Choosing a New Furnace and Air Conditioner

In May, 2008, we needed to replace our furnace, which had failed. At the same time, we decided to replace our air conditioner, which was working during 2007, but had not yet been turned on in 2008. From our discussion with the estimator, we decided to go with an 80% AFUE furnace, with a dual stage burner and variable speed fan, and a 14 SEER air conditioner. These were below the maximum AFUE of 95% for a furnace and 19 SEER for an air conditioner.

Here's what we considered when making our decision:

Efficiency and payout - In discussing options with the estimator, we learned that one component of efficiency is AFUE, which determines the percent of fuel converted to heat energy. Thus a 80% AFUE converts 80% of the fuel to heat and exhausts 20% to the atmosphere. This was the area of efficiency with which we were most familiar. Since our existing furnace was rated 60% AFUE, we would be getting a significant savings with the minimum 80% AFUE furnace.

However, the estimator also shared that a dual stage burner (e.g. low and high) and a continuous variable speed fan could increase efficiency above the AFUE. A single stage burner always burns in high mode. A dual stage burner can use a low mode, when less heat is needed. Thus, while both burner stages may be 80% AFUE, the low stage will use much less fuel. A continuous variable speed fan provides two benefits. It keeps the house temperature more uniform and continues to transfer heat at the lower fan speed. We decided to pay the extra cost for these options.

For the air conditioner, the minimum was 14 SEER for units with the new 410A refrigerant. We briefly considered buying a higher SEER unit. However, the incremental cost did not payout in our climate zone. Thus, we chose the lowest SEER unit, which was still a significant improvement over our existing 8 SEER unit.

Comfort efficiency - We wanted a system that would regulate the temperature well, versus surging past the set point and then allowing the house to equilibrate back to the set point. The dual stage burners and continuous variable speed fan would contribute to maintaining tighter temperature controls.

Installation difficulty - The 80% AFUE furnace could use the existing exhaust venting trough the roof. However, both the 90% and 95% required PVC venting through the basement wall. Unfortunately, we have a finished basement with drywall ceilings, which would have made it difficult to install new venting without damaging the drywall.

Finally, we contracted the work with the company that had installed a furnace and air conditioner in a previous house. From that experience, I knew they would have reasonable costs and do excellent work.

At this point, we are very satisfied with the air conditioning choice. Using the same set point as last summer, we are more comfortable than with the previous system. It is hard to compare the cost savings since this summer has been much cooler that last year. We'll have to wait until winter to do a comparison on our furnace choice.

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This is not financial or home maintenance advice. Please consult a professional advisor.

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