VAV Supply air temperature reset.

[remp]

Can somebody please tell me the best way for the BMS to calculate what the supply air temperature should be set at in a VAV system that has heating and cooling coils in the air handling unit. The cooling coil was designed for an off coil temp of 12 Deg C and the heating coil was designed for an off coil temp of 28 Deg C. This is for max cooling and max heating. How do I reset the temp sensor in the suppl;y duct so as to control the operation of the Heating and cooling valves.

Thanks!

 

[ChasBean1]

It might take some trial and error to get it right because sun load & building tightness can have an effect. I assume there are no reheat coils at the VAVs. Do you have perimeter heat, such as fin tube radiators? If so, I would suggest to try basing the supply air temperature on the following outdoor air temperature reset schedule:

0°C outside = 20°C discharge
20°C outside = 12°C discharge

Initially, then it can be tweaked if needed.

The other way I've seen it done (which almost always results in hot/cold complaints) is by using a thermostat voting scheme, where if a majority call for heat, the AHU will put out the design heating temperature (28°C). At this point, all the VAV boxes are switched to act in reverse, meaning they open during a call for heat and close during a call for cooling. When the majority vote calls for cooling, the boxes go back to controlling normally and the discharge setpoint resets to 12°C.

 

[imok2]

ChasBean1, We normally reset the cold plenum up 5*F as the VAV turns down to about 40/50%. That is from 55*F to 60*F on a single fan system. WE normally use the heating coil for morning warm up.

 

[lilliput1]

To save fan energy w/ VFD keep the peak design discharge temperature until the supply air reduces to about 50% as stated by Imok. Then hold the fan speed & reset discharge air up to 60 or 65°F on further rise of fan set point static. This is to make sure air volume do not get to low as to cause dumping. Also with VFD DX you want to keep air flow at coils above about 250-300 FPM to not get coil frosting.

 

[ChasBean1]

Imok, this can be done but can have ill effects on dehumificiation. Especially during setback periods, when thermal bands are widened. Take the 72°F, 68°F dewpoint summer night. Your building is now a mushroom if you don't keep your DAT setpoint.

Lilliput, right on, but DX makes life difficult. If not staged right, we can blow right past DAT setpoints (which is often the case) if there is not sufficient staging. The operative word in Mr. Billyg's post is "Heating and cooling valves," which leads me to believe this is a HW/CHW system.

 

[lilliput1]

I mentioned DX because overcooling was a typical problem when VAV DX first came about. Now new VAV DX units have a program reset available. Make sure you talk to the factory because some reps are not aware of this. Dehumidification control to control to space set point % RH is also available for McQuay.