Reducing energy consumption with high-temperature heating and ventilation

by Katie Daniel | May 6, 2016 9:44 am

by Marc Braun
The Better Buildings Alliance, an initiative of the United States Department of Energy (DOE), is designed to improve the efficiency of commercial and industrial buildings in the U.S. by driving leadership in energy innovation.

The commercial buildings sector—where 15 percent of the total floor space comprises warehouse and distribution space—consumes nearly 20 percent of all energy used in the United States. As a nation, the U.S. spends more than $200 billion annually to power the country’s commercial buildings. Unfortunately, much of this energy, both electric and natural gas, is wasted because of inefficient or outdated technology. Energy-efficient technologies, if installed, commissioned, and operated properly, are a cost-effective way to save money and reduce pollution (Figure 1).

Over the past 25 years, the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE), 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, has dramatically increased the minimum efficiency requirements of commercial buildings and their associated equipment. The Pacific Northwest National Laboratory (PNNL) reported that over this period there was a 44 percent improvement in lighting efficiency, 27 percent improvement in cooling efficiency, 21 percent improvement in building envelope efficiency, and 37 percent improvement in overall efficiency across commercial buildings built to meet ASHRAE 90.1. Unfortunately, PNNL reported only a one percent improvement in heating efficiencies during those same 25 years.(See M. Rosenberg, et al’s 2014 article, “Roadmap for the Future of Commercial Energy Codes,” for the Pacific Northwest National Laboratory. The 2012 International Fuel Gas Code can be found online at publicecodes.cyberregs.com/icod/ifgc/2012/icod_ifgc_2012_6_sec011.htm[1])

For many of these buildings not requiring space cooling, non-centralized equipment such as unit heaters provide the space-heating. Unit heaters are found in commercial and industrial buildings throughout the United States—most prominently in warehouses, distribution centers, and loading docks. In addition to space-heating, these buildings require ventilation systems to meet the minimum required code ventilation rates of ASHRAE 62.1. For most applications, separate systems provide space-heating and outside air, adding project complexity and energy consumption.

The Department of Energy published a study, “Field Demonstration of High-efficiency Gas Heaters,” where it discussed how high-temperature heating and ventilation (HTHV) direct gas-fired heaters dramatically reduce energy consumption when replacing unit heaters in warehouses and other high-bay buildings.

These direct-fired technologies—often referred to as ‘make-up air heaters’—have been widely adopted as high-efficiency ventilation units. The surprising thing is specialized versions of these products can accomplish high-efficiency space-heating when ventilation is not required, such as during unoccupied times. This is because they feature an innovative high-discharge temperature and high-velocity discharge. In the study, HTHV direct-fired gas heaters demonstrated a 20 percent gas savings when replacing the heating function of standard indirect gas-fired unit heaters. (See J. Young’s 2014 report “Field Demonstration of High Efficiency Gas Heaters,” prepared for Better Buildings Alliance, Building Technologies Office, Office of Energy Efficiency, and Renewable Energy of the U.S. Department of Energy). If widely adopted to replace traditional heating systems, these HTHV technologies could substantially reduce energy consumption and utility bills for warehouses and other types of  high-bay applications throughout the country.

Direct gas-fired heaters meeting American National Standards Institute (ANSI), Z83.4, Non-recirculating Direct Gas-fired Industrial Air Heaters, blend 100 percent outside air directly with a gas flame. Every joule (Btu) of energy created from the gas combustion is delivered directly to the space without any heat exchanger losses. HTHV technologies are those that ANSI Z83.4 certified to a maximum discharge air temperature, greater than or equal to 66 C (150 F), and a maximum temperature rise, greater than or equal to D78 C (D140 F).

During winter operation, the heated outdoor air is delivered at temperatures slightly above the ambient space temperature to a maximum of 77 C (160 F), offsetting air being displaced by an exhaust system or building infiltration. Temperature controls typically maintain a relatively constant discharge air temperature for ventilation applications, or space temperature for heating applications. This is done by modulating the gas flow (i.e. typically, modulation is 20:1) to adjust for varying outdoor temperatures, space-heating, and ventilating needs.

During mild weather, the equipment is generally fitted with an economizer thermostat to disable the burner when the outdoor temperature approaches the desired indoor temperature. Due to the ventilation air and the combustion air being delivered by a common blower, gas combustion heating cannot occur without proper ventilation. This feature causes the safety standards for both Canada and the United States to harmonize with these technologies. Equipment certified to ANSI Z83.4 can be installed with a minimum amount of ductwork, providing an efficient and cost-effective installation. Additionally, the weatherized technology offers installation flexibility as units can be placed indoors or outdoors or in horizontal or vertical configurations while still being always connected to 100 percent fresh outside air.

The 2012 International Fuel Gas Code (IFGC) requires all HTHV direct gas-fired technologies to be certified under ANSI Z83.4. These technologies are covered specifically under Section 611 of the IFGC and by reference in the 2012 International Mechanical Code. The IFGC states ANSI Z83.4 technologies are to be used for commercial and industrial applications for heating and ventilation requirements per the scope of that standard.

Operating modes
HTHV technologies serve commercial buildings in three operating modes:

• ventilation/make-up air;
• heating and ventilation; and
• space-heating.

The ventilation or make-up air mode consists of continuous operation during occupied times, meeting the outside air requirements of the building by tempering the air and bringing it into the building at or near the space temperature.

Ventilation rates can be controlled via variable-frequency drive (VFD) based on an occupancy sensor, pressure control, or any other demand-control ventilation (DCV) strategy. An application many are familiar with is for commercial kitchens, where exhaust is deployed through a kitchen hood and the make-up air unit replenishes this removed air. A second application is meeting ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, minimum ventilation requirements for a commercial building. In this example, ASHRAE 62.1 calls for 0.3 L/s∙m² (0.06 cfm/sf) for a warehouse facility. This outside air can be brought in and exhausted through the use of ANSI Z83.4 technologies. The units are direct gas-fired, causing no flue losses, and all the energy from the heat of combustion to be delivered into the space.

The heating and ventilation mode consists of continuous operation during occupied times to meet the outside air requirements of the building. A fully modulating gas valve adjusts the discharge temperature of the make-up air unit to control space temperature. Every degree above the space temperature provides additional convective heat to be delivered into the space. This additional heat is used to offset any heat loss, including conduction or skin, infiltration, or other process heat losses.

The modulating valve typically controls the discharge between its minimum setting and the maximum discharge temperature allowed through ANSI Z83.4, which is capped at 77 C (160 F). The higher the discharge temperature capability, the larger the amount of available net kW (Btu/hr) to offset additional loads per volume of ventilation air will be. If the amount of net kW (Btu/hr) is higher, it means the units are more flexible to cover large swings in heat loss. This can lead to quicker recovery from set-backs and from any disruption such as opening a garage door at a shipping warehouse or bringing in a large cold mass of steel at a factory.

Additionally, more net Btu/hr (kW) for the same L/s (cfm) can lead to less equipment meeting the heat load of the building. This reduction of heating equipment can lead to lower installation costs due to reduced unit counts, gas piping, electrical runs, and labor.

Space-heating is the third mode of operation and the concentration for much of this article. In this mode, the HTHV units typically operate intermittently under the control of a thermostat to meet the heat loads of the building. The direct connection to 100 percent outside air causes the HTHV unit to bring in the outside air whenever there is a call for heat. While this practice may seem counterintuitive, HTHV technologies have shown improved energy-efficiency over other space-heating technologies for high-bay applications through a recent field demonstration and modeling study.

DOE field demonstration study findings
Over the 2013–2014 heating season, a mechanical equipment supplier collaborated with the DOE to better understand the thermal performance and potential energy savings of HTHV direct-fired heating technology through a field demonstration at a warehouse outside of St. Louis, Missouri. (See J. Young’s 2014 report “Field Demonstration of High Efficiency Gas Heaters,” prepared for Better Buildings Alliance, Building Technologies Office, Office of Energy Efficiency, and Renewable Energy of the U.S. Department of Energy). This demonstration measured the energy consumption of the high-efficiency HTHV direct-fired gas heaters compared to similar, standard-efficiency models in a side-by-side comparison.

The warehouse host site featured several aisles of shelving racks extending to the approximately 7.3-m (24-ft) high ceilings, as well as six loading docks across 3900 m² (42,000 sf). The new HTHV gas heaters were placed in the same general area as the existing unit heaters with the new and existing equipment operating in alternating months. WiFi-enabled thermostats controlled both existing and new equipment with a temperature set point of 16 C (60 F). Monitoring tools collected data on equipment operating hours, temperatures throughout the building and loading dock openings, and other factors. This was supplemented by utility bills and weather data.

As the energy consumption of heating equipment depends on outdoor conditions, energy consumption was normalized over the monitoring period according to the number of heating degree days (HDD). In addition to energy consumption, the study also monitored indoor temperatures at 1.5 and 6 m (5 and 20 ft) off the floor in order to understand temperature stratification within the building.

Results
This field study demonstrated the energy saving and operational benefits of 100 percent outside air and HTHV direct-fired gas heaters. In a side-by-side comparison of alternating months over the 2013–2014 heating season, the new HTHV direct-fired natural gas heaters consumed 20 percent less natural gas than the existing heaters on a normalized basis over the monitoring period (Figure 2). This 20 percent value exceeds the expected energy savings if simply comparing the nominal equipment efficiencies of 92 percent versus 80 percent for gravity-vent units.  This difference was attributed to the overall system efficiency gains through destratification and operational efficiencies.

The HTHV heaters in the noted study operated with a high-pressure supply fan delivering heated air to the floor and creating vertical circulation that reduced temperature stratification between the floor and ceiling and can also deliver outside air requirements for the building. While this strategy reduces thermal energy consumption, the units carry higher fan electricity consumption, which offset a portion of the thermal savings. Nevertheless, over an average heating season for the host site in Bridgeton, Missouri (3705 average HDD, 16 C [60 F]), the new natural gas heaters would save approximately 15 percent on space-heating utility costs and on source energy.

Additionally, the HTHV technologies showed improved temperature control over conventional equipment by reducing thermal stratification between the floor and ceiling. Figure 1 displays the difference in temperature near the floor (i.e. 1.5 m [5 ft] off the floor) and at the ceiling (i.e. 6 m [20 ft] off the floor) during the monitoring period. For buildings with high ceilings, such as the warehouse in this demonstration, warm air naturally rises and can raise the average ceiling air temperature D6 to 11 C (D10 to 20 F) above the thermostat set-point. Due to this temperature gradient, the heating system must run longer and consume more energy to meet the needs of the building’s occupants at or near floor level.

As demonstrated by the temperature differential between floor and ceiling in Figure 3, the HTHV gas heaters eliminate much of the temperature stratification experienced with the existing unit heaters. Generally, the existing gas heaters exhibited vertical temperature differences of more than D3 C (D5 F) higher than those exhibited by the high-efficiency gas heaters. HTHV direct-fired gas heaters and other destratification technologies increase air circulation and provide more uniform temperature distribution throughout the space. Additionally, because the roof is typically the largest area for heat transfer in a warehouse, lowering the temperature of the interior ceiling decreases the heat loss through the roof.

Summary and recommendations
HTHV direct-fired heating equipment can supply both ventilation and space-heating airflow to maintain comfortable conditions for occupants in commercial and industrial buildings. By bringing in outside air, direct-fired equipment does not increase the amount of air entering the building—rather, the airflow brought in by the direct-fired equipment creates a slight positive pressurization and offsets the infiltration that would normally enter through building seams. Through this method, the amount of air entering the building and the related infiltration heat load remains the same. Ventilation-only products, often called make-up air units, replace exhaust air by conditioning outdoor air only to indoor ambient temperatures, and thus require a separate space-heating system to satisfy the conduction heating load of the building. HTHV direct-fired space-heating products supply outdoor air at sufficiently high temperatures to not only supply heated ventilation air, but also satisfy conduction heating loads.

If deployed widely, HTHV gas heaters would significantly decrease natural gas consumption related to space-heating and ventilation for high bay areas such as warehouses, loading areas, distribution centers, and manufacturing facilities. Depending on the exact configuration, HTHV technologies could save 20 percent or more in space-heating energy consumption and utility costs as shown in the field demonstration and EnergyPlus energy modeling. (For more information, see Roger Hedrick’s article, “Energy Performance Comparison of Warehouse Heating Systems,” published in Gard Analytics. View it here www.cambridge-eng.com/Portals/0/pdf/gard-report.pdf[2]). When used to both heat and ventilate spaces, installation cost savings can be achieved by serving the functions of heating, ventilation, and destratification.

Increasing the adoption of HTHV heaters will require a change in the way HVAC contractors, the ASHRAE community of engineers, utilities, and building owners and operators consider non-centralized heating equipment. Proper education on the availability and the potential lifetime energy savings of these technologies may encourage more industry professionals to evaluate HTHV gas heaters for their buildings, and determine whether the systems offer an acceptable payback based on climate, operations, and building design.

The following actions could further support the adoption of HTHV technologies.

Building owners should:

• perform a feasibility study on existing buildings to determine if retrofitting with HTHV technologies would provide acceptable payback; and
• consider upgrading their new construction best practices to incorporate HTHV technologies into their various designs for heating, ventilation, and destratification.

Natural gas utilities should:

• educate commercial customers and internal teams on the lifecycle cost of HTHV heating technologies; and
• include HTHV heating technologies in available grant, incentive, or financing programs.

Rebates
Building owners and facility managers can receive an additional gas utility rebate when purchasing and installing HTHV technology. For years, both electric and natural gas utilities have offered rebates for energy-efficient products ranging from light-emitting diode (LED) lighting to high-efficiency direct gas fired HTHV equipment. For more than a decade, natural gas utilities across the country have been providing custom rebates for HTHV products through their commercial and industrial rebate programs.

Since the DOE has provided solid data via their study on HTHV equipment, many natural gas utilities are adding prescriptive rebates for HTHV products to their commercial and industrial programs. Every month, more and more utilities join the movement and add a prescriptive rebate to their portfolios, thereby making it easier, and more cost effective, to install HTHV technology. The prescriptive rebate amounts can vary by utility. Some offer a set rebate amount for all the different Btu sizes available, while others offer a $3 or $4 per 1000 Btu rebate to help incentivize the purchase of even the largest heaters available. The later rebates can provide as much as $12,800 in rebate funding for a $3.2-million Btu heater.

These rebates, combined with the gas savings that a 92 percent efficiency HTHV heater provides, can help drive down the return on investment (ROI) that today’s building owners and facility managers are required to meet when purchasing capital equipment.

Conclusion
Adoption of energy-efficient technologies such as HTHV can provide significant progress in meeting the goals of the Better Buildings Alliance and will result in overall improvement and growth in heating technology efficiency percentages. As more end-users realize the energy savings and additional ROI available via utility rebates, these technologies will become the norm rather than the exception, helping drive down energy waste and cost within the commercial and industrial sectors.

Marc Braun is the executive vice president of sales and marketing for Cambridge Engineering. Braun holds a bachelor of science in chemical engineering and started his career with Dow Chemical 22 years ago. He joined Cambridge Engineering in the high-efficiency HVAC market six years ago as their chief operating officer and now travels North America speaking to engineers, building owners, and utilities companies on next generation heating and ventilation technologies. He can be reached at mbraun@cambridge-eng.com[3].

Source URL: https://www.constructionspecifier.com/reducing-energy-consumption-with-high-temperature-heating-and-ventilation/

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