Air Management and Pressurization Part 1 - The Rationale Behind Closed System Design

 By Chris Edmondson


Closed loop systems create the need for accurate pressurization and air management in a hydronic heating and cooling system.  Unfortunately, proper air management often turns out to be an Achilles’ heel in a hydronic system’s design.  Design it wrong and a host of problems can erupt, from noisy operation to poor efficiency and shortened equipment life.  It begs the question -- why even design a system to be closed in the first place?  After all, condensate systems and cooling tower systems are designed to be open to the atmosphere – why not heating and cooling water systems?

Closed is BETTER

Many years ago hydronic systems were designed to be open, using expansion tanks that were open to the atmosphere.  Such systems are rare these days and for good reason.  First, open systems tend to collect too many empty soda cans and dead pigeons, which isn’t good for the system or the poor soul responsible for removing them.  Second, opens systems are constantly absorbing oxygen which leads to rapid corrosion of virtually every component in the piping system, from boilers to valves.   Finally, any system that is open is subject to a certain amount of evaporative losses, which means make-up water must constantly be added to the system. 

 Why suffer all of this when you can simply close the system up and extend the life of the equipment indefinitely while never having to add hardly any new water at all?  That’s the reason why most hydronic heating and cooling system you encounter these days are closed – at least they are supposed to be.  Keep in mind, if it’s leaking, it’s not a closed.  If, for some unknown reason, maintenance personnel is occasionally draining water from the system to use it for some other task, it’s also not closed.  Either situation will compromise the integrity of the closed system by introducing oxygen and necessitating fresh make-up water.

Remember –Water GROWS!

So – hot and cold water systems should be closed.  This means that systems must also be designed to maintain a minimum pressure while providing room for expansion and a means to safely relieve excess pressure.  Why?  Because water grows when it is heated and since water can’t be compressed, the piping system must be able to accommodate this extra volume yet still maintain a minimum pressure.  This is the role of the expansion tank. 

 All expansion tanks used in a hydronic system incorporate a pre-charge of air which serves as a cushion that can expand and contract along with the changing volume of water as it is heating and cooled.  The expansion tank is the central component in any closed loop air management system. 

 Over the next several posts, we will discuss the different types of expansion tanks, air control versus air elimination design, expansion tank sizing, and proper design for air management and how to calculate building fill pressure.   Understanding the rationale behind closed system design and why air management is needed is the best starting point for a more in-depth discussion.