HYDRONICS e Continued from p 38
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
40
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 sT) 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 sP (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. l
John V astyan, 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.
Efficient near- boiler piping
Modern hydronic heating systems have evolved in complexity over the last few decades; no doubt we’ll continue to refine and improve upon them. With this evolution, the need for proper piping design and technique has become more important. Simple systems, such as a boiler
and one or two baseboard zones, are pretty forgiving to piping error. However, systems that include radiant heat or fan coils aren’t so tolerant. A single piping error in a radiant system, for example, could lead to a system that refuses to deliver the expected comfort and efficiency. As boiler technology has pushed
operating efficiencies to new heights, the need for correct near-boiler piping has become more critical. These components typically include the system expansion tank, fill (pressure-reducing) valve, air separator, and, in some cases, depending on system design, a circulator and system purge. The arrangement of these
components in the near-boiler piping is a critical point of consideration when installing the system, while also considering ease of access for later service work.
phc august 2011
www.phcnews.com
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