Hydronic Balancing Part 10: Proportionate Balancing with Flow Limiter

By Chad Edmondson

 Flow limiting valve, also known as a automatic balancing valve (ABV)

Flow limiting valve, also known as a automatic balancing valve (ABV)

If you’ve read Parts 1 thru 9 in this Hydronic Balancing Series then you should have a solid understanding of the fundamentals of balancing.  If not – go back and review the previous blogs.  You need these fundamentals to understand what we will discuss here in Part 10, which is balancing using automatic balancing valves (ABVs). If you have read the previous blogs — good news — this is going to be a cakewalk!

First, let’s be clear what type of device we are talking about when we say ABVs, or as we prefer to call them, flow limiters.

Flow limiters include a spring-loaded cartridge, which moves back and forth depending on the amount of differential pressure through the valve. This movement inside the valve changes the size of the orifices inside the valve, which in turn allows more or less water to pass through.  In effect, the valve is continuously stabilizing the flow based differential pressure. These valves are specified to operate under a specific differential range, and therefore it is imperative that the system always sees a specific minimum pressure. Otherwise, there is no movement inside the valve--and no control. That is why we like to refer to these valves as flow limiters, and not necessarily automatic balancing devices.

The following images and graphs illustrate what occurs inside a flow limiting valve when it is operating below, within, and above its control range:

 Flow limiting valve when it is operating below its control range.
 Flow limiting valve when it is operating within its control range.
 Flow limiting valve when it is operating above its control range.

Check Your Critical Zone!

Flow limiters are great devices and eliminate most of the work associated with proportionately balancing a hydronic system.  However, that doesn’t mean you can install them and assume everything will operate as planned.  As we’ve said before, piping systems rarely go in exactly as planned.  That’s why it is imperative that you check the flows through each zone, and (most important) confirm that the pressure drop through the critical zone is no greater than what is required to make the valve operate.

If the pressure drop through the critical zone is significantly beyond the minimum, then either the pump impeller should be trimmed or variable speed drives should be employed to reduce flow.