As zone valves close, a system curve intersects a pump curve at higher and higher pressure differentials. Higher pressure differentials can lead to higher flow velocities, which, in turn, can lead to velocity noise.
One way to deal with velocity noise is to install a pressure-differential bypass valve, which prevents flow when all heating zones are calling. As zone valves close, increasing pressure differential, the bypass valve opens to allow excess pressure and flow to pass back to the suction side of the pump.
A better solution is to use a mid-flow, low-head, flat-curve circulator. With such a pump, system pressure rises minimally, eliminating the need for a bypass valve.
If more head than a circulator can deliver is required, a variable-speed pump should be considered.
“If all of the zones in a system are calling for heat, we may find that the delta-T drops to 16 degrees, not the 20 typically designed for,” Barba said. “Doesn’t sound like much, right? But that also equates to about a 20-percent difference. With only two zones calling, the delta-T drops to about 15 degrees—a 25-percent difference. And with only one zone calling, the delta-T drops to 12 degrees ... a whopping 40-percent difference.”
Those scenarios, Barba added, are under design conditions. If outdoor temperature were, say, 35°F, the delta-T would be much smaller because heat losses would be reduced dramatically. A fixed-speed pump—or a delta-P pump—would be running at the same speed; the result: reduced delta-T and boiler short cycling.
“System designers can solve the dilemma of dropping delta-Ts by using a variable-speed, fixed-delta-T circulator,” Barba said. “You may never have to worry again about oversizing a circ.”
Rather than search for the point where a system curve intersects a pump curve, let the pump curve self-adjust, Barba said.
Back to Delta-P, a Viable Variable
So, it is settled; delta-T is better.
“Not so fast,” Thompson said. “Delta-P is viable and has many just-right applications.”
According to Thompson, constant-speed circulators operate contrary to their intended purpose.
“A circulator is a centrifugal pump with the sole purpose of moving Btus within a hydronic system,” Thompson explained. “Application, not size, should determine the ‘circulator’ descriptor.”
A circulator’s curve slopes down from left to right, meaning more flow, less head energy (what the circulator contributes to move heat). So, as the flow (or system British-thermal-unit requirement/load) goes down, friction loss is reduced. The circulator’s pressure, however, goes up.
A circulator’s maximum head is at zero flow when there is no friction loss.
“It’s actually the opposite of what it really needs to do,” Thompson said.
Discussing the “perfect circulator,” Thompson said: “The system need may call for a small, 1/25-hp model or a much larger 25-hp pump—think application. If it’s true that a circ’s mission in life is to overcome friction loss, doesn’t it make sense that the perfect circulator’s pump curve would look exactly like a system curve? After all, any head above the system curve wastes energy.”
Optimizing energy consumption requires a good control strategy. Thompson offers an example of a commercial pump set at factory defaults of 23 ft at 50-percent proportional. Twenty-three feet refers to the maximum head the pump will produce (adjustable up to 40 ft) at its maximum speed. Fifty-percent proportional means at 0 gpm (dead head or zero fluid velocity).
Assume that, at maximum speed, the flow at 23 ft is 100 gpm (or 1 million Btu at 20°F delta-T). The pump’s head is 50 percent of 23 ft, or 11.5 ft. Thompson draws a straight line between 0 gpm at 11.5 ft and 100 gpm at 23 ft. That is the pump’s curve—an inclining curve. Does it match the system curve exactly? No, but it is cost-effective, is simple (sensorless), and does not overpressure or overflow zones.
Delta-P is a poor fit for:
• Mono-flow systems requiring relatively high, constant velocity/flow.
• Main-system pumps in single-pipe systems because of limited pressure feedback to the delta-P pump.
• Injection systems.
Additionally, a delta-P pump may not be the best fit if used as the main “flow” circulator on a boiler sensitive to low flow, especially with low-mass, high-friction-loss heat exchangers.
So why bother with delta-P? For many other hydronic systems—applications such as reverse return systems and secondary chilled-water distribution circuits—pressure-differential pumps are well-suited.
This brings us back to an overarching theme: It’s all about flow. Hydronic HVAC systems are dynamic in that loads change constantly. Changes in load means changes in pounds of water, based on the law of thermodynamics. Changes in pounds of water means changes in flow. The application dictates delta-T or delta-P control. The bottom line is changing flow and maintaining it smartly, exactly as dictated by demand. Today, pumps and circulators can get the gift of this intelligence as a retrofit improvement. Or, even better, new smart pumps are “born” with the brains to meet system needs precisely.