Every now and then we get asked a question and realize, “the truth is not as straight forward as it seems.” Or “there is a lot more at play here.” Today, that question focuses on why we need to flip on the AC, despite it being perfectly cool outside our homes.
It’s late Spring in Pennsylvania, quickly approaching Summer. Our daytime highs range anywhere from 68 degrees clear to 92 degrees right now and this is considered the ‘cool’ part of the season before things heat up. At night, the lows can range anywhere from 48 to 62 or so for now. This is just pleasant weather. We don’t necessarily think about the air conditioner much yet. Take off your sweaters and you’re going to be pretty comfy outside.
The same however, is not necessarily true for inside the house. Despite it being merely 65 degrees outside, and the heater having sat quietly off all night long, my house was already 80 degrees this morning. We had windows open and fans running to suck in the outside air, but the house was still roasting inside.
The first culprit for all this heat is a key part of any efficient building: insulation. My house, like most others will do its best to resist losing heat. There’s thick insulation in the walls and as we’ve covered before, it works well. Whatever thermal energy is in the house, is going to remain there.
This poses a bigger problem than you think. With the home heating off, there is actually still a significant amount of heat being generated. Every person and computer is bringing heat to the environment. Your body’s core temperature is around 98 degrees, give or take a few degrees. You’re radiating heat to the environment. Your TV, is making heat. Your phone charger is making heat.
Modern housing is like walking into a sauna wearing arctic-grade pants and coats. We’re patching every single leaking bit of energy to make the winters easier, but we’re at the point where small amounts of heat can go a long way.
Not Enough Cooling Power
As a consequence of this, we need to displace a lot of heat to cool a building down. Let’s consider a single room, we’ll say it’s a decent sized bedroom: 15′ x 8′ x 8′. That’s a total volume of 960 cubic feet. Higher end window fans pull air into a home at 400 cubic-feet per minute (CFM). In an ideal world, this means we can replace every bit of air in that room within 3 minutes. In reality, it’s less cut and dry.
The physical fan isn’t going to hit that 400CFM rating most of the time. Every foot of air in generally means another foot of air out. If the room doesn’t have ventilation to support air going out, the fan’s going to bog down trying to compress the air. Air compresses pretty readily, but for a small fan, it’ll lower the RPM and kill its performance.
This still doesn’t explain everything though, does it. A window fan in the fall or earlier spring can easily chill a room. Even an open window with a good breeze and a cool day chills a room. What gives? The needed temperature change is the key issue. We need to not only replace the air with cooler air, but it must be sufficiently cold to to absorb the rest of the heat in the room.
Everything in the room is already warmed up. The surface and material of the furniture, the carpeting, the walls, all has heat. It’s all thermal-mass that must be overcome, plus the actual ongoing heat generation. When we blow in cool air, it can only absorb so much heat. To effectively reduce the room’s temperature, we either need more air or much colder air.
Where the Air Conditioner is King
This is exactly the setting that demands an air conditioner. We’ve hit the tipping point where regular cool-air is not cool enough to offset the existing heat and heat sources. It doesn’t matter that it’ll be 58 degrees at 6 am. You need it to be colder for longer spans of time to effect a change. Nature’s not cooperative with this, but air conditioners are, readily generating temperatures near-freezing, cooling the room, and then relying on the room’s own heat to keep defrosted and running well.