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mhoush: Coil Bypass Overview
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<h1>
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Coil Bypass Overview
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</h1>
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<div class="-mt-6">
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<div class="text-gray gray-links text-sm">
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<span class="border-r border-gray pr-2 mr-2">August 10, 2023</span>1010 words, posted in <a href="/articles/tag/hvac/">HVAC</a> and <a href="/articles/tag/design/">design</a>
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<img alt="banner" src="http://localhost:3000/articles/images/2023-08-10-coil-bypass-overview.png"/>
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<p>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.</p>
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<h2>What is a Coil Bypass</h2>
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<p>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.</p>
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<p>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.</p>
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<p>The coil bypass strategy, as far as I know, was pioneered by <a href="https://www.linkedin.com/in/harry-boody-9b8a4366/">Harry Boody</a> 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.</p>
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<h2>The Problem</h2>
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<table>
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<thead>
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<tr>
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<th>Why</th>
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<th></th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>Question</td>
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<td>Why would we want to utilize a strategy such as the coil bypass?</td>
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</tr>
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<tr>
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<td>Answer</td>
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<td>Improved indoor air quality (IAQ)</td>
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</tr>
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</tbody>
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</table>
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<p>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.</p>
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<p>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).</p>
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<p>We determine the volume of the conditioned space.</p>
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<p>2,500 x 9 = 22,500 ft^3</p>
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<p><img src="/articles/images/2023-08-10-volume-equation.png" alt="volume-equation" /></p>
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<table>
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<thead>
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<tr>
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<th><strong>Where:</strong></th>
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<th></th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td><strong>V</strong></td>
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<td><em>is the volume of the home</em></td>
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</tr>
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<tr>
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<td><strong>ACH</strong></td>
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<td><em>is the desired air changes per hour</em></td>
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</tr>
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<tr>
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<td><strong>60</strong></td>
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<td><em>conversion from hours to minutes</em></td>
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</tr>
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</tbody>
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</table>
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<p>Below is a table of the required CFM to meet the different air changes per hour.</p>
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<table>
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<thead>
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<tr>
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<th></th>
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<th align="center">CFM</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>(22,500 x 3)/60</td>
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<td align="center"><strong><em>1,125 @ 3 ACH</em></strong></td>
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</tr>
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<tr>
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<td>(22,500 x 4)/60</td>
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<td align="center"><strong><em>1,500 @ 4 ACH</em></strong></td>
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</tr>
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<tr>
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<td>(22,500 x 5)/60</td>
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<td align="center"><strong><em>1,875 @ 5 ACH</em></strong></td>
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</tr>
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</tbody>
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</table>
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<p>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.</p>
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<h3>Multi-Stage Systems</h3>
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<p>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.</p>
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<p>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.</p>
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<p>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).</p>
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<p><img src="/articles/images/2023-08-10-register-throw.png" alt="register-throw" /></p>
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<p>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.</p>
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<p>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.</p>
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<h2>Conclusion</h2>
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<p>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.</p>
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<p>In future articles we will continue to explore some of the features, benefits, and challenges presented by such a strategy.</p>
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<h2>Related Resources</h2>
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<p><a href="https://hvacrschool.com/bypass-dehumidification-airflow-hvac-design/">HVAC School - Bypass Dehumidification / Airflow HVAC Design</a></p>
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</article>
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<div class="border-t border-light mt-8 pt-8">
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<h2 class="text-4xl font-extrabold mb-8">
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Written by
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</h2>
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<div class="prose">
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<h3 class="!m-0">
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Michael Housh
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</h3>
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<p class="text-gray">
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HVAC business owner with over 27 years of experience. Writes articles about HVAC,
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Programming, Home-Performance, and Building Science
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</p>
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</div>
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</div>
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<div class="mt-16">
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<h2 class="text-4xl font-extrabold mb-8">
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<section>
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<h2 class="text-2xl font-bold mb-2">
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<a class="[&:hover]:border-b border-orange" href="/articles/2025/vapor-htmx-todo-app/">Vapor + HTMX</a>
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</h2>
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<div class="text-gray gray-links text-sm mb-4">
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<span class="border-r border-gray pr-2 mr-2">January 05, 2025</span><a href="/articles/tag/general/">general</a>, <a href="/articles/tag/programming/">programming</a> and <a href="/articles/tag/software/">software</a>
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<p>
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<a href="/articles/2025/vapor-htmx-todo-app/"><div>
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Build an example application using Vapor and HTMX.
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</div></a>
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</p>
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</section>
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<section>
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<h2 class="text-2xl font-bold mb-2">
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<a class="[&:hover]:border-b border-orange" href="/articles/2024/free-as-in-freedom/">Free As In Freedom</a>
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</h2>
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<div class="text-gray gray-links text-sm mb-4">
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<span class="border-r border-gray pr-2 mr-2">April 09, 2024</span><a href="/articles/tag/general/">general</a>, <a href="/articles/tag/open-source/">open-source</a> and <a href="/articles/tag/software/">software</a>
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</div>
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<p>
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<a href="/articles/2024/free-as-in-freedom/"><div>
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Salute to open-source software engineers
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</div></a>
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</p>
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</section>
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<p class="prose mt-8">
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