Impact of Packaged Terminal Air Conditioning Units on Energy Performance

Susan MacDougall, Wesley Bowley

Focal Engineering, Canada

This study examines the effect of Packaged Terminal Air Conditioning (PTAC) units on the energy performance of a building due to increased thermal bridging and air leakage. Through-wall PTAC units have poor insulating qualities and make a large thermal bridge through the building envelope, increasing heating loads. Additionally, there can be air gaps between the PTAC unit and its sleeve, between the sleeve and the framed opening in the wall, and through the unit itself. An alternative form of PTAC called a Ducted PTAC is also examined and shows promising results compared to traditional PTAC units.

The impact of accounting for increased thermal bridging and air leakage due to PTAC units was simulated for a modular housing building with installed PTAC units. The building was built to Step 3 requirements of the BC Energy Step Code and is located in Climate Zone 4. Energy performance was calculated with and without accounting for the heat losses due to the thermal bridging through the PTAC, and the increased air leakage, and the results compared.

The R value for PTAC units is around R1 °F·ft2·h/BTU, so although the surface area of the PTACs is small, their impact on the effective wall performance is significant. Including the PTAC in the thermal bridging calculation reduced the clearfield wall insulating value by 68% and reduced the thermally bridged wall by 27%. Although the PTAC external surface area only made up 2.2% of wall area, it accounted for 25% of its heat transfer. Blower door testing was conducted on an apartment unit with PTAC installed, and it was determined that including the PTAC air leakage caused the building infiltration rate to increase by 55%.

The building’s energy performance was significantly worse when the PTAC units’ increased thermal bridging and air leakage were accounted for. The building without PTACs had a Thermal Energy Demand Intensity (TEDI) value of 28.1 kWh/m2/yr, which meets the TEDI requirements for Step 3 of the BC Step Code. When PTACs were modeled, the TEDI increased to 35.6 kWh/m2/yr (a 26% increase), which means that the building no longer meets Step 3 Requirements. The Total Energy Use Intensity (TEUI) value with and without PTACS is 90.4 kWh/m2/yr and 97.7 kWh/m2/yr respectively, and while this still meets Step 3 requirements, it is a significant increase. In colder climates the effect of PTACS is even more significant due to the building having a higher performance building envelope to compromise.

Ducted PTAC units use small circular ducts to penetrate the building envelope to bring outdoor air to and from a PTAC unit that is located within the thermal envelope, so the overall external area is much smaller than a through-wall unit. For this project, the increase in TEDI from a ducted unit was minimal at 1.5%, making it a promising alternative to traditional PTACs. This increase does not include the effects of air tightness as no leakage rates for ducted PTACs were available.

More information: https://www.conftool.com/esim2020/index.php?page=browseSessions&form_session=53&presentations=show