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Evaluating low-temperature water-heating options

To take advantage of HVAC energy savings, engineers must change their thinking on heating-water temperatures, as the trend drives toward greater sustainability.

By George Marshall, PE, EYP Architecture & Engineering, Albany, N.Y. October 27, 2016

Learning Objectives:

  • Understand that energy efficiency demands have pushed HVAC design to new levels.
  • Review the various products and systems that help achieve efficiency in water-heating systems.
  • Analyze low-temperature hot water (LTHW) and its benefits.

Figure 1: The graph (boiler efficiency versus return water temperature) illustrates the increase in boiler efficiency as the return water is lowered. Also notice the peak boiler efficiency is obtained when the boiler is running on low-fire and low-returnFrom the campfires we use to warm ourselves to modern condensing boilers, heating usually requires burning something to create thermal energy. Decades ago, the United States’ oil-production infrastructure used to fuel World War II was left in place after the war, and cheap oil was abundant. By this time (the late 1940s through the early ‘50s), Americans decided they no longer wanted to shovel coal into a boiler or furnace; it was much easier to simply adjust a thermostat and burn oil. Many coal-fired cast iron boilers were then retrofitted with oil-fired burners to create this convenience.

Most of these early heating systems (commercial and residential) were low-pressure steam systems and earlier gravity-fed hydronic systems, some of which are still in operation today. When the Organization of Petroleum Exporting Countries (OPEC) energy crisis hit in the early 1970s, however, oil became scarce and prices rose drastically for the first time in 25 years (from $21/barrel to $52/barrel). This was the first time Americans took a serious look at energy efficiency, and the phrase “boiler efficiency” was born.

From the 1970s on, the industry started relying more on hydronic heating systems, typically a combination of oil- and gas-fired equipment. These systems used 180 to 200°F (with a 20°F delta T), heating hot water for all terminal devices (including air-handling units, fan coil units, unit ventilators, and unit heaters). Revolutionary at the time, they typically reached the 80% efficiency range for hydronic heating systems with atmospheric or forced-draft boilers. Some of these boilers can even approach the 82% annual fuel-utilization efficiency (AFUE) threshold, but were held back by the higher return-water temperature needed to prevent condensation in the boilers.

Figure 2: The diagram shows a typical dual low-water temperature boiler piping return. The lower the return water, the higher the boiler efficiency.Energy efficiency

Over the past 15 years, the HVAC industry has seen a dramatic shift in design from conventional boilers and other HVAC equipment to anything and everything characterized by high efficiency. This primarily includes motors, use of variable frequency drives (VFDs), chillers, condensing units, boilers, furnaces, heat recovery, and controls. Interestingly, new energy codes have strict guidelines for the efficiencies of motors and cooling equipment, but many have not yet caught up to the full potential of modern boiler design. As an example, the 2012 International Energy Conservation Code (with a 2014 amendment) still allows the use of gas-fired boilers and furnaces with AFUE as low as 78%. Understandably, this is acceptable for small steam boilers to operate in this range, but not for a large plant.

There is a vast amount of opportunities and strategies for enhancing a building’s overall energy usage. For instance, the invention and widespread implementation of direct digital controls (DDC) and VFDs are an engineer’s dream come true, allowing him or her to design a control sequence of operation that is only limited by imagination. Heat recovery is another, with enthalpy wheels and 75%-efficient high-performance hydronic base heat-recovery systems now widely available.

Figure 3: This shows a typical 12 million Btuh condensing boiler plant in Albany, N.Y. Condensing boilers typically have a very small footprint compared to the boiler output.Today’s industry buzzwords include net zero energy (NZE) and high-performance building design (HPBD), both of which take into consideration not just mechanical systems design but also the building as a whole—including envelope construction, daylighting studies, LED lighting, water consumption, etc. This type of holistic approach is requested by clients more often than not, in a proactive attempt to save money and improve overall building and occupant performance. A typical office-building owner in the Northeastern United States 10 years ago, for instance, would have been happy to use 80 to 120 kBtu/sq ft/year. Today, the same building would be expected to use as little as 30 to 60 kBtu/sq ft/year.

Even with all of our advancements and innovations, much of the time we still have to burn something to bridge the gap and meet the NZE ideal. This brings us back to the condensing boiler. With an ever-expanding nationwide natural gas network, we have a clean-burning fuel that can be used effectively in condensing boilers. By using lower temperature return water, we can efficiently capture more latent heat when operating the boiler in the condensing mode.

To take advantage of this energy-saving potential, engineers and designers must change their thinking on heating-water temperatures. An example is the recent engineering design for several different types of large commercial buildings using a 130°F low-temperature hot-water supply with a 20° to 30°F delta T, as opposed to the traditional 180°F water supply. This small design change bumps up overall boiler plant efficiency from percentages in the low 80s to the mid-90s, depending on boiler-firing rate and heating-water-supply (HWS) temperature.

Boiler efficiency

The lower the return-water temperature, the higher the boiler efficiency. Some boiler manufacturers have actually incorporated a dual-return connection at the boiler to accommodate a heating-water return and a domestic water-heater or snow-melt system return, further lowering the overall return-water temperature to the boiler. Additionally, running multiple boilers at lower firing rates to match the load can produce as much as a 2% increase in boiler plant efficiency.

The complications of using low-temperature hot water (LTHW) include design challenges, operational issues, and increased cost of the terminal equipment. One of the most significant design challenges/obstacles to date has been selecting and obtaining the most effective equipment. Most of the major air handling unit (AHU) manufacturers can provide coil selections using LTHW.

Some of the variable air volume (VAV) box manufacturers use larger coils, which usually require a transition at the end of the box (making the overall size of the VAV larger) or actually duct-mounting the coil. Unit heaters, cabinet unit heaters, blower coils, and fan coil units must be upsized to use the increased coils area and reduced fan speeds to obtain design capacities. In some cases, the design team must then use small AHUs instead of fan coil/blower coil units to obtain the desired output capacity. Since LTHW requires larger coils/terminal units, the cost of the equipment is slightly higher.

However, it is imperative to remain careful about using convectors, finned-tube radiation, and panel radiators when applying LTHW, as the output capacity of this equipment using LTHW is not typically listed or obtainable. This is why most LTHW systems (except for radiant floor or snow-melt systems) tend to be mostly air-side-type systems.

Figure 4: This condensing boiler control panel is shown during winter operation. The boiler is providing 126°F low-temperature hot water, while the outside temperature is 25°F.

Using 180°F water provides very high-approach temperatures on coils. This is desirable, since it allows the coil surface area to be smaller while creating a built-in safety factor to offset those momentary hours when the outside temperature drops below the ASHRAE weather data. LTHW systems, on the other hand, produce very low-approach temperatures and effectively eliminate this built-in safety factor. Vigilantly double-checking calculations and equipment selections will usually keep the designer out of the woods, but there are always other construction-related factors involved that can potentially challenge or even defeat a good system design. This is where the control system and a well-thought-out, strategic sequence of operation can step in to save the day.

From an operational standpoint, in most cases, the LTHW building will operate as designed until the outdoor air (OA) temperatures drop 10° to 30°F below OA design temperature. To offset this handful of hours per year, engineers should consider allowing the control system to incrementally increase to the LTHW temperature to match the building load. Other strategies include allowing VAV boxes to momentarily float above their minimum setpoint and starting morning warmup earlier. This minor deviation in operation yields significant benefits and does not create any noticeable energy penalties.

LTHW is quickly becoming the industry standard, leading equipment manufacturers to redesign their equipment to accommodate future system needs. In addition, most gas utility companies and state energy authorities offer rebates to help offset the increased systems cost of using LTHW. These LTHW systems can effectively increase boiler plant efficiency by as much 15%, depending on baseline comparison.

This is a significant contribution to HPBD in relation to reducing energy cost and carbon dioxide emission reductions. Central-campus-type steam and high-temperature hot-water distribution systems are starting to become decentralized, taking advantage of this potential. All signs point to even more clean-energy sources being developed in the near future, but for now, LTHW is truly starting to make a difference.


George Marshall is a senior mechanical engineer with EYP Architecture & Engineering.