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How does Pelican control a parallel fan powered box during a heating cycle?


For a Pelican thermostat to be configured as a zone damper thermostat and provide the following sequence of operation (SOO), a Pelican Z8 or Z24 must be installed, powered On, and connected to the Pelican wireless network. For the following SOO to operate, the Z8 or Z24 must also control the central HVAC equipment.

Configuring a Pelican Thermostat for Parallel Fan Powered Box:

When a Pelican thermostat’s Reheat Type is configuration to Fan Powered, Pelican Technical Support must set for you if the Fan Control is Series or Parallel. Please email or call Pelican Technical Support for the following Parallel Fan Powered Box SOO.

Control Outputs to a Parallel Fan Powered Box:

When a Pelican thermostat is configured as the following:

  • Damper Type: Open/Close or Floating
  • Reheat Type: Fan Powered
  • Fan Control: Parallel (technical support configuration)

The thermostat controls the zone damper, local reheat, and parallel fan with the following outputs:

  • (Y): 24VAC Output – Damper Open
  • (Y2): 24VAC Output – Damper Close (if an open/closed or floating damper actuator is installed)
  • (W): 24 VAC Output – Reheat Enable
  • (G): 24 VAC Output – Parallel Fan Enable
  • (W2): Unused

Sequence of Operation for a Parallel Fan Powered Box with Heating Demand:

A zone has “Low” heat demand*: The zone thermostat will energize its parallel fan and enable the local reheat. The zone damper will remain shut. This will occur even if there is a central cooling or ventilation cycle active for other zones. This will remain true until one of the following occur:

  1. The zone thermostat reaches its heat set point.
  2. The zone thermostat goes into “High” heating demand: the below “High” heating demand sequence will occur.

A zone has “High” heating demand: The zone thermostat will send a “high” heating demand request to the zone controller at the central HVAC equipment. The following option becomes available if there IS central heat at the HVAC equipment:

 The zone thermostat with heating demand will energize its parallel fan, enable the local reheat, and open its damper. All zones that do not have heating demand will close their dampers*. Once the zones with heating demand dampers are open, the zone controller will enable the central unit’s supply fan, maintain the Target Operating Static pressure**, follow any of its ventilation sequences, and enable the central heat to assist in heating the zone.  Once the zone thermostat detects that its zone has transitioned from high heating demand to low heating demand: it will send a “low” heating demand signal to the zone controller and, if there are no other “high” heating demands, the zone controller will disable central heat. The zone thermostat with heating demand will shut its damper, but keep its parallel fan and reheat active to continue to heat its zone. Because central heat has been disabled, other zones requesting ventilation cycles or cooling cycles will be served by the zone controller.

The above sequence will remain true until one of the following occur:
  1. The zone thermostat reaches its heat set point.
  2. The zone thermostat goes into “low” heating demand: the above “low” heating demand sequence will occur.
  3. If there is a central heating cycle active and the heat/cool change-over occurs (the zone controller has calculated that a central cooling needs to begin): The thermostat will receive a “change-over” notification from the zone controller, it will shut its damper and use its  reheat to continue to heat its zone.

*During a central heating cycle, if a zone keeps its damper open, but does not have heating demand: This zone has been setup as a “Dump” zone using Pelican’s Capacity Management Algorithm.

**In most cases, during a central heating cycle the zone controller will maintain the Target Operating Static pressure. But, there is also a configuration for Target Heating Static Pressure than can be set by Pelican Technical Support upon request.

Can I reverse the logic and have central heat be primary (low demand) and the local reheat be secondary (high demand)?

Pelican Technical Support can reverse the logic and make central heat primary (low demand heat) and the local reheat secondary (high demand heat). Although this is an option, we do not recommend this for parallel fan powered box applications.

The decision on what should be the primary heat is mostly depended on what the lowest BTU output of the central furnace is versus the energy cost of the local reheat? Pelican assumes that the original design would size the local reheat to be more energy efficient during short (low heating demand) periods, especially because the box is pulling warm plenum air into it, versus turning on the central furnace and pushing a large amount of CFM through that unit just to heat zone’s with low heating demand.

What if I have multiple stages of reheat?

Traditionally parallel fan powered boxes are controlled similar to standalone VAV boxes. In this article we are not going to go into all the details on the sequences for stand-alone VAV box controls with parallel fans (and do not hold the below short explanation, in any form, to meeting the full sequences available on the market). This article is just to help provide a comparison to why traditional VAV sequences use multiple reheats and Pelican does not.

In standalone VAV operation with a parallel fan, a local zone damper controller requires enough reheat to counteract a cold (60°F or less) supply air from the central HVAC equipment. During a zone reheat sequence, the standalone VAV box controller modulates its damper to maintain a minimum CFM, enables the parallel fan, and stages the reheat to maintain a target supply air temperature to heat the zone. Because there is cold air bring brought into the box from the primary duct, the efficiency factor for a standalone VAV box is focused on it’s standalone ability to adjust the number of active reheat stages. Less stages equal more energy efficiency, but this can also generate a more challenging approach to get a consistence supply air temperature delivered into the zone. And, because the primary duct is supplying cold air into the box, the overall efficiency of the entire building gets ignored.

Because Pelican’s sequence of operation is different than standalone VAV systems, the mechanical need of the solutions are different. A Pelican zoning solution does not push cold air into the box during a reheat cycle. This eliminate a few things: first it gets rid of the largest inefficiency in a traditional VAV system (reheating already conditioned air), second it remove the need of the zone to have enough reheats to counteract a cold supply air temperature (less reheats needed to heat the zone), and finally it creates a more consistent zone temperature rate-of-change because the air temperature is always the same during each reheat cycle.

In our data analysis, Pelican’s sequence generates a quicker zone heating rate at a lower reheat stage requirement, in comparison to traditional VAV. It is also a sequence that focuses on the overall efficiency of the building versus just the standalone zone, without sacrificing on comfort.

So, how should you decide how many stages of reheat should be wired to your Pelican zone thermostat?

If you have multiple reheat stages, the way to decide how many stages should be wired to a Pelican zone thermostats should be based on the required reheat output BTUs needed to maintain comfort in relation to the zone parallel fan’s CFM output, and the size/heating load the zone might have.

The above can be considered a complex decision, but a good one to follow. To simplify, many of our customers choose to use just one of the reheat stages, while others choose to use multiple. What you need to figure out is how many reheat stages the zone needs to get a good heating rate into the zone, but not too high of a supply air temperature that it becomes uncomfortable for the occupant.

From an energy usage factor, we have found it does not matter much which option you selection (as long as you are meeting the above needs of the zone). This is because the energy calculation between the two options becomes very simple with the Pelican solution. Here is an explanation*: If you use just one stage of reheat: the energy usage during a reheat cycle would equal half of the energy consumption of running two stages of reheat, but the one reheat would need to be active for twice as long. Therefore, the total energy consumption over the run-time is equal to each other (I.E.: It uses the same energy in both situations).

Now obviously we do not get a 100% heat conversion rate, but even if there is an 85% transfer rate, it does not create huge variances in overall building energy usage. It really just becomes a decision on what zone rate-of-change you want to maintain comfort.

The greatest energy difference with Pelican versus traditional VAV controls is that Pelican does NOT reheat already conditioned air. This is an energy efficiency factor multiple times higher than how many reheats are active.

*This explanation assumes both reheat stages are the same size with the same heat transfer rate. But, the ratio of energy consumptions to zone heat rate-of-change remains the same even if the reheat stages are different sizes. With Pelican it always comes back to choosing the number of stages that provides the zone rate-of-change you want/need to maintain comfort.