The choice of piping system can make a significant difference in operating cost.

Figure 1


If you’ve been reading my PM columns over the last year, you’ve probably detected a penchant for reducing the power used to circulate fluid through a hydronic distribution system. I see this as “the final frontier” in the evolution of hydronic systems. Given that boilers now yield thermal efficiencies in the mid- to upper-90 percent range (when properly applied in low-temperature systems), the potential for further reduction in system energy use must come from reduced electrical power consumption.

There are several factors that influence the total electrical power used by a hydronic distribution system. These include the choice to zone with circulators vs. electric zone valves vs. thermostatic radiator valves, choice of heat emitters, circulator efficiency, operating temperature and choice of piping layout.

This month, I want to discuss the latter in the context of a simple residential system using fintube baseboard to supply a 50,000 Btu design heating load.

Figure 2

Lower Than You Thought?

I often ask audiences of hydronic professionals at seminars to estimate the efficiency of a current generation wet-rotor circulator in converting electrical energy to head energy. Number guesses often range between 60 and 80 percent. Such numbers are probably influenced by the relatively high thermal efficiencies of modern boilers.

It’s always interesting to watch the reaction when the group learns that the wire-to-water efficiency of current-generation wet-rotor circulators is typically in the low 20-percent range, as can be seen on the red curve in Figure 1. I was equally surprised when first informed about these wire-to-water efficiencies.

Notice that peak wire-to-water efficiency is achieved when the circulator operates near the middle of its pump curve. If the piping system forces the circulator to operate near the outer fringes of the pump curve, the corresponding wire-to-water efficiency can approach single digits, as shown in Figure 2. Clearly, such situations must be avoided.

Figure 3

Traditional Tactics

Consider the series-loop baseboard system shown in Figure 3. It has been sized to provide 10,000 Btu/hr. of heat output at each baseboard. Baseboard lengths increase in the downstream direction to compensate for decreasing water temperature. The lengths have been based on a flow rate of 5 gallons per minute, and a circuit supply temperature of 180 degrees F. This produces a nominal 20-degree F temperature drop around the circuit when it delivers 50,000 Btu/hr. The circuit is constructed of 3/4-inch, type M copper, with a total equivalent length (allowing for assumed fittings) of 280 feet.

The head loss of this circuit can be determined using tables, graphs or software. For this example, I used the “Hydronics Design Studio” software to get an estimated head loss of 13.2 feet. This corresponds to a pressure drop of 5.51 psi.

The electrical wattage that must be supplied to a circulator to sustain a given flow rate and corresponding circuit pressure drop can be calculated using Formula 1.

Where:
we = electrical wattage input to circulator (watts)
f = flow rate (gpm)
∆P = pressure drop of circuit (psi)
n = wire-to-water efficiency of circulator (decimal percentage)
0.4344 = a unit conversion factor necessary for the stated units

The electrical wattage required for a circulator operating at the peak 22 percent wire-to-water efficiency shown in Figure 1, and supplying the flow and differential pressure requirements of this system, would be:

This number suggests a small wet-rotor circulator operating on its medium or low speed setting can provide the necessary hydraulic operating conditions.

Figure 4

Parallel Perks

Although fin-tube baseboard is traditionally installed in series circuits using rigid copper tubing, this is not the only act in town. Consider the homerun distribution system shown in Figure 4. It consists of a manifold station and five homerun circuits of 1/2-inch PEX tubing, one to each baseboard. The equivalent length of each homerun circuit is assumed to be 120 feet of 1/2-inch PEX tubing.

This distribution system supplies the same water temperature to each baseboard. Thus, to supply the 10,000 Btu/hr. load to each room, all baseboard lengths are 20 feet.

Analyzing the hydraulic characteristics of this parallel system requires a bit more mathematics than does a series loop, but such analysis is well within the scope of the “Hydronics Design Studio” software. In this case, the homerun distribution system, operating at 5 gpm, produces a head loss of 3.98 feet, which corresponds to a circuit pressure drop of 1.68 psi.

Assuming a circulator with the same 22 percent wire-to-water efficiency is used, the required electrical power input to the circulator is:

This lower wattage is directly attributable to the lower head loss of the parallel distribution system in comparison to the traditional series circuit.

Although the difference in operating power of these two systems seems small - 54.4-16.6 = 37.8 watts - it can have a significant effect on the operating cost of the system over a design life of 20 years.

For example: Assume both systems require the circulator to operate 3,000 hours per year. Also assume a current cost for electricity of 14 cents/kwhr, and that this cost inflates at 4 percent per year. Under this scenario, the series loop would have an electrical operating cost $473 higher than that of the homerun system over a period of 20 years.

The calculations leading to this number are shown below.

This comparison assumes a circulator of the required operating wattage can be located and used in either system. This is not always the case, especially with fixed-speed circulators. However, variable-speed ECM-based circulators with a wide range in input power adjustment are becoming available in the United States. These products now allow closer matching of circulator power with the requirements of the distribution system.

In general, parallel distribution systems such as the homerun system described here will have lower head loss for the same rate of heat conveyance compared to series circuits, or quasi-series circuits such as one-pipe systems using diverter tees. Taking advantage of such systems is increasing important as the North American hydronics industry strives to further reduce energy use without sacrificing performance.

A watt is a terrible thing to waste …

Links