Pressure and high school chemistry

The residential expansion tank is one of the most important and least understood components of a properly functioning hydronic heating system. We speak about it indirectly and by now everyone in the industry can repeat the following in their sleep: “always pump away from the expansion tank” and “the expansion tank is the point of no pressure change.” But how many of us truly understand the expansion tank and how it relates to system pressure?
And where does high school chemistry come in?

We will come to high school chemistry in a bit, but first, let’s start by asking a few questions such as: Why do we need an expansion tank and what size of tank do we need?

Here are some basic answers. When water or a glycol/water mixture is heated it expands. If a liquid expands in a confined space, such as a closed hydronic system, the pressure in the confined space will increase and it will increase drastically. The increased pressure will look for the easiest path
for release. Hopefully, this will be a pressure relief valve and not a weak component in the system that may be of vital importance.

We use expansion tanks to prevent this drastic build up of pressure in closed hydronic systems. But every once in a while, it does not seem to work the way we expected it to. Sometimes, the pressure still changes more than we think it should. This is when we need to take a look at the expansion tank and try to understand it better.

SIZE MATTERS

In the age of modern hydronics, expansion tanks come in many sizes and shapes. How do we make sure that we have the right expansion tank for our system? What does it mean to have the “right” expansion tank? Essentially “right” in this case is not wrong, and the only wrong tank is an undersized tank. So size does matter. As for shape, that comes down to personal preference.

An expansion tank typically comes with instructions, which describe what type and size of system that it is suitable for. There may be a table that shows some typical system types such as radiant floor, baseboard, or radiators along with typical boiler sizes. Now here is the interesting part. The table could have been put together by a group of high school chemistry students.

Think back to when you were sitting in your high school chemistry class while your teacher rambled on about guys by the names of Boyle, Charles, Gay-Lussac and Avogadro, along with a couple of things called the Ideal Gas Law and the Combined Gas Law. Do not worry if you can’t remember all of this stuff. I didn’t either, until I walked past my son as he was studying his high school chemistry and suddenly realized that some of that stuff was actually useful in real life. (I also stopped to take a look because it is such a rare sight to see a high school student actually studying.)

There may have even been a question in your textbook that read something like this. “If you have a closed system with 15 gallons of water and two gallons of air separated by a rubber diaphragm at room temperature with an initial pressure of 15 psi, what will be the final pressure in the system if the temperature is raised to 180 degrees Fahrenheit?” That is right, this question is a cleverly disguised hydronic heating system with an expansion tank. At age 16, we had no clue. At age 16 plus a lot, it now makes sense.

Go and dig out your old chemistry text and find the right formulas − you will find that the end pressure is about 22 psi. If you were to increase the volume of air in the system from two to six gallons, the end pressure would be only about 17 psi. So the larger the expansion tank, the smaller the change in the pressure of the system as it heats up.

If we increase the volume of water in the system from 15 to 30 gallons, the two-gallon expansion tank would have an end pressure of 35 psi. At which point, the typical 30 psi relief valve would have opened to relieve the excess pressure. The six-gallon expansion tank would have a final pressure of 20 psi. All of this can be figured out using a high school chemistry text.

WHAT ABOUT THE PRE-CHARGE?

Now that we can see how the volume of our expansion tank, the volume of the system and the change in the temperature are all related, let’s take a moment to look at the pre-charge of the expansion tank. How many installers actually check or adjust the pre-charge of the expansion tanks in their systems? I would say probably not very many. The good news is that for the majority of the residential systems that are installed, you do not need to adjust your expansion tank.

Fortunately, the expansion tank makers, the boiler fill makers and the glycol feed makers are all on the same page. In a typical installation, the boiler fill or glycol feed is connected to the system piping at almost the same point as the expansion tank. Expansion tanks typically come with a “pre-charge” of 12 to 14 psi and the boiler fill makers and glycol feed makers also pre-set their equipment to about 12 psi. Now 12 psi is good for hydronic systems in which the highest point in the system is up to 20 feet above the expansion tank. If the highest point in the system happens to be more than 20 feet above your expansion tank, then you will need to consider increasing the fill pressure of your system and increasing the pre-charge on the expansion tank accordingly. The biggest thing to remember when changing the pre-charge on an expansion tank is to do it before it is connected to the system. If you increase the system pressure while the expansion tank is connected, system fluid will be forced into the expansion tank, effectively reducing the tank’s overall capacity.

LONGEVITY

Here is a question that I have heard in the past. Why don’t expansion tanks last as long as they used to in the good old days? This is an excellent question since the basic design of an expansion tank has not changed since the good old days so here is something to think about. Let us take a look at how a system operated back then and how a system operates today.

Systems used to operate hot. We turned them on in the fall, the boiler came up to temperature and then cycled up and down 10 or 20 degrees for six, eight or 10 months at a time. The expansion tank would make one big flex when the system turned on and heated up from room temperature to its operating temperature. It would then move slightly back and forth with the small changes in temperature. In essence, the expansion tank did not have to flex very much.

In today’s systems, we use outdoor reset controls, cold start boilers, priority for DHW, load shedding and a number of other energy saving features. These developments are great for our utility bills, but the expansion tanks have to flex a lot more. To avoid that increased flex, consider moving to a larger-sized expansion tank.

As we continue to change how we operate systems, it may be time for us to revisit some more of our high school textbooks. Who knows what useful tidbits we might find. <>

Cliff McNeill is with Equipco Ltd. at its Calgary, AB office. He joined the manufacturers rep firm in May 2008, bringing with him an extensive background in hydronics, heating, controls and plumbing. A graduate of UBC, McNeill has been a speaker at trade shows across Canada.