Q&A - Evap Placement - Short #221 · HVAC School - For Techs, By Techs summary

Key takeaways

Evaporator coils are placed on the positive side of furnaces to protect heat exchangers from moisture, while they are typically on the negative side in air handlers to enhance energy efficiency.
Placing the evaporator coil on the negative side can lead to better airflow and reduced BTU loss due to leakage, despite potentially creating more turbulence.
Careful consideration of coil placement is crucial in dual fuel systems to avoid high head pressure and maintain system efficiency.
Listeners are encouraged to submit their questions for future episodes, enhancing the collaborative learning environment.

Summary

Evaporator Coil Placement Explained

In this episode, Brian addresses John’s question about the placement of evaporator coils in HVAC systems, particularly in the context of air handlers versus furnaces. For furnaces, coils are placed on the positive side to protect the heat exchanger from moist air that can lead to rust during summer months. Conversely, in air handlers, coils are typically placed on the negative side, which enhances energy efficiency because it minimizes BTU loss due to leakage. This design choice also helps maintain a cooler blower motor, though the performance gain is minimal. Brian notes that positioning the coil on the negative side can potentially lead to increased turbulence, affecting airflow. He discusses different configurations including the challenges faced with dual fuel systems that require careful management of coil placement to avoid head pressure issues.

Challenges with Gas Furnaces and Dual Fuel Systems

Brian discusses the specific challenges that arise with evaporator coil placement when integrating gas furnaces with heat pump systems. One major concern is the high head pressure that results from trying to run gas heat over a heat pump coil, which can potentially damage the system. He suggests that an ideal setup would have a parallel configuration that allows both systems to operate without interference, thus optimizing performance and addressing potential problems with heat exchanger corrosion. This segment highlights the importance of thoughtful design in HVAC system configurations for effective dual fuel operation.

Listener Engagement and Future Questions

To wrap up the episode, Brian encourages listeners to actively engage by submitting their own questions related to HVAC systems, furthering the educational aspect of the podcast. He reminds listeners how they can reach out via email and also provides links for tickets to upcoming HVAC events. This not only helps in fostering a community of learners but also enhances the podcast’s content through listener-generated inquiries.

Show notes

In this short podcast, Bryan talks about evaporator coil placement based on a question submitted by a listener, John. Evap coil placement differs in furnace applications versus air handlers.

When we use air conditioners with furnaces, we usually put evaporator coils on the positive side to protect the heat exchanger from rusting out during the summer (due to condensation and the chemical reactions that can occur with the metal heat exchanger). However, when it comes to air handlers, they're usually on the negative side. The coil is usually on the negative side of a fan coil or air handler because it keeps the blower motor cooler (though that doesn't make a huge difference in terms of performance).

The HVAC system will lose fewer BTUs to leakage when the coil is on the negative side, which is better for energy efficiency; those BTUs can reach the conditioned space rather than the closet, garage, or wherever it is stored. Pulling air over the evaporator coil on the negative side, however, could potentially lead to greater turbulence.

We run into challenges with evap coil placement on gas furnaces when we have dual fuel systems (a heat pump and furnace working together). You can't run gas heat over a heat pump coil due to the high head pressure, so a parallel setup would avoid the risks of high head pressure (downstream) or rusted-out heat exchangers (upstream).

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Q&A - Evap Placement - Short #221