102 lines
6.2 KiB
Markdown
102 lines
6.2 KiB
Markdown
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tags: HVAC, design
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---
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# Coil Bypass Overview
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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
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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
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system.
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## What is a Coil Bypass
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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
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return. Instead, a coil bypass diverts a portion of the airflow around the coil using a bypass damper(s). The bypass can serve several
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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
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of the coil can be shifted towards more or less dehumidification. In other words, it decouples the total system airflow from the coil
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airflow.
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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
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sensible load to the structure, instead it just brings the supply air temperature closer to the existing home's temperature while still
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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
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reduces the risk for duct condensation.
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The coil bypass strategy, as far as I know, was pioneered by [Harry Boody](https://www.linkedin.com/in/harry-boody-9b8a4366/) of Energy
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Innovations and Scientific Environmental Design, Inc. However their websites are no longer active, so I'm not sure if they are still active
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in the HVAC design space or not.
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## The Problem
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| Why | |
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| -------- | ---------------------------------------------------------------- |
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| Question | Why would we want to utilize a strategy such as the coil bypass? |
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| Answer | Improved indoor air quality (IAQ) |
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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
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general the higher the better, along with a MERV 13+ filter. In some / most cases the system airflow does not meet that criteria, especially
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low load homes or high volume homes.
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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
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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
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climate that needs some priority on dehumidification and requires an airflow of 350 CFM/Ton (875 CFM).
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We determine the volume of the conditioned space.
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2,500 x 9 = 22,500 ft^3
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| **Where:** | |
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| ---------- | ------------------------------------- |
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| **V** | _is the volume of the home_ |
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| **ACH** | _is the desired air changes per hour_ |
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| **60** | _conversion from hours to minutes_ |
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Below is a table of the required CFM to meet the different air changes per hour.
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| | CFM |
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| --------------- | :-----------------: |
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| (22,500 x 3)/60 | **_1,125 @ 3 ACH_** |
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| (22,500 x 4)/60 | **_1,500 @ 4 ACH_** |
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| (22,500 x 5)/60 | **_1,875 @ 5 ACH_** |
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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
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system. The coil bypass strategy is one viable way, by decoupling the total system airflow from the coil airflow without, which eliminates
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the need of an auxilary fan / system that circulates air through some sort of filtration system.
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### Multi-Stage Systems
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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
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time in lower stages. The lower stages often do worse at dehumidification than when running at full load.
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When the equipment runs in lower stages on a traditional system the total system airflow is reduced even further from the recommended air
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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
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stratification, poor air mixing, and increased potential for poor mean radiant temperatures (MRT) of the surfaces. The decreased airflow in
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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
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possibility of condensation on the ducts when they're located outside of the thermal envelope of the building.
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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
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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
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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
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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
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rectangle).
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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
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can cause the room to feel uncomfortable since the air never reaches the exterior wall and window.
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By decoupling the fan from the coil airflow it is possible to run in low stages, still have adequate dehumidification performance out of the
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system, and achieve the proper throw from the registers.
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## Conclusion
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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
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that it can help solve. We've learned about why we may desire to decouple the total system airflow from the coil airflow.
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In future articles we will continue to explore some of the features, benefits, and challenges presented by such a strategy.
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## Related Resources
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[HVAC School - Bypass Dehumidification / Airflow HVAC Design](https://hvacrschool.com/bypass-dehumidification-airflow-hvac-design/)
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