My HVAC Unit is Running Too Much

Today on the Reschexpert blog we look at troubleshooting an HVAC unit in construction.  In this instance a unit is not keeping up.  What are some good ideas and concepts to consider when trying to find a why an HVAC unit is having trouble keeping up.

I’m trying to figure out what’s going on with my HVAC system, the house was finished recently. The house will sometimes maintain a 70°-72° internal temperature, but will run for 13hrs+ in doing so. Sometimes the house will struggle to maintain those temperatures and it will be 75°+ for hours while the AC is supposedly cooling. During the latter event the thermostat says the system is cooling but the air coming out of the vents is only a couple degrees below room temperature.

Here are some good next steps on new construction in regards to HVAC performance.

Have the HVAC unit inspected. It should be under warranty.

Have the ducts inspected for tightness. Is there a loose fitting somewhere in the attic or crawl space that conditioned air is escaping.

Make sure the ducts are insulated. R8 on insulation on a duct in an uninsulated area like a crawl space or attic will allow more of the conditioned air to reach your living space.

Install blinds, drapes, or curtains on the windows. The summer sun can heat up a living area to the point where an HVAC unit will have difficulty keeping up on days when the temperature is high.

Inspect the insulation thickness in the attic. If you are supposed to have 10″ or 12″ of blown in insulation in the attic is there that much insulation. Are there spots where the insulation has settled to below this threshold. Are there bare spots with little or no insulation?

The run times you mention are pretty typical of newer units and this current July weather pattern. I attached typical run times over the last 10 days of 2 separate units where the average daily high has been around 80-90 degrees. These are my personal units, but 12-18 hour run times during a day are not uncommon this time of year. The units today are much more energy efficient so while they may have longer run times they use less energy than the units from just 5-10 years ago.

One of the HVAC unit’s main functions is to limit the amount of humidity. So longer run times can be better in controlling mold and mildew than having an oversized unit that runs infrequently and does not have time to run a full cycle and remove the moisture inside a structure.

Are all your windows and doors sealed properly? Can you see daylight around any of them or on a windy day can you feel a breeze? Sometimes on new construction it is possible that several windows or doors may have not been fully sealed during install.

Thanks for reading the Reschexpert blog.  If you need a Rescheck, Manual J, Manual S, or Manual D we can help.  Email PDF plans, jobsite address, and square footage to service@rescheck.info and we will get you taken care of.