Oversized HVAC Systems and Design Temperatures

In recent years, there has been a shift away from oversize HVAC systems that are simply too much for the space they need to heat and cool. This is because more emphasis in building and energy codes is towards energy efficiency. Buildings are required to have more accurate cooling and heating load calculations to prevent the problems associated with oversized systems. These problems can include short cycles and poor dehumidification, among other things. However, sometimes systems are still not sized correctly.

Things Are Heating Up

One of the problems that lead to oversized HVAC systems is that the installer used the incorrect design temperatures when making their load calculation. Here is how that happens. In the winter, heat bleeds from the building to the outside. In the summer, the building absorbs heat. How much heat is gained or lost, and how fast it heats or cools, is dependant on the difference in temperature from the outside to the inside. There is a table that provides the industry standard for the proper design temperatures for each season. However, too many contractors ignore or tweak those recommended temperatures when they are making their calculations.

Temperatures are always in flux, whether inside or outside a building. How much heat a building games or loses is in flux as well, even from hour to hour and day to day. If the goal is to keep the temperature of a building at a constant, such as 72 degrees, then the difference in temperature from outside and inside will be less in the evening and morning during the summer when it is cooler. When the temperature difference goes up, then the cooling load will have to rise as well. In most cases, buildings have air conditioners that are not designed to change the amount of cooling based on the change in temperature. THis would require split heat pumps, which are not common.

Design Temperatures

This is where design temperatures come in. Since most HVAC equipment is fixed capacity, and not variable, the design temperature will help make sure your system is operating at its best. The question is, what do these temperatures represent? In Winter, the 99% temperature refers to the level that the outdoor temperature will be above for 99% of the time. This is based on a 30-year average. This means that in the climate in which the building is located, the temperature will only be below that point for 1 % of the time. For example, in hotter cites, the 99% winter design temperature may be 25 degrees or so.

In the summer, the 1% design temperature is the point that your climate will be above for just 1% of the time. In a hot place, that point could be in the 80’s, for example. In the past, there used to be 97.5  and 2.5% temperatures. Those are no longer used since the current design temperatures have become the new standard.

Contactors and Design Temperatures

In general, contractors do not like having to return to the scene of a job to adjust things if the customer complains about being uncomfortable. This is why they will install oversized systems to make sure there is always enough cooling and heating. They do not know how about the other factors that come into play with the temperature of a building, such as insulation and infiltration. Despite this, if you are having a new system installed, make sure that the contractor uses the correct design temperatures so that you will not only be comfortable, but you can avoid any issues with having an oversized system for the space that you have.

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