As we have said many times before, most people don’t think about their water heaters until something goes wrong and then, of course, it is top of mind until the issue is fixed. That also goes for any parts that are connected to the water heater such as expansion bottles. Most people don’t know what these are, much less whether they have one or even need one. In this blog post, we are going to discuss thermal expansion bottles and what exactly it is that they do and to do that we should first briefly explain what thermal expansion is and that will lead to understanding the need for an expansion tank.


Real simply, when water is heated, it expands. When water is heated in a 40 ga. water heater, depending on the starting point temperature, the water can expand up to one-half gallon or even more for a bigger water heater. That’s because when water is heated, its density decreases and its volume expands. Obviously, this is pretty common information that anyone who remembers their basic science will understand. But what many people might not think about is how this might affect their water heater and plumbing system in general.

The effects on your water heater may range from minimal to major depending on the specific situation. The thermal expansion of water in a closed plumbing system (one in which there is a pressure reducing valve as is often the case throughout the San Diego area) can cause the buildup of unusually high pressure in the system, pressure surges and even be responsible for a constantly dripping pressure relief valve on your water heater or even dripping from your faucets in your home. This pressure can then get so bad that it can lead to the collapse of some of the internal parts of your water heater such as the flue, fittings or even the water connections. If the flue collapses, that could lead to a carbon monoxide leak, since the flue is where the gas is vented out of the water heater. In a worse-case scenario, thermal expansion could warp or even rupture your water heater. And this is where the expansion bottle comes into play.


While there are several safety features to help relieve pressure in your system including the pressure relief valve, these usually result in water dripping or even pouring out and that could cause problems. The expansion bottles are designed to hold the hot water and then, when the water is cooled and the pressure in the system has gone down, the water will go back into the water heater. Most expansion bottles have a diaphragm in them and there is pre-charged air inside. The hot water goes into the expansion bottle and there will be room for it because air can be compressed, unlike water. It’s a pretty simple system but can really help avoid major damage to your water heater and plumbing. To help figure out what size expansion bottle you may need, here is a link to the Watts website about expansion bottles.

Just fill in the information and the chart will help you decide what size you may need. Watts is a company and brand that we trust and use frequently; however, it is recommended that you have a professional come out and take a look at your water heater and your plumbing in general. We would be happy to come out and give you an idea of what you might need and also identify any issues you may be having with high water pressure. Just give us a call at 1-800-833-4571 24 hours a day, 7 days a week and one of our water heater experts will be happy to help you set up an appointment and answer any questions you might have.


First, we should start this section off by saying that all of our repairs and installations are done in accordance with the California Plumbing Code requirements and according to the code, an expansion bottle is required for all “closed loop plumbing systems”. These systems typically have a backflow preventer and a pressure regulator located between the house and the water main. Nowadays, most homes in the San Diego area have one or both of these. However, each city and municipality can enforce some or even all of the California Plumbing Code. Here is a list of recent San Diego water heater codes requirements being enforced in the San Diego area.

So, when you are having your water heater installed, you may or may not need to have an expansion bottle put on the cold-water side. It just depends on the inspector in that area and what they choose to enforce. We make sure our technicians and water heater experts all know the latest code requirements and that is just one of the many reasons that you should consider having Water Heaters Only, Inc. come out and take a look at your water heater and plumbing.

Though not every residential water heater installation may require an expansion bottle, every commercial water heater does require an expansion bottle. There are other situations that we run across where a homeowner should definitely consider putting in an expansion bottle. Here are just a few:

  • If your plumbing uses polypropylene, you may want to install an expansion bottle. That’s because the pressure that can build up is too much for polypropylene to handle and plumbing issues can occur.
  • If your water heater is failing every in less than 5 years, it could be due to excessive pressure in your plumbing and, in that case, we would strongly recommend you have us put in an expansion bottle.
  • If your water pressure goes up and down very erratically and dramatically, then you may consider an expansion bottle. Specifically, when we check your water pressure after installing a water heater and it jumps more than 20 psi after the heater kicks on, then you should definitely get and expansion bottle.

But, of course, as we always like to tell people, if you aren’t sure whether you need an expansion bottle, let one of our technicians come out and take a look at your system. Again, just call us at 1-800-833-4571 24 hours a day, 7 days a week and we will be happy to answer any questions you may have or set up an appointment. Regardless, we hope this blog post has been helpful and perhaps given you a good understanding of expansion bottles and whether you may need one in your home.