History of Gas Furnaces - Redux · HVAC School - For Techs, By Techs summary

Key takeaways

Gas furnaces have evolved significantly, yet the fundamental combustion efficiency has not improved substantially since the 1930s.
Safety features in modern furnaces are crucial, especially as buildings become tighter and natural ventilation decreases.
Understanding the differences between combustion efficiency and appliance efficiency can help in selecting and servicing HVAC equipment.
The future of HVAC may include more adaptable technologies and systems that enhance both efficiency and comfort.

Summary

The Evolution of Furnaces

The podcast dives deep into the history of gas furnaces, tracing their evolution from the early days of coal and wood heating. The discussion highlights how the original gas furnaces came into existence by modifying existing coal models. Back in the day, the risks associated with these furnaces primarily centered on fire hazards rather than carbon monoxide poisoning, which was less of a concern due to complete combustion in those older appliances.

Technological Advances and Safety Features

The podcast emphasizes the significant technological advancements in furnace design, especially during the gas crisis of the 1970s. As efficiencies increased, so did the implementation of safety features such as spill switches and draft-induced motors. The hosts discuss the negative impact of drafting appliances in tightly sealed modern homes, where air supply for combustion becomes an issue, leading to potential carbon monoxide risks.

Combustion Efficiency vs. Appliance Efficiency

An important discussion point is the distinction between combustion efficiency and overall appliance efficiency. While modern furnaces have improved in terms of appliance efficiency, the actual combustion efficiency remains relatively unchanged since the 1930s. The conversation uncovers the importance of modulating systems that balance combustion with comfort, while also addressing how excess air impacts efficiency and safety.

Future of HVAC Technology

Towards the end of the conversation, the hosts speculate on future trends in the HVAC industry, particularly regarding variable speed inducer motors and their ability to adapt to different venting conditions. The talk also revisits the importance of correct installation practices to meet industry standards and ensure safety.

Show notes

Jim takes us all the way through the history of furnaces, from the Stone Age when he was a child to modern modulating condensing types.

The goal of a furnace is to move heat, so a furnace uses heat exchangers to facilitate heat transfer. Furnaces have primary and secondary air. The primary air goes through the burner, and the secondary air goes around the flame and is pulled in around the heat exchanger inlet. So, the flame's heat creates a draft that pulls air in.

Natural gas and oil (LP/propane) furnaces are common nowadays, but we initially burned wood and coal in furnaces. The first gas furnaces came into existence by modifying coal, not from the gas lines we see nowadays. Long ago, the flue gases were also exhausted to the basement; CO poisoning was less of a concern back then, as combustion was usually complete. Burning the building was a much more severe risk.

The first "gas crisis" in the 1970s forced us to focus on gas furnace efficiency. In that time, we developed spill switches and retrofit kits that converted furnaces over to spark ignition. In the 1980s, we came out with the draft-induced 80% furnaces we see nowadays. We also eliminated standing pilots and draft diverters. Even though the appliances became more efficient, we didn't actually burn the gas any more efficiently.

So, despite the technological advancements we've made over the years, we don't actually burn gas any more efficiently than we did in the 1930s. However, our modern furnace technology has eliminated standby losses, controlled ignition, and focused on the role of latent heat in combustion.

Jim also discusses:

Flame color and types
Draft hoods and diverters
Products of complete combustion
Excess air: a double-edged sword
Natural ventilation
Efficiency percentages
Furnace testing and ratings
Turbulators
Modulation

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History of Gas Furnaces - Redux