problem he insisted that it was improperly installed and it didn’t’ make a difference which side it went on; and all that was needed was for me to drain the hot water heater and replace the expansion tank and everything would be fine. Now, the plumber that was sent by the insurance company said the hot water heater needs to be replaced installed correctly or the problem would continue. Everything I have read online tells me the tank should have been installed on the cold valve. So which should I believe and what is the best course of action for my problem?

    • Larry, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  5. I think the tank should be pre-pressurized by you to match the input water pressure in your system. Using a manual tire pump is the best way to increase the presure. You should not assume the tank has the correct air pressure from the factory for your home.

  6. Our home has a circulating pump on the water heater with a timer on the pump , works great instant hot water on the first floor to all fixtures baths and kitchen with the loop system. The problem is the second floor bathroom takes at least 1-2 long minutes to get any hot water at all in the shower or sink.
    I noticed the hot water supply for the 2nd floor bath is tee'd off the first floor return loop about 10' from the water heater. Why does it take so long to get hot water? The 2nd floor bathroom is only about 25' from the first floor outside water heater. I also notice there is not a check value on the loop pipe in front of the circulation pump. Please help if you can any suggestions.

  7. My house is 11 yrs. old and I was just told by a plumber that the hot water heater expansion tank should be changed since it is that old. Is that correct? I have not noticed any problems but will be glad to change it if necessary to avoid damage to my water heater or plumbing system.

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