Setting SelectedValue For One DropDownList Also Updates The SelectedValue For Another DropDownLIst

I have a WebForms page that has two DropDownList controls on it that both contain a range of temperatures from 60-80 degrees, one for heating the other for cooling. They are declared in the .aspx as:

<asp:DropDownList ID="heating" runat="server" />
<asp:DropDownList ID="cooling" runat="server" />

The values for each list are populated in the code-behind using:

for(int i = 60; i <= 80; i++)
{
var listItem = new ListItem(i + " degrees", i.ToString());

heating.Items.Add(listItem);
cooling.Items.Add(listItem);
}

When I try to set the selected value for each DropDownList using the values in an object containing data loaded from the database using:heating.SelectedValue = myHome.avgHeatingTemp.ToString();
cooling.SelectedValue = myHome.avgCoolingTemp.ToString();

The SelectedValue for both lists is set first to the value in myHome.avgHeatingTemp, then to the value in myHome.avgCoolingTemp. Heres what the values are for each variable after each step of the process of setting the SelectedValues:

Initial State
heating.SelectedValue: 60
cooling.SelectedValue: 60 [code]....