Air Management and Pressurization – Part 3 How to Calculate System Fill Pressure

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By Chris Edmondson

Want to do a building owner or facility manager of a new building a BIG favor? 

 If you’re a mechanical engineer responsible for designing the hydronic system for a new building, one of the kindest things you can do for your client is include the initial system fill pressure on your drawings.  This one accurately calculated value could help your client avoid a lot of operational problems in the future.

What is system fill pressure?

Frequently referred to as the “cold fill pressure” the system fill pressure is the pressure required to fill a hydronic system with water and still leave enough pressure at the top of the system to vent air off. 

 System fill pressure will take into account the:

(1) System height, which is the vertical distance from the bottom of the piping system to the highest piece of piping in the system, and

(2) Any additional pressure in a heating system that may, in rare cases, be required to prevent water from flashing to steam.

 The fill pressure must be high enough to fill the system with water and still have enough pressure to vent off air.  In some cases it must also be high enough at all points in the system to prevent flashing in the piping or pump cavitation.

How To Calculate System Fill Pressure

Calculating the system fill pressure is pretty simple. Let’s say you have a system that is 30 feet in height.  (Remember, this is the system height, not the building height!)   Since we know that 2.31 feet of elevation equals 1 psi we simply divide 30 feet by 2.31 to determine the static head pressure: 

Thus, our static pressure is 13 psi.  This will get our system filled, but it will not be sufficient to meet the operational demands of the system once it starts operating.  We must add some additional pressure (typically 4 psi is enough) to make sure there is enough pressure at the top of the system to make an air vent open.  Since 13 psi + 4psi equals 17 psi, this is the pressure that the pressure reducing valve shall be sat at upon system fill.  This is the value that should be included on the plans.

Sometimes you need a little more

4 psi at the top of the system is generally enough surplus pressure for most hydronic systems to operate.  However, systems that are designed to operate above 220°F may require more.  This is because water boils at a lower temperature when it is under low pressure.  Thus, in order to keep water from flashing to steam in a higher temperature closed system, it is necessary to increase the system pressure.  TABLE 1 provides a reference for what the minimum cold fill pressures at the top a system should be based on system height and maximum operating design temperature.

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Minimum Cold Pressurization at the Top of Closed Low Temperature Hot-Water Heating System (Pumps Off)

 Note that for higher temperature applications, if the pump is installed near the top of the system, the pressure-reducing valve may have to be set at a higher pressure.  Also, it is important to know what the Net Positive Suction Head Requirement (NPSHR) is for the pump.  A high NPSHR is another reason you may have to increase the fill pressure. 

 Once you’ve made these simple calculations, and double-checked the NPSHR on the pump, you are ready to pass this crucial information onto the installing contractor by including it on the plans!