The Other Efficiency
John Siegenthaler, PE
It's been practiced for years. A boiler with an AFUE rating of 86 percent is presented to a prospective customer as clearly superior to a competing boiler having an AFUE of 84 percent. Better yet, a condensing boiler with an ultra high efficiency of 95 percent makes a conventional “dry smoker” at 86 percent efficiency look like a real dog.
Now that the price of crude oil has coasted over $70/barrel, the word “efficiency” is on the minds of heating professionals as well as those seeking their services. Everyone understands that higher efficiency is better - although there's considerable disagreement on what the most efficient approach to space heating often is.
I'm often asked, “Is a radiant heating system more efficient than a forced-air system?” Or, “Is a slab-type system more efficient than a staple-up tube and plate system?” I'm sure many of you field similar questions on a daily basis.
Defining EfficiencyDefining Efficiency: It's pointless to state that one process is more efficient than another without first defining efficiency. Here's a good general definition:
Let's say, for example, that a given boiler puts out 80,000 Btu/hr., while consuming 0.8 gallons of No. 2 fuel oil per hour of operation.
To make the resulting efficiency come out as a percentage, the top and bottom of the fraction have to be in the same units. In this case, we'll convert fuel input into Btu/hr.
Now the input and output quantities can be put into the formula:
It's All RelativeConscientious heating pros expect high efficiency from the heat sources they install. To them, a boiler offering 2 or 3 percent higher efficiency deserves a second look as a possible replacement for their current offering.
Interestingly, many of these pros are not as concerned about the “delivery efficiency” of the hydronic systems they design or install.
Let's define delivery efficiency:
Since the two efficiencies have the same units, and are both based on the same concept of energy required to deliver heat, they can be compared.
It probably comes as no surprise that the hydronic system in this comparison has a delivery efficiency almost four times greater than the forced-air system. Water is vastly superior to air when it comes to being a conveyor belt for Btus.
We're Not There YetSince condensing boiler technology now can give us thermal efficiencies into the mid-90 percent range, you might think there's not much room for improvement. We're wringing out just about all the thermal energy possible from a given amount of gas. The current thermal efficiency numbers for oil-fired equipment is not far behind, and will inevitably make incremental gains in the future.
One might conclude that we've gone just about as far as possible with the efficiency issue. However, the thermal efficiency of a boiler is not the same as system delivery efficiency, and there's considerable room for improvement with the latter.
Here's an example: A couple of years ago I got to play “radiant doctor” for a 10,000-sq.-ft. house with 40 circulators in its hydronic system. Some of these circulators were small zone circs, others were rated at 1/12 horsepower. Let's say the average wattage of the circulators was about 90 watts each.
At design load, that system's two boilers put out 400,000 Btu/hr. Assuming all circulators are operating at design load, the system's delivery efficiency is:
That's not much better than the previously cited forced-air scenario. Just the circulators in that system could add 40 x 90 x 3.413 = 12,300 Btu/hr. of heat input to the building.
That system brought back memories of an RPA keynote address from Richard Trethewey, the resident hydronics pro on PBS's “This Old House.” He was comparing European hydronic system design to standard practices in North America. One of his observations was that the North American hydronics industry tends to “overpump” its systems. I absolutely agree. Thanks Richard, for getting this concept out on the table for further discussion.
So what's going to improve the hydraulic efficiency of future hydronic systems? Here are some things I think will help the North American hydronics industry get more Btu/hr. delivered per watt of electrical pump power. Some are based on pending new technology; others simply require a change in how we apply present-generation hardware.
1. New motor technology. The PSC (permanent split capacitor) motors used in most current-generation wet-rotor circulators are reliable and inexpensive to build, but their overall efficiency is not very good. A typical wet rotor zone circulator has a peak wire-to-water efficiency in the range of 18-22 percent. You've seen new motor technology with other devices; you'll soon see it with small circulators. The potential gains in wire-to-water efficiency will surely get your attention.
2. Variable-speed pumping. Just as modulating boilers have gained a significant share of the new boiler market, variable-speed circulators will soon be displacing a significant share of fixed-speed circulators. These new circulators will likely be multifunction, programmable devices that vary speed based on their operating mode. Constant differential pressure control or one of its variants will become standard for systems with zone valves, valve actuators or thermostatic radiator valves. The result will be “cruise control” for circulators. Set it and forget it. Differential pressure bypass valves will eventually take their place next to tankless coils in the museum of hydronic heating technology.
3. Higher temperature drops. We have to stop thinking that water “wants” to or needs to drop 20 degrees F as it flows around every hydronic piping loop we design. Instead, we should design for 30- to 40-degree F temperature drops where they are appropriate (e.g., in certain primary loops, panel radiators, high mass boilers and air handler coils). Doubling the circuit's delta T cuts the flow required for a given rate of heat transfer in half! The Europeans figured this out years ago and routinely use higher delta Ts to reduce tube size, circulator size and, most importantly, pumping power.
4. Reduced head loss. As hydronic system designers, we make daily “trade-off” decisions between tube size and circulator power (i.e., should I stick with 1-inch copper for this circuit and use the 1/12 horsepower circulator, or go to 1.25 copper and drop down to a 1/25 horsepower circulator?).
The answer depends on what yields the lowest life-cycle cost, which includes not only installation cost but also the operating cost over an assumed design life. Certainly you'll spend more for the 1.25-inch copper, and somewhat less for the smaller circulator, but few of us bother to factor in the savings in operating cost over the life of the system. The latter is where the big savings potential exists.
Assuming electricity costs $0.10 per kw/hr., and that the primary loop circulator operates for an average of 3,000 hours per year, the additional operating cost for the larger circulator over a 20-year design life is:
Keep in mind that this calculation doesn't factor in any inflation for the cost of electricity. With even modest assumptions on such inflation, the added operating cost of the larger circulator over the 20-year period could easily exceed $1,000! What do you think his customer might conclude if he knew how to make these calculations?
A Modest GoalSuppose you could build a hydronic system that delivered 100,000 Btu/hr. to the heat emitters using only 50 watts of pumping power. The delivery efficiency would be 2,000 Btu/hr./watt, several times higher than the hydronic systems we've used as examples, and a quantum leap above the delivery efficiency of a forced-air system.
This somewhat random target efficiency could be achieved by simply doubling the wire-to-water efficiency provided by a wet-rotor circulator in some currently installed and well-performing systems. No changes in piping or controls. I feel such delivery efficiency is not only achievable in the next few years, it could very well be exceeded.
The North American hydronics industry must get serious about conserving the electrical energy used to move heat from where it's produced to where it's needed. As you read this, manufacturers already are working on the next generation of hydronic circulators. However, you don't have to wait for new high-tech products to arrive to get with the program.
Consider upsizing your piping one size and see what a difference that makes in circulator sizing. Use accurate design tools to evaluate these trade-offs. Use copper or polymer tubing rather than threaded piping and fittings to reduce head loss. Don't “overpump” then correct for it with throttling valves, especially in series loops. Compare the wire-to-water efficiency of currently available circulators, especially when larger circulators are needed. Try to operate the circulator in the middle of its pump curve near the “best efficiency point.” Don't get carried away with zone circulators. Just because you can create 25 individually-pumped zones in a house doesn't mean you should.
Will we ever achieve “perpetual flow” in hydronic piping systems? Nope. But it's always good to work in that direction. Providing higher delivery efficiency builds an ever-stronger case for hydronic systems, especially as potential customers deal with tumultuous energy prices. It's a situation rife with opportunity.