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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.

RE: Parallel Fan powered VAV box with primary heating

Assuming I followed what you are saying correct, you wouldn't have a problem with overheating.

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

You don't explain your control volume very well.

Is this an interior space or does it have exterior exposure?

What is your climate?

RE: Parallel Fan powered VAV box with primary heating

The problem here is the gas heated RTU is a single zone unit. It can only do either heating or cooling. So you should zone them so each perimeter exposure (N, S, E, W) get separate roof top each. Then the interior w/o roof and the interior w/ roof also get a RTU each.

RE: Parallel Fan powered VAV box with primary heating

Start with your loads. You must calculate your maximum SAT w/o overheating interior zones. This may be an iterative approach – 70F sounds too high. Second, review the operation of the VAV RTU, each mfg has a controller that must be addressed to use gas heat in OCC mode. This may be called Supply Air Tempering or OCC heating sp. SAT is normally employed when excessive OA lowers mixed air below LAT setpoint. In OCC cooling mode and requiring heating, requires the correct combination of controller settings, sensors and machine options. Most VAV RTUs such as IPac1 1 have gas heat for MWU only. Mfgs are concerned about minimum air flow on the heat exchanger during MWU. You may be asking for conditions the machine cannot perform. Most RTUs mfg controllers will command the unit to lock into a mode such as cooling, heating, mwu or occ muw. I doubt you will carry the cooling load of an interior conference room with SAT of 70F!! The highest I see is 65/64F with 2.5 cfm/ft. If you need the 70 SAT on the perimeter to carry the design heating, your terminals do NOT have the correct heat selection.
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 maintains discharge at around 55 deg f or less. The VAV box primary damper modulates from minimum to maximum to maintain cooling set point. When heat is required, the damper modulates to minimum, the fan initiates and uses plenum air to achieve heating set point. If not met, the electric reheat coil energizes. As space temperature rises, the coil de-energizes, the fan de-energizes and the damper modulates open to achieve cooling set point.

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.

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