Variable-speed hydronic circulation has been around for years. With the advent of packaged controls on pumps, it is easier than ever to implement.

To take a closer look at the concept of variable-speed pumping, I asked professional engineer and manager of application engineering for Taco Inc. Greg Cunniff and Warwick, R.I.-based plumbing and heating specialist William J. Riley to explain the best applications and key benefits of the technology.

Convection-Heat-Transfer Equation
The speed of a variable-speed pump is adjusted automatically based on heating- or cooling-load demand. To understand how, let's take a quick look at the convection-heat-transfer equation, which, for water, is:

gpm = Btuh ÷ (∆T × 500)

where:

gpm = the flow rate, in gallons per minute, needed to meet heating- or cooling-load demand

Btuh = the heat or cooling, in British thermal units per hour, required for a zone

∆T = the delta-T, or designed temperature drop, across a piping circuit (for heating, design delta-T typically is 20°F; in many radiant-floor-heating and chilled-water-cooling systems, however, it usually is about 10°F)

500 = the specific heat of water, in British thermal units per minute per gallon per hour per Fahrenheit degree (8.33 lb per gallon times 60 min per hour times 1 Btu per pound per Fahrenheit degree)

Sample Project
Consider the example of a small restaurant with a heat gain of 75,000 Btuh and an outdoor design temperature of 95°F. Three zones of fan coils, each with a cooling load of 25,000 Btuh, are needed. Each zone is designed to a 10°F delta-T and has a flow rate of 5 gpm. With this information, the boiler and chiller supply and return pipes, distribution header, and zone piping can be sized.

Pipe-sizing guidelines are based on minimum and maximum flow velocity and maximum head loss. Recommended design parameters are velocities of 2 fps to 8 fps at a head loss of no more than 4 ft per 100 ft. In smaller pipe, head loss of 4 ft per 100 ft rules, resulting in a maximum velocity of 4 fps, above which noise is likely. In larger pipe, a velocity of 8 fps rules, resulting in pressure drops below 4 ft per 100 ft. Larger pipe can withstand higher velocities without noise.

According to Riley, president of William J. Riley Plumbing & Heating Company Inc., determining the piping arrangement is next. He said our example calls for 11/4-in. pipe and 15-gpm flow. He said he would branch into 1-in. lines for each fan-coil zone at the chiller header before doing the same, only in reverse, for the return side of the system.

Next up: estimating the head loss of the piping system. Riley measures the longest zone from the discharge side of the pump all of the way around the system, through the chiller, and back to the suction side of the pump. For this application, the longest run is 150 ft of pipe, including the fan coil.

To allow for additional pressure drop through fittings, Riley said he multiplies the length of the longest run of pipe by 1.5. Returning to our example, 150 ft multiplied by 1.5 equals 225 ft. That total equivalent length, then, is multiplied by 0.04 (representing 4 ft of head loss per 100 ft of straight, properly sized pipe, based on the maximum pressure drop of 4 ft per 100 ft), yielding 9 ft of head loss.