VAV ReHeat

[Rayohray]

I have a AHU with no hot water coil and 6ea ducts coming off of the ahu with 6 individual vavs with HW heating coils. I have a room that's cold. I have never heard of an ahu with our heating coil. The AHU is providing 44-49 F currently, and the heating coil providing 70-72 F. The room is about 64- 66 F. So, I do not have the specs of the heating coil but I find it hard to believe that the delta T can be much higher. The coil is a little dirty but I don't think it is a major issue.

 

[dbill74]

First observation is that the air off the AHU is unusually cold, It should be about 55-58F.
Is the cold room an exterior room?
Off hand I'd say you have a controls problem. You describe the AHU as a multi-zone unit, an older style of VAV system. These systems typically have pneumatic controls and are never used these days. Check all your controls linkages, not just on the air side, but the water side as well.
How is the AHU air flow being modulated?

There are too many things in an old system like this that can cause a cold room and you've not given enough information for us to help much.

 

[Rayohray]

All Digital controls. The ahu has no hot water coil, which I have never seen before. Which leaves me confused on why would it be built that way, so the only way I can get the supply temp up is by raising the the mix air temp.

 

[MedicineEng]

Not sure of your location, but assuming that your outside air is also cold, have a look on the quantity of fresh air that you are bringing in.
If you are pulling too much fresh air into the space instead of recirculating, then your system might also struggle with keeping the temperature.
Be attentive to ASHRAE (or CIBSE) requirements terms of fresh air though.

 

[dbill74]

What are your water temperatures?
Are the water valves modulating as needed?
Is the AHU blow thru or draw thru?