Condensing Boiler Plant Piping Design & Control Part 3: Variable Primary/Variable Secondary Flow Systems

By Chad Edmondson

In our last blog we discussed how to design and control a variable primary condensing boiler system.  Now it’s time to go to the next level – Variable Primary/Variable Secondary.

Variable primary/variable secondary flow is the top tier strategy for optimizing your condensing boiler system.  You’re not only precisely load-matching your supply with demand, you’re setting up your primary pumps for pretty easy duty.  Under low load situations you may not even need to fire the boilers or operate the primary pumps, as the secondary loop may be able to meet demand by using up the residual heat in the system.  Finally, when the boilers are operating, they are operating at their lowest fire rate with a return water temperature that is low enough to ensure condensing.

How would such a system be piped?  A variable primary/variable secondary boiler system is piped very similarly to a variable primary flow system, actually, with a few modifications.

First, each boiler would have its own pump.  Right off, this eliminates the need for the flow limiters and two-way valves on the boilers.  And because the pumps are only going to operate when the boilers are fired (with a slight delay in shut-off simply to remove residual heat from the boilers) you automatically meet the requirements of ASHRAE 90.1 – 2010 and 2013*.

Figure 1

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The primary boiler pumps will also have their own set of variable speed drives, as will the secondary pumps.  Instead of a common pipe, which you commonly see in a constant speed primary/variable speed secondary (Figure 1), a buffer tank might need to be installed between the primary and secondary loop (Figure 2).  Buffer tanks are frequently used in conjunction with water tube-boilers because water tube boilers have substantially less volume than fire-tube boilers.  The buffer tank stores BTU’s that can be used by the building when demand is low and the boiler and primary pumps are off.  This increases the run time of the boilers when they are needed and keeps them from short cycling under low load conditions. 

Figure 2

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The secondary piping is set up exactly as it would be in any variable secondary system.  The secondary pumps have their own variable speed drives, which operate based on the differential pressure in the critical zone. 

One thing remains the same whether you are varying flow through one or both loops.  There needs to be at least one 3-way valve to maintain a minimum flow through the secondary pumps whenever they are operating.  This valve also provides a pathway for the water to flow back to the buffer tank when the coils are not operating and their own 2-way valves are closed.  Otherwise you could deadhead your secondary pumps.

Operationally, the primary and secondary loops are decoupled from one another and vary in flow and in temperature according to their own set of control parameters.  Each loop also has its own Delta T, the primary pumps operating to maintain a constant Delta T across each boiler and the secondary pumps operating to maintain a constant differential at the end of the secondary loop. 

For more information watch this video:

Condensing Boiler Variable Primary System Design

* ASHRAE 90.1-2010 and 2013 state that when a boiler plant includes more than one boiler, provisions shall be made so that the flow in the boiler plant can be automatically reduced, correspondingly, when a boiler is shut down.