Do Sensors Make $ense?
June 21, 2019
Many options exist in the marketplace to control variable speed pumps in hydronic systems. The increased capability and price reduction of microprocessors has allowed the basic centrifugal pump to become smart. Some manufacturers have decreed, with no explanation, that the new ways are the best ways. While new methods offer some advantages, in the end they still come up short in many ways.
Control logic has always been a key element to any successful variable speed fan or pump. Many systems didn’t reduce speed in fear of not supplying enough flow to a system, thereby not achieving the primary goal of an HVAC system—comfort. Systems wasted energy to avoid the possibilityof complaints. The industry now realizes that this approach is unacceptable and doesn’t meet owner expectations or the energy code. So, how do we control hot water and chilled water pumps?
The Old Way
In this system, a differential pressure sensor is installed on the critical circuit which maintains a minimum differential pressure across that point (remember, the pump head is based upon the friction of that flow path). If the critical path is satisfied, all other paths are satisfied. This system is very simple to understand and operate. A major advantage of a sensory system is that it automatically removes the excess capacity of the pump, whether it’s due to oversizing or reduced system demand.
There are two common enhancements that can improve this efficiency of this system:
- A multiple sensor system is used when a system’s load is so diverse that its critical path could change based on usage. For example, a school auditorium is empty while students are in classrooms, but hours later the reverse could be true. Parts of the system could be fully loaded while others remain at minimum load.
- Resetting the differential setpoint was recently added to ASHRAE Standard 90.1. Building Automation Systems can monitor the position of all the control valves in a system and make real-time decisions to alter the differential pressure setpoint in order to get all modulating control valves to be nearly wide open. If a circuit is slightly throttled, it is under control.
The New Way
Although control curves can be employed with different approaches, all follow the same theory—at a certain flow the pump will be operated at a specific pump head. Some operate off a flow meter or DP sensors at the pump, some use VFDs programmed with the pump’s operating data. No matter the method, a microprocessor decides the pump speed based on system flow. The problem is that these systems don’t know where the flow is going. Close loads? Far loads? Split? In the end, a control curve system’s only feedback comes in the form of a comfort complaint which results in the system being adjusted to operate at a higher speed.
The Punch Line
We all must follow the laws of physics and one simple formula ends up determining the largest cost of ownership for a pump—energy consumption.
head * gpm
Pump Power = ______________
3,960 – (pump eff)
GPM is how much heat transfer a system needs. Head is the friction needed to move that flow through the system. Any system that minimizes those factors will draw less HP and cost less to own. Pumps with 2-3 HP can be demonstrated to save enough energy to justify the cost of the sensor.
All design choices are a series of tradeoffs. DP sensors cost a little more money and effort to install, but allow a pump to operate at the lowest HP for a given set of conditions. However, it’s not always cost effective to install them. For a low HP system or a system that is difficult to access, a control curve might be the best choice.