Engineers, today, tend to lean towards primary-variable piping systems in hydronic heating and cooling systems for commercial applications. The simpler piping and elimination of a pump seems like an automatic win. There are times where the owner’s potential operational issues caused by the variable-primary heating system may suggest a primary-secondary system as the better design choice.
A Quick Review of Hydronic Boiler Piping Schemes
Take a moment to review the piping schemes below so we are using the same terms.
Primary-secondary hydronic boiler piping systems use a set of variable speed pumps, called the secondary, to pump the terminal units. There are also pumps for each boiler and these are called the primary pumps. As the two-way valves open and close, the flow in the secondary changes but it does not affect the boiler flow rate. The boiler flow rate is controlled by the primary pumps which may be constant speed or variable speed.
Primary variable hydronic boiler piping systems use a set of variable speed pumps to pump the terminal units. These same pumps flow through the boilers. As the two-way valves open and close, the boiler flow rate changes. Any minimum flow requirements of the boilers must be accomplished using three-way or bypass valves.
The Issue with Primary-Variable and Boiler Minimum Flow Rate
In part one of this series, we introduced a space-saving fire-tube boiler with a 10:1 burner turndown and a minimum flow rate requirement. Let’s use one of those boilers for our example.
You have a system with a requirement of 500,000 BTUH. You select an NTI Boilers model TFTN600. This boiler will have an output of about 510,000 BTUH when the return water is at the design non-condensing temperature of 150°F. If the system has a 30°F ΔT, the design flow rate will be 34 GPM. See the chart in part one for all the numbers. Due to its space-saving design, this fire-tube boiler has a minimum flow rate of 21 GPM.
The boiler can throttle the burner all the way down to about 51,000 BTUH using the 10:1 turndown. This turndown helps you avoid cycling and wasted energy. But, what about the flow rate? The two-way valves would throttle severely when down below 100,000 BTUH. If we do the math and assume a 30°F ΔT here is what we end up with for flow rate.
51,000/ 30 x 500 = 3.4 GPM
Although the math indicates a need for only 3.4 GPM, the boiler has a minimum flow requirement of 21 GPM. The two-way valves are not aware of the boiler requirement, they are only reacting to room conditions. In a primary-variable piping system, the only way to assure the 21 GPM demanded by the boiler is to add a lot of three-way valves or bypasses.
The factory IOM has the following statement, “To ensure the minimum flow rate is attained, NTI strongly recommends installing the boiler in a “Primary/Secondary” plumbing configuration utilizing “Closely Spaced Tees” or a “Low Loss Header” to de-couple the Boiler-Primary loop from the System-Secondary loop(s).”
In a primary-secondary piping system, the two-way valves can throttle and not affect the primary flow rate. The common piping will see a great deal of mixing at the low secondary flow rate, but the NTI fire-tube boiler can turn down enough to handle the change.
Primary-Variable with Multiple Boilers in Parallel
The NTI Boilers TFTN series has control options to handle up to 8 boilers in parallel. Look at the primary-variable sketch above. What would happen if there were three boilers in parallel and the system temperature was reset to take advantage of the condensing technology? The system would have a design capability of:
510,000 BTUH X 3=1,530,000 BTUH
The boiler system could turn down 30:1 assuming three boilers at design and only one needed in the warm spring and fall. With condensing boilers, we like to run as many boilers as possible operating at lower loads. This improves boiler plant efficiency. But there is a big problem. If the load was 510,000 BTUH, we would want to run three boilers at a third of the load each. We would not run one boiler at 100% in our condensing boiler world.
If we run three boilers, the minimum flow would be 63 GPM. Each boiler requires 21 GPM regardless of the firing rate. The math might indicate a 34 GPM requirement, but the equipment requires 63 GPM. The two-way valves don’t know this.
Imagine the staging trauma. One boiler operates with a minimum system requirement of 21 GPM. The load changes and we need a second boiler to come online. The minimum flow rate jumps from 21 GPM to 42 GPM in an instant.
Once again, the factory IOM has the following statement, “To ensure the minimum flow rate is attained, NTI strongly recommends installing the boiler in a “Primary/Secondary” plumbing configuration utilizing “Closely Spaced Tees” or a “Low Loss Header” to de-couple the Boiler-Primary loop from the System-Secondary loop(s).”
You may choose constant speed or variable speed primary pumps, but you can see the issue with the primary variable design and these smaller commercial boilers that don’t have a large flow rate range unlike the larger commercial boilers. There are times where the owner’s potential operational issues caused by the variable-primary heating system may suggest a primary-secondary system as the better design choice.
The choice of primary-variable or primary-secondary boiler piping will depend on the many factors mentioned in these blogs. Invite R.L. Deppmann to discuss your project and make sure the boiler selection and system type provide the best solution for your clients needs.
PART ONE: Condensing Fire-tube Boiler Heating Systems: Why Primary-Secondary Pumping May Make Sense
