Though long a standard in hydronic-heating-system design, a 20°F difference between supply- and return-water temperature — usually involving 180°F supply water and 160°F return water — does not yield the highest-possible efficiency. This article will discuss how a higher temperature differential (ΔT) and return-water temperatures well below 160°F can reduce the capital cost of mechanical systems and significantly increase operating efficiency. Additionally, it will discuss how further savings can be realized if a design incorporates modulating and condensing boilers.
One explanation for why a 20°F ΔT has become standard is that with water temperatures so close, there are no significant, rapid, or repeated expansions and contractions of a heat exchanger. Thus, the danger of thermal shock is minimal.
Another explanation for the standardization of a 20°F ΔT has to do with mathematics and design engineers' comfort with an easily conceived and implemented system. Consider the formula for measuring energy output:
Btuh = 500 × ΔT × gpm
Plugging in a 20°F ΔT yields:
Btuh = 500 × 20°F × gpm
Btuh = 10,000 × gpm
10,000 Btuh = 1 gpm at 20°F ΔT
In commercially sized applications, this translates easily to:
1,000,000 Btuh = 100 gpm
3,000,000 Btuh = 300 gpm
In short, maintaining a 20°F ΔT keeps the numbers simple and reduces the chance of mathematical error. Unfortunately, this simplicity does not translate to efficiency. While a 20°F-ΔT system makes for an easy and safe design, it does so at the expense of short- and long-term cost-effectiveness.
Looking at the formula for measuring energy output, as ΔT increases, lower flows produce the same energy output. For example, raising ΔT to 40°F cuts gallons per minute in half (Figure 1). This is true each time ΔT is doubled.
Plugging in a 40°F ΔT yields:
Btuh = 500 × 40°F × gpm
Btuh = 20,000 × gpm
20,000 Btuh = 1 gpm at 40°F ΔT
In commercially sized applications, this translates to:
1,000,000 Btuh = 50 gpm
3,000,000 Btuh = 150 gpm
The ability to output the same energy with a lower flow rate has a tremendous positive impact on the economics of a system. Pumping capacity is reduced, making smaller equipment with less electrical draw appropriate. For example, with a 50-percent reduction in flow, a 10-hp pump can be replaced with a 1.25-hp pump. Similarly, smaller pipes can be used throughout the system to reduce materials and installation costs.
Even though a greater number of copper heating coils may be required in terminal units in a high-ΔT system, the effect on initial project costs often is minimal. In warm climates, where terminal units primarily are sized to accommodate a greater number of cooling coils, unit size may not increase. And in cold climates, where larger units may be needed, the cost can be significantly less than that of larger piping and pumps.
Older “conventional” boilers had only one level of power: 100 percent. They either were on or off. This created cycling losses each time they were shut down. Their heat exchangers cooled off and had to be fully “reheated” before heat transfer could begin again. Once the units were re-started, the 100-percent firing rate may have been far more than what was required to meet a building's load.
With turndown ratios of 4-to-1, 5-to-1 — even 20-to-1 — fully modulating boilers are designed to operate at less than 100-percent input. For example, a boiler with 20-to-1 turndown can operate at just 5 percent of its maximum capacity and increase output in precise 1-percent increments up to 100-percent capacity. The firing rate of a modulating boiler is precisely matched to actual demand in real time. Instead of cycling on and off, these units fire continuously at lower capacity, minimizing cycling losses. This steady operation helps to maintain a constant temperature throughout a system, minimizing overshoot.
The thermal efficiency of modulating boilers increases at part load, offering greater seasonal fuel savings. Not only is less fuel burned, when these units operate at “part load,” heat transfer is enhanced. Temperature within the heat exchanger is maintained, while the time combustion gases are in contact with the heat-exchanger surface is increased.
For every pound of water vapor that is forced into a liquid state, approximately 1,000 Btu of latent energy is released. Known as condensing operation, this increases the efficiency of boilers and water heaters by about 11 to 12 percent — turning more of a unit's fuel into usable heat. What's more, condensing occurs naturally — when the vapors cool below their dew point, which typically is 135°F. With the flow of cold (less than 130°F) water into a heat exchanger, just about any boiler will begin to condense.
With a conventional boiler, condensing operation is avoided because when latent energy is extracted from water vapor, acidic condensate remains on the surface of the heat exchanger. Unless the heat exchanger was built from the highest-quality materials and designed to drain freely, it will be destroyed over time. To protect conventional boilers, which are made from less-expensive materials, from condensing operation, dedicated boiler pumps, mixing valves, and temperature-averaging components are utilized to pre-warm entering water to above 140°F.
Engineers are encouraged to design systems that utilize low (less than 130°F) return-water temperatures to foster the condensing process. In such an environment, the water vapor found in exhaust gases has the opportunity to drop below its approximately 135°F dew point and begins to condense when it contacts the heat-exchanger surface. This releases additional energy that otherwise would be lost up the flue.
The ability of modulating condensing boilers to handle variable flow rates (even no-flow conditions) and extremely low water-side pressure drop reduces project costs even further. With their condensing capabilities, they can be installed directly on a heating loop with no risk of thermal shock. Eliminating primary/secondary piping and pumping and mixing valves further simplifies overall systems and lowers materials costs.
By increasing ΔT and decreasing flow, designers can build smaller, more-streamlined systems. When low-temperature water is returned to a condensing and modulating boiler, long-term fuel savings are even more significant.
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Currently director of global sales, Neil Pilaar has contributed to product- and market-development initiatives for AERCO International for more than 18 years. A member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the American Society of Plumbing Engineers, he holds a bachelor's degree in mechanical engineering from the New Jersey Institute of Technology.