According to Dalhoff, the on-board intelligence of VFDs and ECMs has improved to the point that system engineers and pump manufacturers can optimally meet the needs of either control scheme required: delta-T or delta-P.

Delta-T. John Barba, one of Taco’s variable-speed experts, is a proponent of pump or circulator application based on delta-T, the difference between supply and return temperatures.

For variable-speed circulators, delta-T relates directly to the amount of British thermal units being taken out of a system or zone.

The Universal Hydronics Formula states:

gpm = Btuh ÷ (∆T × 500)


gpm = the flow rate, in gallons per minute, required to deliver a specific amount of heat at a specific time

Btuh = the heating load, or the British-thermal-unit-per-hour requirement at a given point in time

∆T = the design temperature drop across a piping circuit

500 = the weight of 1 gal. of 100-percent water (8.33 lb) multiplied by the 60 min in an hour multiplied by the specific heat characteristic of the water (1, for 100 percent)

“I don’t see delta-P in the equation,” Barba said. “What I do see is that delta-P seeks out gaps in the system and changes speed to fill the gaps.

“But it doesn’t take into account what happens out there in the loops because it doesn’t know how many Btus a zone is using when ambient temps are 5°F or 25°F,” Barba continued. “It doesn’t know what combination of zones is calling or if two zones of equal length—but unequal heating load—are calling. A delta-T circulator will monitor the temperature differential between the supply and return piping and will speed up or slow down to maintain the delta-T that’s selected.”

In the Universal Hydronics Formula, if delta-T is fixed, then flow rate will adjust based on the heat required.

“What’s seen in systems with a delta-T of 1°F is water rushing through the system,” Fina said. “The delta-T that’s seen is a result of a fixed-speed circulator slamming water through the system at a fixed gpm, while the Btu load has decreased. The math says the variable now isn’t the gpm, but the delta-T.”

Barba added: “In a case like this, the system is getting the Btus it needs, but the flow rate is way too high, so the delta-T shrinks. Now, how would that affect, say, a system with a modulating-condensing boiler? Would it work more efficiently if the water returning to the boiler was 109 degrees or 90 degrees? If the delta-T pump was set at 20 degrees, the water temperature coming back to the boiler would be lower, and that would help the boiler operate far more efficiently.”

Consider the effect of short cycling. A boiler certainly will run longer to raise water temperature 20 degrees than it will to raise water temperature 1 or 2 degrees. Unfortunately, short cycling is a fact of life with modulating-condensing boilers because of low water content. Many experts recommend use of a buffer tank to build volume.

“Ideally, a broad delta-T works well for many hydronic systems,” Barba said. “Let’s say we have a light-commercial building with baseboard radiation. Fin-tube baseboard is generally sized and rated at a 20-degree delta-T. There’s nothing magic about a 20-degree delta-T; that’s just what it’s rated at. If you wanted to set it for 10 degrees, you’ll have a higher flow rate through the system, and the pump will run faster. Set it for 30, and the flow rates will be lower, and the pump will run slower.

“So, for a fin-tube system, 20°F delta-T makes sense,” Barba continued. “Outdoor reset doesn’t make much difference to either delta-T or delta-P. It’s all about the Btus, so a 20-degree temperature drop at 150 and a 20-degree temperature drop at 110 is still a 20-degree temperature drop.”

The key, Barba said, is the circulator offers the flow required to deliver the heat needed simply by noting supply and return temperatures. These tell the circulator how much heat is being removed from the system.

Delta-P. Optimizing pump performance based on delta-P is accomplished by setting and maintaining a prescribed pressure within a piped system.

“To move fluid, the pump or circulator adds energy (head) to the fluid to overcome friction loss—what occurs when the moving fluid comes in contact with the inner walls of the pipe,” Thompson explained. “The faster the fluid moves, the higher the friction loss is—like driving your car faster into the wind.

“The graphical representation of this event, of course, is called a system curve—and it goes up in an arc, starting at 0 gpm (no velocity, no friction) to where it intersects with the circulator’s performance curve,” Thompson added.