HVAC Recirculating AHU are Reheat Coil and Face-Bypass Damper

[LAV21770]

Is necessary for an 80% recirculation AHU installed inside a building the steam coil and face and bypass damper?

 

[BronYrAur]

not sure what you are asking. Do you want to remove the face and bypass dampers and let full air flow through the steam coil at all times? You may need to change some components (steam valve, trap, vacuum breaker). Are you currently lifting condensate up out of the trap? I am assuming you have full steam pressure on the coil at all times now. That won't be the case if you switch. Need more info.

 

[LAV21770]

Bron,
I am buying a new recirculating AHU but the engineering designing the mechanical side of the project is proposing this unit without face and bypass damper and steam coil, I am just wondering why is he doing that.

 

[willard3]

Likely using face and bypass dampers for temperature control.

 

[BronYrAur]

I rarely see face and bypass dampers on systems that are not 100% outside air. Recirculating systems usually don't have that arrangement in my experience. They do have heat, however. The heating coil is just in series with the cooling coil. Sounds like your unit might not have any heat, which could be concerning. Are you in a mild climate?

 

[GT-EGR]

It sounds like you are asking if a unit that mostly recirculates needs heat or not. This is somewhat common in warmer climates, as long as your coldest outdoor air temp mixed with your return air temp gets you to the discharge air temperature you need - which at only 20% outside air is very possible. You also need to make sure your system has either reheat coils or supplement heating in the spaces since you AHU is no longer able to deliver warm air.