A quick update to the attic fan cooling experiment (check the previous post to read about the setup). I ran the fan for a few days in a row, from around noon to 5PM, at which point the HVAC kicked in until late into the evening. There were some HOT days and some WARM days, with a hot day with PM showers which produced some interesting data. from the WeatherShield sensor motes in the attic and the master bedroom below it. Here’s conceptually how an attic fan is supposed to work, in theory:
My cheap fan I installed in the attic hatch draws 85W on the high speed and uses ~0.45kWh for a 5h run time), not bad. Here’s the temperature graph after several days with different weather conditions, read on for explanations.Notice the huge temp swings of the attic, and the relatively small changes in the master below it. July 29th was the HOT day with PM showers and moderate wind which cooled down the attic. I also had the fan going but the rain/wind cooled the attic quickly through the built in soffits and vents. On the rest of the days the fan did a good job of chopping the “hotbergs” tips off, they look like mouse bites :). To be more effective it has to be started soon after the sun starts to blast the roof around 10am. The master temps were zoomed in for more detail. Here are the humidity measurements:
Notice the humidity drops to the lowest point when the temps are at the peak, also true for the master. Anyway the surprising part is the master temperature. It is relatively flat. The temp swings are only about 7 degrees at the extremes. During the day it rises to the upper 80s and it feels uncomfortable. However even after the HVAC takes over in the afternoons it still has a hard time cooling the upper level. Why? I begin to realize there are several factors, most notably:
- I added about 12″ of blown insulation in the attic after the first winter after moving in. This does a great job keeping the master warmer than a cold attic, and cooler than a hot attic (46F max difference I recorded in this period on the hottest 90F day). So even if the attic is getting cooled down by the fan, the master bedroom is still getting rather warm and a 20F cooler attic in the course of 5 hours won’t make it much cooler in the master below which is around 45F cooler anyway on a hot day.
- the master bedroom has large exterior walls that are warmed by the sun heat most of the second part of the day when it’s hottest outside. The walls are not insulated very well and they act as radiators, retaining and releasing heat into the house for a long time after the sun starts to set.
Now to make sense of all this. I think the fan does the job and maybe with more effort the system could be improved to move more air etc. The end goal was to cool the living space (which isn’t as effective as I had hoped because of the factors mentioned above) by first cooling the attic above it (which it does a good job actually). Here’s a breakdown:
- It’s a daily chore to cover/uncover the hatch.
- The fan start/stop times could be automated via a SwitchMote
- the fan is pretty low power, up to 1kWh/10hrs runtime
- makes a big dent in the attic temperature
- makes small dent in the master bedroom below, it’s ultimately another few hours of HVAC conditioning that brings temps down in the master
So is it worth it? It feels almost like it’s not, although it certainly doesn’t feel like a fail since the fan does make a difference, at least in the attic. Running the HVAC throughout the afternoon hours makes it run at relatively low duty cycle and keeps the second level pretty comfy so it doesn’t have to run full cycle in the evening hours. It eats more kWhs but it’s convenient compared to the attic fan. I realize not having a new or custom built home with the high R foam insulated walls and attic means small efforts like these will hardly make a dent, although I have tried this and many other things. The stock homes are built to sell and make the builder the maximum amount of profit, not create the maximum amount of efficiency, energy savings and comfort for the owner. The next big step would be spending $5k or more on a high efficiency HVAC/furnace. But doing the math again points to the same conclusion, it will take the next 10 years or more to recoup that upfront investment, since the extra 15% efficiency will take so very long to amortize in electric/gas bills because the house is still poorly insulated and fixing that is yet another daunting task if not impossible. Just like hybrid/electric cars, you’re moving the emissions at the power plant, and paying a huge premium upfront for it being hybrid, sounds like a terrible deal. Same for the high efficiency HVAC. At my last tune-up the HVAC guy told me I should forget about upgrading to an expensive high efficiency HVAC since it’s not too old (2001) and I should let it die before I change it since the gain will be far too low to make it worth it. He was probably right.
Another observation from this is that despite the wild temperature drifts, the frequency of the radio in the attic does not drift enough to fall outside the receiver Moteino bandwidth (which is in the basement at about 74F when it’s hottest in the attic, hence a ~60F difference!!!) and cause a broken link. This means the crystal used is high quality and low temperature drift, even at the far end of 915mhz where the drift effect would be doubled compared to the 433mhz band. This endorses the same observations for the mailbox notifier which also suffers from very high temperature swings because of its location (exposed sun beat metal mail box).
So this wraps up my attic cooling experiments. I will use the fan on very hot days to cool the attic but it won’t make a huge difference down below. If you’ve done similar experiments and had better results or I made a an epic mistake somewhere in my setup let me know!