Hotel Room HVAC Energy Saving
When determining HVAC set points for unoccupied rooms, it is important to balance the need between guest comfort and energy efficiency. On the one hand you do not want the set point so low that the room requires a long time to warm up, and on the other hand you don’t want to set the minimum temperature to a point where the HVAC system will run for significant periods to maintain the set point.
The factors that affect room HVAC energy usage are:
- The thermal efficiency of the room. In other words, the rate at which the space gains or loses heat.
- The difference between the space temperature and the outside air temperature.
- The capacity of the HVAC equipment in the room.
- The room occupancy percentage.
- The precision of the temperature regulating device; usually the room thermostat.
- The heat load in the space. This includes lighting and appliances such as TV’s, refrigerators, and the like.
The thermal properties of the room and the capacity of the space HVAC equipment are determined in the design/build phase and are beyond the ability of the hotel operator to control. Likewise the occupancy percentage is generally dependent on outside factors although during extended periods of relatively low occupancy guests can be blocked in certain areas and other rooms can be allowed to ‘lie fallow’ as it were, thereby allowing the HVAC systems in those spaces to be turned off.
It may be assumed that the hotel operator has initiated the obvious energy saving methods of replacing room lighting with low-wattage bulbs and given some thought to replacing appliances with energy-efficient devices as the old ones wear out. Besides the obvious advantages such steps reduce the total heat load on the space.
Depending on the size of the property it may be advantageous to replace the room thermostats. The average mercury-bulb thermostat can be wildly imprecise, with deviations as high as 9 °F or more. These thermostats are dependent on correct and level mounting, and it is likely over time that they have been bumped and put out of alignment. Electronic thermostats are generally much more precise than their mercury –bulb cousins and are not dependent on level mounting and are thus much more resistant to hotel room wear and tear.
The area where the hotel operator can have the largest influence on room HVAC energy usage is choosing where the set points are for unoccupied rooms. The proper set points will depend on:
- Season
- Design conditions for the area in which the property is located.
- Guest perceptions
The American Society of Heating, Refrigeration, and Air conditioning Engineers (ASHRAE) has developed standard optimal temperatures for various buildings and occupancies at which most people feel comfortable. For the summer season in most of the U.S. these temperatures are between 74.3 °F and 77.9 °F and during the winter the range is between 70 °F and 73.5 °F. It may be expected that most guests will desire a room temperature in these ranges. The goal of the property operator is to find an unoccupied room set point that minimizes room HVAC energy consumption when the room is unoccupied while the time required to heat or cool the room to comfort levels is reasonable from the guest point of view.
It should be pointed out that during the summer months in most of the U.S. the unoccupied room set points can be kept closer to the outside air temperature than they can in the winter months, thus reducing energy costs. If the outside temperature is 90 °F then walking into room at 80 °F is not unreasonable. However, most guests would complain if they walked into a room set at 50 °F when the outside temperature is 40 °F.
Regardless of the individual factors that determine the thermal performance of a given space, heat gain and loss will follow a natural log (ln) curve once the climate control is inactive. This is illustrated in the graph below.
In this graph Time increases left to right and Temperature decreases top to bottom. The Temperature gridlines represent equally spaced intervals.
One can see that the temperature rate of change is highest early and markedly decreases as time passes. This is because the rate of heat transfer decreases as the differential between inside and outside temperatures decreases and rate of heat flow decreases. The same principle holds true for heat gain in a space. The space will gain heat at a greater rate early and the curve will level off as the temperature in the space increases.
In order to minimize HVAC energy costs a property operator should choose to set the unoccupied room temperatures at a point on the curve where the rate of temperature change is lower and thus the time the equipment is maintaining the room set point is reduced.
To illustrate the point let us look at a representative hotel room located in Portland, Oregon.
The room is a national average 350 ft2 with an 8-foot ceiling and is located on the ground floor of a multi-story hotel where the ground slab is 6” concrete and the exterior walls are 4” concrete faced with 1” insulation board and stucco on the exterior and ½” drywall on the interior. The interior walls are framed with wood 2 x 4’s and faced with ½” drywall filled with fiberglass insulation. The room window is double-glazed with an area of 31.5 ft2and the entry door is a standard size solid core wood door. The floor is covered with carpet over a fiber carpet pad.
The average design temperature for the six month period from October to March is 43 °F. The exterior wind speed was taken to be 7.5 mph. The room HVAC equipment is a package unit (PTAC) rated at 9900 Btu/hr (BTUH) in heating. An occupancy rate of 70% was assumed and the public space outside the room was assumed to be kept at a temperature of 72 °F.
Assuming an initial room temperature of 72 °F, the room cools fairly rapidly to 65 °F and then much more slowly to 60 °F. Because of the nature of the heat loss curve, the room HVAC will run nearly twice as long to maintain the space at 65 °F as it will to maintain the space at 60 °F. Given that most people prefer a room temperature in low 70’s during the winter, an unoccupied room set point of 60 °F is not unreasonable. For a room HVAC unit of the stated size in the given room it would take about 20 minutes to warm the room from 60 °F to the low 70’s and about 30 minutes to warm the space to the mid to upper 70’s.
Of course, if the room is kept at 65 °F while unoccupied, the room warm-up time will be reduced, but at the expense of increased cycling time to maintain the higher temperature while the room is unoccupied. As stated previously, part of the hotel operator’s decision at where to maintain set points will be driven by guest perceptions. At higher-end properties people will expect their room to be reasonably warm (or cool) when they enter. Hotel management may wish to manage guest expectations in the interest of reducing costs.
In summary, hotel operators should seek to set unoccupied room temperatures at a point on the heat loss curve where the slope of the curve is flatter. This reduces HVAC cycling time while maintaining reasonable room temperatures. The unoccupied room set points are dependent on several factors and electronic thermostats allow for exact programming and provide for more precise temperature control.
The hospitality industry is in the business of maximizing guest comfort so managing guest expectations is a key part of managing hotel energy costs to prevent guest complaints.