Winter is Coming: Is Your Building Ready?

by Jeff Yirak, P.E., CPMP, LEED AP BD+C, O+M

This article is part of Wood Harbinger’s newsletter series.

When seasons change, so does the behavior of a building’s heating, ventilation, and air conditioning (HVAC) system. Heating in winter and cooling in summer results in very different HVAC equipment demands. The spring and the fall aren’t particularly easy either, with part load performance and switchover from heating in the mornings to cooling in the afternoon keeping the HVAC system on its toes. Season changes are a critical time to evaluate the performance of your building systems. This article explores what to watch out for in the heating (winter) season, cooling (summer) season, and shoulder seasons, as well as what the operator can do to keep systems in top shape.

Analyze the Trends

One of the easiest and most effective ways to verify building system performance is to do a quick analysis of trend log data. This data allows operators to review the performance of systems and inter-system interactions in response to the real-world behavior of the building, including external factors (the weather) as well internal contributors (the occupants).

Modern direct digital control (DDC) building automation systems can collect and store as much historical control input and output data as there is hard drive space provided. A year’s worth of data takes up a relatively small amount of hard drive space when sample points are collected on a reasonable interval, such as every fifteen minutes. Conducting a seasonal analysis reduces the need to store the raw data, as the analysis and summary provide the historical record for future reference.

Cooling Season Trends

The oceanic climate of Western Washington enables cooling via ventilation only. Because of this, buildings are often designed without mechanical cooling equipment, instead utilizing “heat and vent” systems. These systems are simple and effective but require that the building envelope be constructed to limit solar and thermal gain during long summer days.

The trend log graph above shows an effective heat and vent system at a middle school, operating on a September day. The system maintained the room near 70 degrees F (represented by the return air temperature, shown in green circles). When the system started at 6:30 AM, it first postured for some economizer cooling, then quickly assumed a minimum outside air damper position of 25% (represented by the yellow triangles).

Carbon dioxide (CO₂) levels (represented by the blue squares) began to rise as students came into the classroom. It rose first to about 500 parts per million (PPM), then as high as 750 ppm around 11:00 AM. Reviewing this trend shows that temperature and demand control ventilation programming is working, although the dampers are pretty active and may wear out prematurely. The control loop tuning could be slowed.

Heating Season Trends

The graph below is for a rooftop unit air handler (RTU) at a high school operating in February.

This unit is shown to start at approximately 3:15 AM for a pre-occupancy warm-up. The heating water valve (yellow line) had been held open overnight at 50% to prevent freezing, although the supply fan was not operating. When the fan did start, the heating water valve closed because the zone did not require heating. The occupied schedule started at 5:00 AM, so the unit opened the outside air damper (cyan line), which lowered the mixed air temperature and supply air temperature (dark and light blue lines, respectively). This unit has a mixed air temperature low limit built into the software to prevent supply air temperatures from being uncomfortable and to prevent freezing of the heating coil. In this case, the mixed air temperature low limit is 50 degrees F, and the heating coil is providing about a 10-degree temperature rise, so the supply air temperature is about 10 degrees higher than the mixed air temperature.

Throughout this day, the zone was slightly above the heating setpoint; this was caused by a high amount of solar heat gain (sunshine) entering the room, so the unit was actually trying to cool the zone by opening the outside air damper to increase the amount of outdoor air being brought into the room. This increased outdoor air lowered the mixed air temperature to the lower limit, which caused the outside air damper to return to minimum position and the heating water valve to open and heat the room. This resulted in a cyclical pattern that lasted all day.

The occupants likely wouldn’t have noticed this system behavior, but the facilities maintenance staff would notice the wear on the outside air damper actuator and heating water valve when all this cycling activity causes them to fail earlier than expected. These trends revealed that the control loops needed to be re-tuned, and the mixed air temperature limit revisited, in order to make this a more stable system while maintaining occupant comfort.

Shoulder Season Trends

Buildings systems are usually most stable during high-demand situations, whether heating or cooling. Mild days during the shoulder season often present challenges to the mechanical system. Shoulder days reveal weaknesses or gaps in system functionality, whether due to part-load instabilities or heating to cooling changeovers.

The graph above was created to illustrate a demand-control reset of the heating water supply temperature setpoint. Instead of using the outside air temperature, the sequence looks at the average zone temperature and the average terminal unit heating valve position to determine the heating water demand of the building.

On this day, the boiler started at 5:00 AM, and all heating water valves were open; please note that on this system, valve position is recorded as “percent closed, as the actuators spring open. By 6:00 AM, when the morning warm-up period was ending and the occupancy schedule was beginning, the average valve position was between 40% and 50% closed, and the heating water supply temperature set point was about 158 degrees F. By 8:30 AM, the average valve position was about 70% closed and the heating water supply temperature was 151 degrees F. Just before 10:30 AM, the building had warmed up and the average heating water valve position was more than 90% closed. This signaled the boiler plant to shut down.

You’ll notice that the heating water supply temperature curve (represented by the purple line) resembles the curve of the outside air temperature data (represented by the lightest green line). This is fairly typical and explains why this building functioned successfully using an outdoor air temperature reset table to control boiler supply water temperature. The move to a demand-based reset allows the boilers to operate when needed and shut down when not needed. The outside air temperature changed from a low of 54 degrees F at 5:45 AM to a high of 68 degrees F at 2:15 PM. If the boilers had been programmed to shut down when the outdoor air temperature exceeded 65 degrees, then they would have shut off at 10:00 AM, which would have been before all of the zones would have been satisfied. This graph shows that comfort and energy consumption can be managed on shoulder days, with heating in the morning but not all day.

Trend Analysis for Better Building Performance

Your building undergoes varying demand that corresponds to changes in the season. Just as you wear a coat in winter and shorts in summer, so does your building vary its performance to maintain the indoor environmental conditions requested. Take a moment to review the behavior of your building during these seasonal changes to ensure smooth and comfortable performance this season.