By Max Rohr
View the full presentation on “High Performance Air, Dirt, and Magnetic Separation in Hydronic Design.”
Primary and secondary piping is a vital concept to understand to install modern hydronic equipment. There are many ways to achieve the goal of hydraulic separation in a system. Depending on the project, installers could use hydraulic separators, closely spaced tees, or a buffer tank to accomplish the task. How do we explain to a less-technical audience what is happening inside of these components?
The goal of a primary/secondary system is to keep two different symbolic customers happy. The first customer is really picky. This customer is the energy plant, composed of one to many boilers, chillers and/or heat pumps. The mechanical room customer has very specific flow requirements, per the heat exchanger designs of each component. That is why a lot of boilers and heat pumps ship with a circulator. Manufacturers want to ensure that they are not leaving much to chance with the proper flow rate for the equipment.
They typically don’t want flexibility or modulation that isn’t controlled by the equipment itself.
Complex piping
The second customer’s needs are hard to understand. This customer is the entire distribution network of piping in the building with many zones opening and closing, likely paired with a variable speed pump. Without a flow meter or indication on the circulator screen, it would be difficult to know what the achieved flow rate is in a dynamic system, as it adjusts to the heat loss in a complex building. This customer requires a wide range of flow rates.
How do we keep both of these customers happy at the same time? They aren’t going to agree on a numeric value of units of flow very often. Maybe once or twice a year on a design day scenario, in a multi-zone building when every zone is open, will both customers be calling for the same GPM. This is also assuming the energy plant design Delta-T is the same as the distribution side.
We need to separate these customers. Here are a couple analogies that can be helpful for non-technical audiences. In a home, there can be fights within the residents over what the temperature should be set to. In the same way that we don’t want these types of fights to occur, we don’t want the primary/secondary loops fighting over the ideal flow rate. Primary/secondary piping is like giving them each their own zone and thermostat.
Imagine you have a bicycle hanging from a frame rack in a shop. If it is suspended in the air, you could spin each wheel freely at any speed you want. Maybe one wheel has completely stopped, and the other is moving fast. While suspended in the air, both wheels are able to spin independently. If you spin both wheels and then set the bike on the ground, the wheels will have to spin at the same speed. As long as they both have traction, you can’t ride a bike with the back tire rotating 50 per cent faster than the front. It isn’t possible. Primary/secondary is a bike on a rack with wheels free to spin at any speed.
For car enthusiasts, try this one. The energy plant is the flywheel in a car engine. Let’s say the engine can turn the flywheel between 1,000 RPM and 8,000 RPM. Primary/secondary is like the clutch. It allows us to change gears and spin the tires on the distribution side at an RPM that translates to 10 KPH or 80 KPH. The flywheel and the tires are not required to spin the same RPM.
Exception, not rule
Mechanically, primary y/secondar y is important to keep our flow customers happy. However, sometimes the building occupants don’t want to pay for things they don’t understand. Having multiple ways to explain primary/secondary can be helpful to win the bid, protect the energy plant and keep everything running smoothly. If you have another analogy that works for you, send us an email.
There are instances where a primary/secondary piping couldn’t be needed at all. An example where the system would only require one line of piping could be snow melt. This is due to the fact that with these types of systems, it has a simple on/off functionality. This remains the same for any type of system that would be designed using only one zone. Although the system would still need to make sure that it can handle any pressure drops. It should be noted that most systems are going to have multiple zones.