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.