Working Together

Written on: May 1, 2021 by George Carey

Over this past year, Zoom presentations have become a normal way of offering training classes while the country (and world) has grappled with the COVID-19 pandemic. One of our more popular classes is on the subject of hydronic system components.
Of all the components we use in a hydronic system, the most frequent questions I get during Zoom presentations are about diaphragm expansion tanks and pressure-reducing valves (PRVs) and how they interact with each other. Consistently, the contractors have been asking about what the proper settings are for each device and why.
Before we start applying answers to those questions, it is important to really understand what the functions of these components in a hydronic heating system are.
Pressure-reducing valves
A pressure-reducing valve is pretty much like its name implies—it takes the incoming street pressure and reduces it down to what is needed inside a particular building where it has been installed. The hot water heating system functions when the boiler and all of the piping and radiation are filled with water that comes from the city water main located in the basement.
This question comes up often…how do we know how much water is needed to fill the system? By using the pressure gauge on the boiler, we can determine when the system is completely filled with water. Water has weight and as you stack more water on top of itself, it weighs more. By using the pressure gauge on the boiler, we can determine how high up into the system the water has gone. The pressure gauge reads in pounds per square inch or psi. We know a column of water that is 2.31′ (or 28″) tall weighs 1lb per square inch at the bottom of that column.

The key to using a pressure gauge in determining the height of a column of water is the expression pounds “per square inch.” Whether the piping system has ¾” copper pipes or 4″ steel pipes, the measurement on the pressure gauge is the same…per square inch. A square inch is a square inch, it doesn’t change; therefore, a column of water 2.31′ tall weighs one pound per square inch.
To properly fill a hydronic system, measure from the boiler pressure gauge/PRV location up to the highest piece of piping or radiation (whichever is highest) in the building. Then take that number and divide by 2.31′ to convert to pressure in pounds per square inch.
However, don’t stop there—the pressure reading would ensure that the water is all the way to the top of the system, but what would the pressure be in the system at the highest point? It would be zero pounds per square inch, and if you had any high vents located at the top, how effective would they be? There would be 0lbs of pressure inside the system and 0lbs of pressure (atmosphere) on the outside of the system.
There is no motive force for any air bubbles to vent out of the system. To ensure that high vents will be able to do their job, the industry has standardized on adding an additional 4psi to the number required to get water up to the highest point. To establish the proper PRV setting for each application, measure in feet the distance between the boiler pressure gauge/PRV location to the highest pipe/radiation in the building, divide by 2.31′ and to that number add 4psi. The result will be the proper cold water fill pressure for that system. The key is to fill the system when it’s cold so that you will have an accurate reading from the pressure gauge.
Expansion tanks
Expansion tanks play a very important role in the proper operation of a hot water heating system; that function is very different from the PRV’s job but for both to be effective, they work together. To appreciate this relationship, you want to have a good understanding of what the expansion tank’s role is and how it does what it does.
When a system is completely filled with water and then is heated to the operating control’s high limit, there is anywhere from 3.5%–5.0% more water in the system because when heated, it expands. Here’s the problem—water is not compressible and so when this increase occurs, if there is no place to put this extra water, the relief valve on the boiler will open up and dump system water onto the floor.
This is where those diaphragm tanks come into play; air is a gas which is compressible and so the expansion tank becomes the place where the expanded water goes while keeping the pressure in the system below the relief valve’s setting. The air in the diaphragm tank acts like a spring, allowing the system water to push against it as it is heated and expands. The air in the tank is separated from the system water by using a butyl rubber that is flexible.
These tanks are different from the older-style steel compression tanks—in those tanks, the system water and air cushion came into direct contact with each other, and because of that, the tanks were larger than the diaphragm tanks. With a diaphragm tank, the air side is fully expanded, pushing the rubber diaphragm all the way against the other side of the tank. When connected to the system, the air side pressure is now seeing the system’s fill pressure. Remember, when cold, there should be no system water in the tank; for that to occur, the diaphragm tank’s air charge pressure must match the system’s fill pressure.
Diaphragm tanks
Diaphragm tanks are sized to accept the volume of expanding water in the system while keeping the pressure in the system below the relief valve’s setting. Normally PRVs and diaphragm expansion tanks come pre-set at 12 psi since most of the applications are for two-story buildings. If you have a system in a building that requires a higher pressure setting, the expansion tank must be pre-charged to the higher fill pressure setting.
If you did not match the air charge to the fill pressure, once the tank was attached to the system, a certain amount of cold system water would enter the tank. Remember, there should be no water in the expansion tank when the system is cold. The net result is the expansion tank acts like it is too small, causing the relief valve to open, discharging the excess pressure.
The only proper way to check the tank’s pre-charge setting is while it is disconnected from the system. If you were to check the pressure while the tank is attached to the system, it would be a faulty reading because the water pressure from the system is “squeezing” against the diaphragm. The gauge would just be reading the system pressure.
As you can see, each of these components has its own “job to do,” but to do them properly, they have to work together.
If you have any questions or comments, e-mail me at, call me at FIA 1- 800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM