Make Up Air Unit?

[Zeroman]

I am a recent graduate in need of help from you, the experienced HVAC&R engineers/technologists. Here is the situation. There is a small building with an existing electric heating unit in it. Although it DOES supply the correct CFM it does not supply the correct amount of heat. (15kW versus 40kW). They have asked me to add an additional heating element to the existing system (the guys are freezin). I have completed the cfm calculations and heating load. I need to supply an additional 25 kW to the building, but there is no vacant space in the existing unit to place another heating element. Should I install a Make up air unit that supplies the additional heating with the minimum cfm that coincides with that size of unit? I would appreciate someones opinion. Thanks.

 

[KenRad]

Is the existing unit a ducted air handler? If so, you could add duct heaters as additional stages of heat to what is already in the unit, provided your CFM is adequate.

You mentioned the possibility of adding a makeup air unit; these bring in outside air to the building, which you wouldn't want to do unless you needed it for ventilation or room pressure requirements. Obviously, it would take a lot more to heat outside air than recirculated room air. Fan-powered unit heaters or additional air handlers would do the job better.

What kind of building is this (warehouse, factory, office, etc.)?

---KenRad

 

[Zeroman]

Thx Kenrad for replying,

Yes the existing unit is a ducted air supplier. Would it be a difficult job to install duct heaters? And do they have their own sensors? I was considering the makeup air unit because the room does require pressurization and the existing fan is not meeting the requirements. The building is a small control building.

"The design of the pressurization system shall meet the requirements of the latest edition of NFPA 496: Standard for Purged and Pressurized Enclosures for Electrical equipment."

Because the existing unit is not pressurizing the building properly I believe I will have to install a makeup air unit, for the small amount of extra cfm. Do you think that an additional air handler would be a better idea than a makeup air unit?

 

[ramnam]

since it seems you have two issues here. first is pressurization. second is heating. First the pressurization. check and see if you have outside air directly connected to the fan coil unit. is there a volume damper on it? is there a volume damper on the return? can they rebalance either to get some more pressurization? say 100 cfm + at the entrance door.

second, the heating issue. Perform your calcs for the additional outside air. (cfm x 1.08 x (Tin-Tout)) then add that to the total heat you calculated. several options are:
1) add the additional elements in the Fan coil unit if it can handle them (see KenRad). 2) Add a heater in the duct. if the cfm is correct. otherwise you could burn out the elements. 3)add unit heaters in the space near the ceiling. add baseboard heat near windows. 4) add radiant heat if none of the above and people are working there. it could keep them quiet for a while.
good luck!

 

[mt31]

you could simply install a duct heater into the supply box, directly after the air handler. you can wire in sequence with units controls. this can also have a variable speed fan motor for cfm requirments. Make up air is a good idea, you could probably tie a fresh iar duct into yhe return box. Just some ideas, a new unit could save cost, and sized properly could solve both problems.