Optimizing your home and air conditioning system for comfort
If you’re on the tail end of a super-hot summer, you can breathe a sigh of relief, but recognize that it’s the perfect time to plan for a cooler, more efficient home next year by optimizing your home and AC system.
If your AC was barely keeping up with cooling and humidity-removal, it could actually be because of “holes” in your home’s envelope. Think of it like a boat with holes in the bottom. You have 2 options for less water inside the boat: plug most or all of the holes, or invest in a bigger bilge pump! The same is true with your AC. It’s best to start with holes in the envelope of the home which allow humidity and heat to come in. This inspection and repair will make it more comfortable in the winter, too, which is why the fall is the perfect time to get busy. Let’s look at how these holes work because of the stack effect. In summer, air in the home heats up during the day. This causes a pressure differential: hot air rises and creates a negative pressure on the first floor, which causes air to be sucked into cracks in the lower part of the home, and pushed out as hot air in the upper part, similar to the diagram below. The same thing happens in winter, but to a greater degree: When it’s cold outside, the pressure created by the stack effect is 4 pascals per story of height; when it’s hot, about 1.5 pascals per story of height. It’s even worse in multi-floor residential buildings, because people on the ground floor get cold, and turn on heaters, while people on the top floor open windows to let the heat out, and air literally rushes through buildings (especially elevator shafts). This was the reason revolving doors were invented. “When skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn’t open the front door due to the stack effect pressure. The cold air was rushing in with so much pressure that it was difficult to push the exit doors open.” (Understanding the Stack Effect)
Reason says that if we seal the holes at the bottom of the building, then most of the stack effect will be thwarted! Although the diagram shows air leaking in around windows (it certainly can), we suggest looking for bigger holes first, like gaps in an air conditioning duct below the home, or unsealed penetrations going through a raised floor or exterior wall (like a gas line), or an open ash pit in the bottom of your fireplace (an opening where the ashes are dumped into a chute under the home).
If you are having trouble visualizing where air and humidity are leaking in, find an energy consultant in your area who does blower door tests. A “blower door” test measures the amount of air that leaks through your building envelope per unit time, in units of “Air Changes per Hour at 50 Pascals (ACH50)”. In general, for existing homes, a score below 5 ACH50 (Air Changes per Hour at 50 pascals) is considered good, while 3 ACH50 or lower is excellent. (What is a Blower Door Test: Everything You Need to Know) This video is also an excellent visual on how these tests are done.
Here are some more envelope leaks that are often overlooked. In many cases a can of spray foam will plug up holes in walls; just make sure you check if “fire block” spray foam is needed (fire block foam is recommended around electrical boxes and wires, or other penetrations in a firewall):
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Damaged, loose-fitting or missing flappers on exhaust vents, like dryers, kitchen and bathroom exhaust vents will allow free passage of air into/out of the home
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Gaps around the chimney chase, either in the basement/crawlspace or into the vented attic (Top 10 Air Leaks in Existing Homes — Part 1)
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On the first story of raised, older homes, often the bathtub is set directly onto leaky floorboards, allowing humid air from the crawlspace to communicate with the wall behind it.
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Plumbing pipes and drains are notorious areas for having big, leaky holes through the home’s envelope. (Top 10 Air Leaks in Existing Homes – Part 2)
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The attic door is most often not sealed well, and a ¼-inch gap around the perimeter of an attic access can potentially leak the same amount of air supplied by a typical bedroom heating duct (~100 CFM)! It’s time to stop the hemorrhage of heat and cool. (ATTIC ACCESS)
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Wood to wood connections anywhere: Just because wood touches wood, does not mean there is an airtight seal, and all around the top and bottom “plates” are opportunities for air leakage into/out of the home. (Top 10 Air Leaks in Existing Homes – Part 2)
Next, if you have an old or historic home and the windows are single-pane (brrr!) or you can move one pane back and forth against the other because of loose-fitting sashes, then sealing completely over them or replacing the windows are options to stop these leaks. Here are some articles for window sealing:
Now that you have clamped off most of the leaks that fuel the stack effect, you can take a look at your HVAC to make it more efficient all year round. First of all, if your HVAC has not been maintenanced in more than a year, you’ll want to find a reliable company and request a maintenance call. This maintenance is not only tied to the efficiency of the system, but also your air quality! The technician should take a look at the indoor (evaporator) and outdoor (condenser) coils, which exchange heat. Inside, dirty evaporator coils (which usually comes from not having or not changing the filter on a regular basis) hold dust that becomes the food for mold. The coils are constantly wet when the system is running in cooling mode, so mold can grow quickly. If the technician sees that the coils need cleaning, he can clean them in place, or if they are very dirty and contaminated, remove them to clean them outside (for more cost, of course).
The second way that maintenance affects your air quality is by checking the condensate pan and drain below the evaporator coil inside. This pan catches and drains all the water that drips off the evaporator coil. If slime starts to grow in your pan or drain line, it can quickly back up and overflow the pan, causing a water damage incident in your home, and, you guessed it–potential mold. If your air handler is in the attic or over any ceiling, this type of damage is ugly and costly!
While the maintenance technician is attending your home, ask him/her about any lingering humidity or air quality problems you have in the home. For example:
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If the unit runs in cool mode for only a few minutes and shuts off, it probably does not run long enough to remove humidity from the air. There may be a fan adjustment that can prolong the run time.
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If there’s a strange smell coming from certain registers, they can investigate the ducts for dust and debris, which can allow mold to grow.
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If there’s water dripping from the register onto the floor, this is a problem that should be addressed (also see our article on this problem).
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If the whole home stays humid, there may be hidden damage to the ducts that could allow humid air to be pulled in.
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Does one area of your home stay warmer/cooler than the rest? It’s possible to “balance” the system better using dampers, to redistribute the conditioned air where it’s needed most.
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Do you want to make your HVAC do the work of an air purifier? This is a great capability/use of one of the most expensive parts of your home, especially in areas that are prone to wildfire smoke or urban smog. It requires upgrading your furnace filter to MERV 13 or higher, which should only be done if you have sufficient return filter area (square inches). Many techs don’t know about this requirement, but if you educate yourself (articles here and here) to calculate the area, you can ask the tech on where you can add another return grille/filter if necessary.
These are just some of the benefits of connecting with a knowledgeable technician at least annually (many HVAC manufacturers suggest it should be semi-annually). To learn more about getting more comfort out of your home and HVAC, check out an expert’s free guides.
