Coil Bypass Overview
Series: Coil Bypass
This is the first article in a series that explores the idea of a coil bypass strategy in an HVAC system. This article introduces you to a coil bypass strategy at a high level, future posts will dive deeper into the features, benefits, as well as the challenges of this style of system.
What is a Coil Bypass
A coil bypass is not to be mistaken for a zoning system bypass, where airflow is “relieved” from the supply side of the system back into the return. Instead, a coil bypass diverts a portion of the airflow around the coil using a bypass damper(s). The bypass can serve several functions depending on the application, but in general it allows for a constant volume of air to be delivered to the space while the output of the coil can be shifted towards more or less dehumidification. In other words, it decouples the total system airflow from the coil airflow.
The bypassed air mixes with the supply air stream to act as a reheat source, however unlike a typical reheat source it does not add more sensible load to the structure, instead it just brings the supply air temperature closer to the existing home’s temperature while still covering the latent and sensible loads of the home. A warmer duct system reduces the losses of the duct to unconditioned spaces as well as reduces the risk for duct condensation.
The coil bypass strategy, as far as I know, was pioneered by Harry Boody of Energy Innovations and Scientific Environmental Design, Inc. However their websites are no longer active, so I’m not sure if they are still active in the HVAC design space or not.
The Problem
| Why | |
|---|---|
| Question | Why would we want to utilize a strategy such as the coil bypass? |
| Answer | Improved indoor air quality (IAQ) |
ASHRAE’s recommandation for the amount of air changes per hour (ACH) in a residential structure to be in the range of 3-5 ACH, and in general the higher the better, along with a MERV 13+ filter. In some / most cases the system airflow does not meet that criteria, especially low load homes or high volume homes.
For example, let’s imagine a single story ranch home that is 2,500 square feet with 9 foot ceilings. This home is relatively tight construction and after doing the heating and cooling loads we’ve selected a 2.5 Ton system for this home. It is located in a green grass climate that needs some priority on dehumidification and requires an airflow of 350 CFM/Ton (875 CFM).
We determine the volume of the conditioned space.
$ 2,500 \times 9 = 22,500 $ $ ft^3 $
We can then determine the CFM for the given air changes per hour (ACH) using the following formula.
$$ \frac{(V \times ACH)}{60} $$
| Where: | |
|---|---|
| V | is the volume of the home |
| ACH | is the desired air changes per hour |
| 60 | conversion from hours to minutes |
Below is a table of the required CFM to meet the different air changes per hour.
| CFM | |
|---|---|
| $ (22,500 \times 3)/60 $ | 1,125 @ 3 ACH |
| $ (22,500 \times 4)/60 $ | 1,500 @ 4 ACH |
| $ (22,500 \times 5)/60 $ | 1,875 @ 5 ACH |
As you can see we have a discrepency of meeting even the low end of 3 ACH. The high end of 5 ACH is over 2x the airflow for our 2.5 Ton system. The coil bypass strategy is one viable way, by decoupling the total system airflow from the coil airflow without, which eliminates the need of an auxilary fan / system that circulates air through some sort of filtration system.
Multi-Stage Systems
A challenge with multi-stage systems, even when sized properly, is that we often run at part-load conditions, and spend the majority of the time in lower stages. The lower stages often do worse at dehumidification than when running at full load.
When the equipment runs in lower stages on a traditional system the total system airflow is reduced even further from the recommended air changes per hour. This reduced airflow also causes the throw of the air from the registers to be reduced which can lead to increased odds of stratification, poor air mixing, and increased potential for poor mean radiant temperatures (MRT) of the surfaces. The decreased airflow in low stages, lowers the velocity in the duct system, while low velocity is not a concern, it does increase the duct gains and increase the possibility of condensation on the ducts when they’re located outside of the thermal envelope of the building.
Let’s imagine we have a duct system that has high wall registers located in a soffit at the interior wall that moves 100 CFM and we are trying to throw the air to the exterior wall which includes a window. The wall is @ 12 feet from the register. We’ve selected a register that meets the criteria, at high stage airflow it has a throw of 11.5 feet (shown as the green rectangle). When the system runs in low stage, the airflow is reduced to 70% of high stage (70 CFM), which would give us a throw from the register of @ 7 feet (shown as the red rectangle).
The reduced flow through the register causes the air to only make it about 60% across the room before reaching it’s terminal velocity, which can cause the room to feel uncomfortable since the air never reaches the exterior wall and window.
By decoupling the fan from the coil airflow it is possible to run in low stages, still have adequate dehumidification performance out of the system, and achieve the proper throw from the registers.
Conclusion
In this article we’ve begun to scratch the surface of what a coil bypass strategy is in an HVAC system, as well as some of the challenges that it can help solve. We’ve learned about why we may desire to decouple the total system airflow from the coil airflow.
In future articles we will continue to explore some of the features, benefits, and challenges presented by such a strategy.
Related Resources
HVAC School - Bypass Dehumidification / Airflow HVAC Design
