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.
