Parallel Fan powered VAV box with primary heating
Parallel Fan powered VAV box with primary heating
(OP)
I have a parallel fan powered VAV box. It has a minimum primary CFM to provide ventilation. In cooling mode, electric reheat is used to prevent subcooling.
My packaged rooftop has a gas fired heating coil so it delivers primary heating air to the VAV box as well.
Question is, how do I prevent overheating in heating mode? When space temp is satisfied, and primary damper is at minimum position. I think the answer is just that I run the plenum fan to create a lower discharge air temp, but isn't my plenum eventually going to heat up? If my discharge air temp is, for example 70 when my plenum is 65, after some amount of time of 70 degree discharge air, the plenum will warm up to 70 as well. now I'm mixing 70 degree air with my primary air instead of 65, so my discharge is going to go up. See my dilemma?
This facility has no DDC, so the boxes and the rooftop have their own controllers and aren't networked together.
My packaged rooftop has a gas fired heating coil so it delivers primary heating air to the VAV box as well.
Question is, how do I prevent overheating in heating mode? When space temp is satisfied, and primary damper is at minimum position. I think the answer is just that I run the plenum fan to create a lower discharge air temp, but isn't my plenum eventually going to heat up? If my discharge air temp is, for example 70 when my plenum is 65, after some amount of time of 70 degree discharge air, the plenum will warm up to 70 as well. now I'm mixing 70 degree air with my primary air instead of 65, so my discharge is going to go up. See my dilemma?
This facility has no DDC, so the boxes and the rooftop have their own controllers and aren't networked together.





RE: Parallel Fan powered VAV box with primary heating
Take a look at the primary AHU (in this case an RTU), it's providing 70F air, correct? It is your source of outside air, so plenum air that is returned to the AHU is going to be mixed with OA (presumably it is less than 70F since you are in heating mode), the mixed air temp will be less than 70F and the AHU heats it to discharge at 70F, thus primary air to the VAV will always be 70F. So the RTU should be controlled and operated to maintain a 'primary air' temperature of 70F. (Heating mode only.)
Now at the boxes. I assume they are controlled via a thermostat only. It is also safe to assume that your plenum temperature is the same as the temperature of the room (thermostat) or slightly higher. So what is going on (or should be) is dependent upon what the thermostat is needing. If the thermostat is satisfied, the box fan should be off and the primary damper at a minimum; the result is you are introducing "neutral" air from the RTU into the space that is neither heating or cooling the space. (This assumes a thermostat setpoint of 70F.)
When the thermostat calls for heating the first thing that should happen is the primary air damper opens to full. If the thermostat is still not satisfied then the fan and reheat coil in the box needs to be energized. Overheating is not a problem because at this point the plenum air will be less than the thermostat set point (<70F) and when mixed with primary air, the mixed air will also be less than 70F.
When the thermostat becomes satisfied, everything goes back to the initial state (box fan off, reheat coil off, primary air damper at minimum) and you are just introducing neutral ventilation air from the RTU to the space again.
Another note, Because you are introducing air from outside the space/plenum area (from the RTU), the same amount of air has to leave the area, whether it returns to the RTU or is exhausted out of the building entirely; this will keep the temperature of the plenum in check.
I hope this makes sense and clarifies your dilemma.
RE: Parallel Fan powered VAV box with primary heating
Is this an interior space or does it have exterior exposure?
What is your climate?
RE: Parallel Fan powered VAV box with primary heating
RE: Parallel Fan powered VAV box with primary heating
Aaon and McQuay have good designs for modulating stainless steel high turndown gas heating that will give you ok temperature control when you can enable in a SAT. Make sure you read the notes on minimum air flow. A standard Ipac1 or Carrier 48/50 series will limit you to 1 or 2 stages of heat , drive all boxes open in a MWU and IDF runs at 100% speed. You can reset LAT in the reset schedule. This will help you move LAT up based upon external input such as OAT or bulk /zone temperatures. On an Ipac, these are settings are set at the HI and Carrier at the comfort link controller.
Non-DDC building can perform but you lose the ability to adjust the ability to correct for poor performing zones. You may be forced to 55 F LAT and excessive re-heat and higher operating costs.
RE: Parallel Fan powered VAV box with primary heating
The rooftop gas heat is often configured to provide morning warm-up by delivering a supply temperature to the VAV boxes indexed to full open damper position. When a representative zone or average space set point is achieved, the system reverts to above.
Minimum damper position/volume must provide required outside air per ASHRAE Standard 62 ventilation rate procedure or IAQ Procedure.
The rooftop doesn't care about exterior or interior zones. The terminals deal with that.
If you do an energy analysis, this system's heating energy consumption is mostly electric at the terminals.