The Washington, DC area is in the midst of a sizzling heat wave, with triple-digit highs. Air conditioning units are being stretched to the limit, and some residents are finding out that their systems are just not up to the task of keeping the house cool. But is it the air conditioner, or something else?
Air Leaks
Some common sources of air leaks
Unintentional air leaks can suck cool air out of living spaces, drawing in more hot air from outdoors. Even tiny gaps around recessed light fixtures, vents and other penetrations can add up to a lot of leakage. A simple way to track down air leaks is to light a stick of incense and go through the house, checking around every wall penetration (electrical switches and outlets, exterior doors and windows, ceiling lights, etc.). When you find a draft, plug it with insulation, caulk or weatherstripping.
Insufficient Insulation
Insulation was swept aside for access and not replaced.
In the DC area, the minimum recommended insulation level is R-38. R-40/50 is better, and that is 14” of Fiberglas or 10” of cellulose at a minimum, with no paths, gaps or voids. A single small void can reduce the overall R value in the entire attic.
Contractors working in attic spaces sometimes clear footpaths through insulation and fail to replace it or fluff it back up as they leave. Settling is also an issue; you should inspect your attic regularly to make sure the insulation is in place.
One common weakness in attic insulation is the access hatch. It rarely has the recommended insulation, and that means a big reduction in overall attic protection. One way to insulate the hatch is to construct a simple open-bottomed box out of rigid foam insulation and silver foil tape (which is rated up to 150 degrees). This keeps the access hatch insulated, and is easily moved aside when you need to get into the attic.
Inadequate Ventilation
A powered attic fan can help your HVAC efficiency
If the attic is 150 degrees, and you want the living space at 72, the insulation better be in really, really good shape. I have recorded temperatures above 150 during my inspections. The passive ridge vents just don’t hack it in our area; you really need active ventilation, meaning a powered fan, to keep the attic between 110 and 120 degrees or cooler. Look at it this way: If you have an AC/Heat pump in the attic, and its metal chassis is at 150 degrees, how is it going to pump out 65 degree air without spinning the electric meter really fast?
Lack of Maintenance
If your HVAC unit is in the attic, it needs regular attention. Air filters need to be changed every 30 days. AC units must be serviced each spring to make sure they have the right amount of coolant or you will have high electric bills and the unit won’t stand a chance of keeping up on these triple-digit days.
Design Considerations
The design temperature in the DC area is 95 to 100 degrees. This means that units are designed to be working at 100% capacity when the outside temperature is 95 to 100degrees and the inside “middle of the room” temperature is 78 degrees. If your unit is cycling on and off on a day like today, there is a problem. This could indicate that the unit is too big and is cooling the house too fast and not dehumidifying the house, so it will be cold and clammy because it is not running long enough to remove the humidity. Units need to be running 14-20 minutes before they reach peak dehumidification. If it cycles before that, you have a problem.
If your unit is undersized, you will not be able to cool the house down no matter how long the unit runs. This is why it is critical to have a properly-trained technician assess your home’s heating and cooling needs. Ask for a “Manual J” assessment; this is an industry standard survey that measures the square footage of window and wall space, along with other factors, to determine the house’s requirements.
A lot of the efficiency of an HVAC unit also hinges on proper installation; I have seen too many houses with improperly installed systems and/or faulty ductwork.
Practical Issues
The temperatures we’re seeing in the DC area are outside the normal range, and it really isn’t practical to install HVAC systems that can handle occasionally extreme temperatures. Doing so means the unit won’t operate efficiently on the more normal days, and will just increase the operating and maintenance costs.
