Domestic Hot Water Recirculation Part 7: Balancing Systems with Multiple Risers

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By Chad Edmondson

Ever have to wait 2 or 3 minutes (or longer) for hot water to arrive at the shower in a hotel room?  If you know a little about plumbing design you may assume you’ve had the misfortune of choosing a hotel without a recirculation system.  But chances are the hotel does have a recirculation system, it just isn’t properly balanced.  And you are the lucky hotel guest positioned at the back of the line for hot water.

Why does this happen?  Because water always takes the path of least resistance! So if a recirculation system lacks any sort of manual or automatic flow balancing devices (aka flow limiters), hot water will always serve the closest circuits first and/or, if the hotel has more than one return line, the section with the shortest return line. 

Automatic flow control valves or manual balancing via circuit setters equal out the resistance among all these circuits, so no one gets left out in the cold, regardless of time of day or load.  If the option is there, we always recommend automatic flow control devices for multi-riser systems.  These valves arrive from the factory already customized for a given flow application.  You install them and forget about them.

Regardless of the balancing method you select, here are a few things to keep in mind when it comes any multi-riser system:

Only LEAD FREE devices meet code.  This requirement, originating with the California Assembly Bill 1953 and part of Standard NSF-61, applies to any pipe, plumbing fitting or fixture used to dispense water for human consumption.  As of January 1, 2010 the term “lead free” in this instance refers to the weighted average lead content of the wetted surface area of these devices, which must be not be more than 0.25%.  Circuit setters, flow limiters, etc. must be lead free!

You may need to upsize your circulation pump.  When balancing multiple circuits, you may need to upsize your recirculation pump just slightly in order reach the minimum flow requirement of any automatic flow control devices that are used.  Typically these valves need at least 2 PSI of differential to operate.

Consider using ¾-inch pipe instead of ½ inch.  Although your pipe sizing calculations may indicate that you can use ½-inch piping, sizing up slightly gets you a little closer to a balanced system.   This is because it helps balance out the pressure drop between the various circuits. 

Manual balancing may require some touch.  Whenever possible, JMP always recommends automatic flow balancing devices.  However, when manual balancing must be used, many people use a hands-on balancing method.This means physically touching each riser pipe at start-up, starting with the most distant riser, and tweaking each circuit setter until the pipe feels warm.  Once you know you have hot water at the last riser, you then repeat the process, touching all remaining risers and adjusting circuit setters until every pipe in the system is warm. 

This is especially helpful on low flow systems with ½-inch circuit setters.  Remember, to read a circuit setter, you must first measure the pressure drop between the inlet and the outlet; however, this value may be so slight that it really isn’t a useful point of reference for setting the device.  The touch method is fool-proof.

You can ignore vertical lifts when calculating head losses.   When determining the head loss in individual circuits that involve both single story and multi-story service, remember that vertical lifts need not be considered, as this head loss is recaptured as the recirculation water returns by gravity.  Thus, even though there is an “uphill” service in the multi-story wing of the hotel shown in Figure 1, the single story wing actually has a higher head loss.   There is more friction loss at the end of that circuit, than there is at the end of the multi-story circuit.  Ultimately it comes down to the pressure drop in each length of the recirculation piping.

Figure 1

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