A 34% increase in sensible capacity by simply replacing the evaporator coil?

Surely I jest! But I don't. It doesn't happen often. But the circumstances were just right for it to happen at this customer's house.

If seeing is believing, then the picture below is proof. The coil on the right is what I pulled out. The coil on the left is what I installed. Obviously it's much, much bigger. The extra surface area allows for better heat transfer and better airflow. Plus, the new coil is just a little thinner. That too allows for increased airflow.

The new coil ended up on their furnace as shown below. It's not the greatest picture. But a camera phone was all that was handy.

Why match that beautiful new evaporator coil to the monstrosity shown below? Taxes! Namely, the customer needed to spread out expenses over two different years. So for the summer of 2004 the home was cooled with the old air conditioner. But with a 34% increase in usable capacity, they were VERY happy with its performance.

Now for the nitty-gritty. Sensible capacity equates to temperature reduction. It's all we care about in a dry climate. As such I'll sometimes refer to it as heat removal. The way you calculate sensible capacity is to multiply the airflow times the temperature drop times a constant of 1.08. The rest of an air conditioner's capacity is used for moisture removal. Since you can't see that happening on a thermometer, it's somewhat hidden and as such referred to as latent cooling capacity.

Before the coil was replaced we had an airflow of 800 CFM with a temperature drop of 16 degrees. That means our sensible capacity was 13,824 BTUs per hour, which is only 58% of the air conditioner's total capacity. The other 42% was either removing moisture or doing nothing at all. If you've read the applicable section of this site, then you know a typical system runs about 70/30 sensible to latent. A high performance system will run 80/20 or better. This system was running 58/42!

After the new coil was installed, we had an airflow of 950 CFM with a temperature drop of 18 degrees. Our sensible capacity was then at 18,468 BTUs per hour, which is 77% of the air conditioner's total capacity. The result was a 34% increase in sensible capacity. 77/23 is getting real close to high performance territory. Not bad for an old piece of junk.

In the spring of 2005 I finally put the old clunker out of its misery and installed the beauty below.

Another example of a big coil is below. The new coil at the top is rated for three tons. Below is a 5 ton coil sitting next to a standard 5 ton furnace. When boxed and installed the coil looks like it's 2/3 the size of the furnace. When compared to a low profile furnace the coil IS nearly as big as the furnace.

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Contra Costa

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