Tempered air

[steve.c]

Hi all,

can anyone enlighten me. i have been asked to provide a cooling coil to an existing supply duct after a LPHW AHU. i only want to temper the air as it is drawn from outside. at present the heating stat is inside the AHU and i intend to install a DX coil after it. question is, should the cooling stat be positioned before the everything, between the AHU and the heater battery or just before the cooling coil. also, what temperatures should each stat be set to. my fear is that at any one time both batterys will be on.

[The Viking]

First of all, I would strongly recommend some sort of control interlock to prevent any Heatingtogetherwithcooling situations (unless ofcourse dehum is required).
Position of stat for the DX coil????
I would say put a room stat in or maybe a return air stat and control on that. I would also put a low temp stat before the heater, to ensure the DX stays off if the ambient is cold.

[steve.c]

thanks for that.

[frank]

The correct way to do it is to put the cooling coil before the heater coil. This way, the low limit control can be maintained, also, you have a better control over humidity.

Tempering the air normally means that you heat it up to an acceptable delivery temp.

Control of the dx with a stat (I assume you are using on/off compressors) without capacity control will be difficult if you are trying to maintain an "air off" temperature rather than a room temperature.

[onetastyburger]

Hi all,

can anyone enlighten me. i have been asked to provide a cooling coil to an existing supply duct after a LPHW AHU. i only want to temper the air as it is drawn from outside. at present the heating stat is inside the AHU and i intend to install a DX coil after it. question is, should the cooling stat be positioned before the everything, between the AHU and the heater battery or just before the cooling coil. also, what temperatures should each stat be set to. my fear is that at any one time both batterys will be on.

Installing a cooling coil after the AHU is perfectly acceptable to provide local cooling to an area. Make sure you've got the correct control stragety in place though.

By tempering the air do you mean a fixed supply temperature or are you trying to control the space?

Use room control if your controling the space for both the LTHW and the DX.

Use duct mounted control if your controling the supply temperature for both the LTHW and DX.

Use a stat between the LTHW and DX coils to hold off DX in event of low air on temps. Use a stat before the LTHW coil to provide frost protection.

Dependant on the cooling load required, consider using a multi stage coil and units to provide efficient cooling and capacity control.