Humidity After New AC Install: Do I Need a Manual J

Question:

Hello, our 18-year-old 5-ton unit failed earlier this year. It worked great until then, keeping the house cool and the humidity around 35% to 50%. A contractor installed a new two-stage 5-ton unit in early March. They said the 18 year old ducts had to be resized to support 5 tons, and claimed they were only able to provide about 4 tons of airflow. After the ductwork changes and new AC install, the humidity has usually been between 59% to 71%, mostly in the mid-60% range. Do I need a Manual J, and is this situation okay?

Answer:

It’s understandable to be concerned about the increased humidity after your new AC installation. Let’s break down the potential issues and what steps you can consider.

It can be challenging to accurately assess the insulation, windows, and doors in older homes, which significantly impact cooling load calculations. It is a good thing the ducts were resized. Newer ducts probably included a significant amount of new insulation and any loosened fittings should be tighter with the newly installed duct system. Reduced airflow from smaller older ducts can definitely impact the system’s ability to dehumidify effectively.

Regarding whether you need a Manual J: A Manual J calculation is the industry standard for determining the precise heating and cooling load of a home. This would provide a more accurate understanding of your home’s cooling needs and whether a 5-ton unit is indeed the correct size, especially given the airflow limitations. However, the time to do this Manual J report would be before the new unit was installed.

Are you “okay” with the current humidity levels? Generally, indoor humidity levels between 30% and 60% are considered comfortable and healthy. Humidity consistently in the 60% to 71% range is in the higher range.

As you can see from the graphic I checked my personal humidity levels at my house after receiving this email and the humidity was 59%. Which personally, does not bother me. On mild day, like today when the temperatures outside are in the lower 60’s the AC unit will not run. So you could naturally expect the humidity levels to increases as daily life takes place inside the home.

Humidity After New AC Install: Do I Need a Manual J

Some things that might happen in a home when the humidity stays too high for too long.

• Discomfort: Feeling sticky and less comfortable even at moderate temperatures.
• Mold and Mildew Growth: Excess moisture can encourage the growth of mold and mildew, which can be harmful to your health and home.
• Potential Damage to Belongings: High humidity can damage wood, fabrics, and other materials.

Potential Solutions and Things to Consider:

• Dehumidifier: As I mentioned, if your current contractor is willing to work with you, a dehumidifier might be a viable solution to supplement the AC’s dehumidification capabilities.
• Increased AC Run Time: It’s true that as the weather gets hotter and your AC runs for longer periods, it will naturally remove more humidity from the air. Monitor the humidity levels as the summer progresses to see if there is an improvement.
• Check Ventilation: Ensure your clothes dryer is properly vented to the outside and that the vent line is clean. Run bathroom exhaust fans during and after showers and baths, and use your kitchen hood fan when cooking, especially when boiling liquids. These activities introduce moisture into the air, and proper ventilation helps remove it.

In conclusion, the high humidity levels after a new AC installation, especially with reduced airflow, might be a concern. Exploring dehumidifier options and ensuring proper ventilation are good initial steps. You might find that those steps alone reduce your indoor humidity to a resonable level.  The Manual J typically takes place before the installation along with a Manual S and Manual D to make sure the equipment and ductwork is sized according to the local climate, system, and personal needs.