Varying pump speed based on temperature differential helps match system output to building load
When it comes to maximizing the efficiency of a boiler system, the Holy Grail is precisely matching the output of the system to the load in the building. Fortunately, new technologies and new ways of controlling water flow through the system are making this goal more achievable than ever before.
Modulating boilers and variable-speed pumps have helped make boiler systems highly efficient, new control systems have enhanced that efficiency, and now pumps that vary water flow based on temperature differential, rather than pressure differential, hold promise to take boiler-system efficiency to the next level.
“The control systems tell the boiler how to fire and how many Btus to send out,” Joan Mishou, manager of customer service and applications engineering, LAARS Heating Systems Co. said. “From there the goal is to match the Btus with the water flow that goes through the boiler.”
“Traditionally, much of the flow going through the boiler has been fixed, even if the boiler itself is step-firing or modulating,” Mishou continued. “However, it makes a lot of sense to match the water flow to the Btus. You don't need a lot of flow going out of the boiler if the system doesn't have a need for all of the Btus that the boiler can produce.”
Pumps in a boiler system generally are designed to handle the most extreme design conditions that will be experienced in a given geographical area. This means that in the big picture, pumps rarely — if ever — need to run at full speed. System efficiency is compromised whenever pumps spend more time running at a higher speed than needed to match a system's British thermal unit requirements.
The quest for optimum boiler-system efficiency has led system designers to examine the relationship between flow rate, British thermal units, and system delta-T.
According to John Barba, contractor-training and trade-program manager for Taco Inc., the flow rate (in gallons per minute) required to heat a zone is equal to the British thermal units the zone requires at a given point in time divided by the system's delta-T, times 500. The 500 is a constant that comes from multiplying the weight of 1 gal. of water by the number of minutes in an hour, by the specific gravity, by the specific heat of the fluid.
Precisely matching water flow to boiler firing rate dramatically improves system efficiency by preventing system short-cycling.
When a boiler is firing and pumps are operating at a fixed rate, a reduction in heat load (caused, for example, by warmer-than-design outdoor temperatures) leads to the system satisfying the call for heat quickly. This leads to short-cycling on two levels: The boiler is running for a shorter period of time because it satisfied the zone so quickly, and the water coming back from the zone is warmer, which makes the boiler short-cycle more.
“When boilers short-cycle, the overall efficiency of the system takes a hit,” Barba said. “The burner never gets the opportunity to burn at its peak, steady-state efficiency. It's similar to a car that is always in stop-and-go city driving and never on the highway.”
Barba said new circulator-pump technology that varies flow based on temperature differential, rather than pressure differential, is a key to a highly efficient system.
“If you have a circulator that varies its speed based on the temperature difference of the fluid going out to the radiation and the fluid coming back from the radiation, that delta-T gives you a precise idea of what's going on in that system at any given point in time,” Barba said. “It tells you exactly how many Btus are required in the zone you're trying to heat.”
Greg Cunniff, manager, application engineering, Taco Inc., agreed. He said system designers who are seeking to optimize boiler system efficiency by matching flow through a boiler to the load in the building may be well-served to consider pumps that vary flow based on differential temperature, rather than differential pressure.
“With differential pressure, you're measuring a variable that's secondary to the load — namely, the pressure in the system,” Cunniff said. “As valves open and close, pressure in the system rises and falls. When you use differential pressure, you're not measuring the load directly, you're measuring it indirectly based on how many valves are open and how many are closed.”
Cunniff said that he is not “disrespecting” differential pressure.
“I'm not saying it's not a good way to go, because it's what we've all done for years,” Cunniff said. “It's just that today we have the option of directly measuring the load, which is a more efficient way of doing things.
“If I have a boiler putting out 150°F water, and I have a design for a 10°F delta-T, I know that water is supposed to come back at 140°F,” Cunniff continued. “If it comes back at 145°F, I know that the actual load is one-half of what the system is providing. So I can slow down the pump and still maintain that 10°F delta- T.
Cunniff added that by using differential-temperature measurement, it is possible to shut off a pump if the load goes down to zero. When using differential pressure, the pump must always be on to maintain minimum pressure in the system.
For system designers, the quest for optimal boiler efficiency will continue.
“The push probably started in radiant heating, with variable-speed injection, and then it started carrying over into different parts of hydronics,” Mishou said. “Now the next big cog is getting the boilers themselves to match the system better by varying the water flow through the boiler, since we can already vary the Btus.”
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