In VAV systems, the use of supply air temperature reset can increase comfort levels and save on energy use as well. Supply air temperature reset is a very low cost energy conservation measure. As I visit new customers, more often then not, I find it turned off. So what is supply air temperature reset and why should we use it?
Supply air temperature reset is when we look at one factor, like outdoor temp, to determine if the demand for another, like cooling capacity, can be reduced. In the heat of summer, VAV boxes usually require 55° supply air to be able to meet the demand for cooling. That cold air feels good to occupants when humidity levels are higher and they know it is hot outdoors. When cooler weather and lower humidity prevails, that air feels cold. The boxes are programmed to always allow some of that supply air to pass through as it brings in needed fresh air, but at 55°, it can quickly over cool the space which brings on the heat, if available, to re-warm the space. This over use of heating tends to further reduce humidity levels, which makes the tenants feel cooler, so they ask for the set point to be raised, which causes more heat to be used and increases the heat loss to the outdoors. It is an expensive cycle, and difficult to see coming since most people would think the higher utility bills are what is required to condition the building and keep the tenants happy.
Supply air temperature reset would look at the outside air temp, being below its starting set point, and change the supply air temperature set point up from say 55° to 62°. Now this warmer air doesn’t feel so drafty and doesn’t require so much, if any, reheat to keep the tenants comfortable. Often humidity levels will rise, causing tenants to feel comfortable at lower space temperatures, which reduce heat losses. This is the kind of cycle we want.
This same method can be applied to hot water heating, when outdoor temps or indoor temps indicate that full heating potential is not required, lower the hot water temperature. This will not only cut energy costs by reducing heat losses through pipe insulation, but it will provide a more even controlled and comfortable space for the tenants.
I often get service calls where tenants complain that the system is blowing cold air on them. They sometimes say the air conditioner is running when they need heat. In some systems, if the outdoor air temperature is still too warm for the boiler to be operating, a call for heat causes the VAV box will blow even more cold air on them. The action taken by an inexperienced maintenance man is to lower the minimum fresh air setting for the box or bring the boiler on at a higher outdoor temp. Either way, the tenant is happier and the complaints stop. One of these solutions will reduce the oxygen levels in the office causing decreased alertness and increased risk of contracting a disease, the other raises the energy bill.
Let’s face it, very often the wrong action is taken in response to a problem, it happens in every field. It can be difficult to convince a customer that something that was done by someone they trust now needs to be undone when, at the time, it seemed to solve a problem. If you find yourself facing this customer and this situation, send them to this article, and maybe I can help!