Finely-tuned Hydronic Systems: Sum of Many Parts
By John Vastyan
Go ahead: Ask the hydronic gurus, say Dan Foley, John Abularrage or Paul Pollets, what it is that makes the “perfect boiler system.” Well, I did it for you, and one thing’s for sure: It’s more than just shopping for boilers.
Today, with stratospheric energy rates, homeowners and building managers alike are pushing the need for high efficiency systems. In response, installers must consider overall specifications, budget, boiler options, controls and heat distribution because, after all, high efficiency systems are the sum of many parts.
Condensing boilers, deftly extracting heat from condensate, have now pushed combustion efficiency into the 95 to 99 percent range. System components play a key role too: Variable speed circulators are attuned to precise Delta T and sleek, Btu-spitting zone valves do their part in maximizing overall system efficiency.
That’s smart use of energy. This also becomes a key system advantage when long-term operational cost of systems is scrutinized. When trade professionals can calculate a three- to four-year payback for new equipment, there’s real incentive to move ahead with the project.
So, if the spec dictates high efficiency, the next question may be, “How high?” Non-condensing boiler technology offers a lot, with broad capabilities and fuel efficiency into the mid-to-high 80s. If that's not enough, the conversation often turns to the latest, über-technology.
But, as many of you know, other facets influence the type of system that’s best suited for the job. A key factor in the selection of a boiler has to do with the anticipated temperature of the return water or glycol mix. If the temperatures are low (say, in the range of 60 to 130 F), a condensing boiler will operate most efficiently. If return water/glycol mix temperatures are high – above 130 F – then non-condensing equipment may make more sense.
“Along with the need to consider system temperature, stage-fired or modulating units should be considered,” said John Abularrage, president of Stone Ridge, N.Y.-based Advanced Radiant Design Inc. “These units supply less than full input when the full input of a boiler isn’t required. In many cases, a condensing boiler even gains efficiency when it’s running at a lower firing rate. Multiple boilers can be another way to increase system efficiency, again allowing for lower input to the system when full input isn’t needed. “
The condensing + non-con twist
One of the more interesting approaches to commercial boiler system design is the deliberate joining of condensing and non-condensing boilers. Mixing condensing and “non-con” boilers in the same system is a concept that’s getting more attention these days.
“By setting a condensing boiler to be the lead boiler when the system temperature is at its lowest and/or outdoor reset controls are bringing water temperature down, the installer or system designer can better ensure that the investment made in a condensing boiler will be worth it, in terms of efficiency gain and fuel-savings,” added Dan Foley, president of Foley Mechanical Inc., based in Lorton, Va.
“If more heat is needed in the system, a non-condensing boiler can be the next in line,” he continued. “The second non-con boiler — and additional downstream boilers, if called for — would then be an advantage, considering lower initial cost and suitability for efficient operation with higher return temperatures.”
The basic concept is to operate the condensing boiler when loads are low and, thus, so are supply water temperature needs. As the load increases, such as when weather gets colder, so does the supply temperature requirement with outdoor reset control. The higher the water temperature, the less condensate that’s produced within a condensing boiler, and its efficiency decreases toward that of a substantially less expensive, non-condensing boiler.
“So the idea is to shift load to conventional boilers as the water temperature goes up, since they will have efficiencies comparable to the condensing boiler operating in a non-condensing mode,” said Foley. “This is likely to make sense in larger systems using relatively high design load temperatures, such as those using fan-coils, air handlers, baseboard convectors or radiators.”
There are also regional differences to consider. “If the boiler system will be used as a backup for air-sourced heat pumps, geothermal heat pumps, or solar, these factors will play into the question importantly,” said Paul Pollets, president of Seattle, Wash.-based Advanced Radiant Technology. “Typically, these systems provide lower return water temperature, keeping the condensing boilers in their ‘sweet spot’ most of the time.”
In areas where these systems are most popular, winter seasons are tempered somewhat, so high water temperatures are not routinely needed to heat interior space. These areas are likely to have broad outdoor temperature swings as well. Systems that can change the temperature of water used for heating tend to accommodate these outdoor temperature swings most efficiently.
System efficiency: the sum of many parts
Though the boiler(s) may be the most important single piece of a hydronic system, overall system efficiency depends on the interrelationship of several key parts, all of which are changing and evolving as new ideas and technology influence their role in the mix.
1. Boiler efficiency
Boiler efficiency is determined by two key factors: combustion efficiency and thermal efficiency. “How effectively the boiler interacts with the hydronic system is determined by its ability to deliver heat either quickly, or slowly, depending chiefly on the needs of the system and the ability of the boiler to adjust to changes in the system’s demand for heat,” explained Joan Mishou, manager of applications engineering at Laars Heating Systems Company. The common term is “to size to the load.”
System efficiency is at its best when the equipment works at peak performance, with fuel consumption happening at the highest levels of combustion efficiency, at all levels of heat demand.
“But no doubt, one of the key factors in attaining optimal system efficiency today is the advent of condensing boiler technology,” added Mishou. “Condensing boilers, like our Rheos+ or NeoTherm boilers, are built to extract latent heat from the moisture that forms in the condensing heat exchanger, dramatically enhancing combustion efficiency.”
Laars NeoTherm boiler Rheos+ boiler installations
The use of a condensing boiler can play an even more important role. “Their tough resistance to thermal shock and the ability to accept low return water temperatures puts them in a category of their own and opens up many new possibilities for hi-volume, cold-start systems. One example is a commercial snow melt system,” said Watts Radiant’s John Sweaney. “A condensing boiler takes very low inlet temperatures in stride; in fact, the lower temperature of incoming water (or a water/glycol mix, as is usually the case), the higher the combustion efficiency of the boiler.”
2. System performance = efficiency
“Other important factors in determining system efficiency include modulation, or staged firing, as opposed to On-Off,” added Abularrage. “This demonstrates the giant strides the industry has made during the past several years. Modulating and staged fired boilers reduce fuel consumption by ‘sizing to the load’ so that the amount of heat produced by the system precisely matches the need.”
According to Mishou, another key facet is that more sophisticated controls are now capable of sampling changes over time and “learn” the responses of the system to changes in conditions such as heating load, outdoor air temperatures and firing stages of the boiler(s).
Tying to the earlier statement about outdoor reset controls, a system’s response to outdoor temperatures is important. Other variables include water storage temperatures and system loop temperatures.
3. Heat distribution
Heat distribution and its impact on overall system efficiency consists chiefly of Btu load, the need for high or low temperature distribution and the presence (or lack of) insulation, and its effectiveness. Also, will the system be convection or radiant? High or low mass?
Of course, large radiant systems require a boiler or boilers with high output. A key advantage, however, is that when the thermal mass of a floor or heated surface has reached temperature, shorter and less frequent boiler cycle-times are required.
“Better yet, a boiler system with modulation permits the heating, and, later, steady heat-maintenance, of the heated surfaces,” added Foley. “Either a fully modulating burner or the lead-lag staging of boilers would allow a system to meet ever-changing load requirements for best efficiency. Another option is to add mass to the piping system to increase boiler run times during periods of low demand. For this, insulated storage tanks can be used to contain volumes of water, easily adding mass to a piped system.”
“Snowmelting systems pose a different challenge, high demand and high mass with extremely cold water/glycol temperatures,” added Sweaney. “Here, the challenge is not short-cycling of the boiler. Thermal shock happens when freezing return-water temperatures come crashing into the heat exchanger in a long, hard, cold start.”
The new generation of condensing boilers takes this icy slap in stride. Many modern boilers aren’t susceptible to thermal shock, because their heat exchangers and waterways are built to handle it, especially condensing systems that give their best performance under these conditions.
The role of circulation
One of the most important things in achieving optimal circulation for hydronic systems is for installing contractors to match a pump’s performance, or flow characteristics, to the specific job that it needs to perform within the system.
These needs can be accomplished manually or electronically. The latest advancements are responsive, energy-wise circulators such as Taco’s wet rotor, variable speed (VDT, or variable ?T) circs, and, when coupled with Zone Sentry or iSeries zone valves, circulation happens with extreme efficiency, especially during partial-load operation.
New, variable speed drive VDT circs have an integrated microprocessor-based variable speed differential controller. Installers simply dial in the design delta-T of the system or zone (from 5 – 50 F). The circulator automatically adjusts its performance to match the system’s ideal Btu/hr output, while reducing fuel consumption four to five percent and eliminating velocity noise.
Unlike a ?P (differential pressure) pump that’s always on, always drawing power 24/7/365, a Delta-T circulator shuts off when there is no call for heat. “When it comes to comfort, it’s all about supplying the right amount of Btu to zones at the right time,” explained John Barba, Taco’s training program manager.
“With a VDT circulator, the specific amount of heat delivered to the structure is optimized to match a building’s heating load, regardless of how many zones are calling for heat, or as outdoor temperatures change,” he added.
Return water temperatures play a key role in optimizing the system efficiency and performance of both cast iron and modulating-condensing boilers. Water return temperatures and boiler cycling are optimized by controlling the Delta-T.
The perfect hydronic storm: dropping Delta-Ts
Another concern is pressure differential within the system. As zone valves close, a system curve intersects a pump curve at higher pressure differentials.
One of the best solutions is to use a mid-flow, low head, flat-curve circulator. With such a pump, system pressure rises minimally, eliminating the need for a bypass valve. But – if the job has higher head requirements than the circulator can deliver, a better solution may be a variable speed pump.
“If all of the zones in a system are calling for heat, we may find that the delta-T drops to 16 degrees, not the 20 it is typically designed for,” said Barba. “Doesn't sound like much, right? But that equates to about a 20% difference. With only two zones calling, the delta-T drops to about 15 degrees, a 25% difference. And with only one zone calling, the delta-T drops to 12 degrees, a whopping 40% difference.”
“Solve the dilemma of dropping Delta-Ts by using a fixed delta-T, variable-speed circ,” he says. “You may never have to worry again about over-sizing a circ.”
Rather than searching for the point where the system curve intersects the pump curve, let the pump curve self-adjust every moment and every day of the heating season.
Variable speed circs are easy to set up. Simply dial in the required delta-T. “The simplicity of it — pump choice, installation and performance — is a huge asset for us,” concluded Pollets. “We know exactly what pumps to apply and how to finesse hydronic flow . . . like dialing in comfort for our customers.”
And so the efficiency equation all comes back to the sum of many parts. If the system’s many components are designed to work in concert with one another, the high efficiency grail can be achieved.
John Vastyan, a journalist whose work focuses on the plumbing and mechanical and radiant heat industries, owns Common Ground, a trade communications firm based in Manheim, Pa. He can be reached at 717/664-0535 or cground@ptd.net.