pneumatic vav problem

[licensed to chill]

Our company replaced a dual duct vav box due to age and damper blades sticking. The old box had 1 actuator with opposing linkages for hot deck/cold deck dampers(1 no and 1 nc). The new dual duct vav box came with nc dampers on both hot and cold deck with a kmc velocity controller for each actuator.I had to replace 1 of the velocity controllers so 1 would be reverse acting and 1 is directing acting. Played around with the mininum and maximum cfm adjustments per instructions and calibrated t-stat but customer has called back several times complaing of space temperature being too cold. My thoughts are to ditch the velocity controllers(since their is no electric or hot water heat, only the hot deck) and install a reversing relay on 1 of the dampers. Do any of you pneumatic wizards have any suggestions? Trying to make this as simple as possible. I really dont get to work on pneumatics enough to be very good, only dangerous.

[twisted pair]

I would expect to see a N.O. hot deck damper for heating and a N.C. cold deck damper for cooling working off a reverse acting space thermostat.

cooling 3-8 psi, heating 8-13 psi

Or

N.C. hot deck damper and N.O. cold deck damper with a direct acting thermostat.

Either way on a dual duct box you should have an airflow / velocity pickup & transducer for both ducts

[simux]

Do a system check out as follows.

1. Take a temperature reading and turn the thermostat setpoint to equal the room temperature.

2. Verify the hot and cold duct temperatures.

3. Turn the thermostat down a few degrees and verify your in full heat. Also verify that your getting the right cfm to the registers.

4. Turn the thermostat up a few degrees and verify your in full cooling. Once again verify the cfm.

[twisted pair]

If you plan on removing the velocity controller just remember your changing the VAV Box configuration.

Terminal Units are classified regardless of whether they are single duct, single duct with reheat (hot water coil or electric resistance coil), dual duct, fan powered (series or parallel), type of temperature controls (Pneumatic, Electric, Electronic, DDC) as either Pressure Dependent or Pressure Independent.

The easiest way to explain the difference is Pressure Dependent relies on a signal sent directly from the space / zone thermostat to modulate the damper position.

Pressure Independent utilizes a velocity transducer connected to a pickup in the VAV Box inlet. The PVR will respond to changes with the inlet pressure and will modulate the damper to maintain setpoint.

In Pressure Dependent all you will need using Pneumatic Controls are a damper actuator and a space thermostat.

With Pressure Independent using Pneumatic Controls you still need the damper actuator, space thermostat, as well as the airflow pickup, velocity transducer and controller or pneumatic volume regulator (PVR). For dual duct it is X2.

[clmair2fly]

From the first post, this sound like a constant volume mixing box rather than a VAV. I am assuming this had a spring actuated device inside the box that when over pressured, slowed down the air flow. The single actuator, when at set point had both the Hot and Cold duct open 1/2 way therefore mixing approx 72 DEG_F air for a 72 DEG_F set point (Adj)

they are Energy hogs and I have retrofitted these in the distant past for ESC companies, by separating the system into two distinct "vav" boxes and adding Velocity to both sides and DDC VAV box controllers that will allow the Hot side to shut fully and when in Deadband mode, the cold duct makes up the required min airflows.

Run these as shown by the above threads, you will get complaints from the occupants as now they will "feel" the different temperatures out of the supply duct. You may want to investigate further up the duct because more than likely you have one or two full speed fans serving these without duct static senors.

I would start looking into DDC controls for this building if retrofits keep coming