IES-VE User Forum

Have I done this correct?

I have a DX coil and an LTHW coil serving an UFAD system.

Each coil has a summer independent time switch set to 18 and another winter independent time switch set to 20. Then I have 2 dependent time switches. One is ON if OA>18 and the other is ON if OA<18. If the OA>18 then it activates the switch to 20 supply air. If OA<18 then it activates the switch to 18 supply air.

Is this the way to do it? Or some ham fisted bodge? Please let me know if there is a simpler way to do it.

Should I connect the dependents with an OR switch?

Many thanks for anyone's advice

With the setup you currently have, yes, you need to connect the dependent time switches with the independent time switches using an AND connection to get them to work. There's an easier way to do this, however.

Try using an Independent Controller with Sensor for each coil instead. Both controllers will sense outside air temperature and control the respective coil leaving air temperatures. You can leave the "Time Switch Profile"s to "on continuously" and enable the "On/Off Control" section. In this section, you'll set the controller to sense your 18 deg OA temperature. Set your "Action with High Sensor Input" to be ON for cooling and OFF for heating. Finally, enter the respective LATs in the "Dry-bulb Temp at Max sensed DB Temp" field.

To changeover between heating & cooling operation at exactly 18 deg, leave your deadband on each controller at 0. If you find you have issues with the sudden changeover in your results, consider separating your sensed temperatures and giving them an appropriate deadband.