Attic fan tests: 1 week of data

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:Benefits of an attic fan

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!

6 thoughts on “Attic fan tests: 1 week of data

  1. well the attic fan may not make a difference on heating and cooling, but the cooler temps will help prolong roof life for an asphalt based roof. One interesting idea that I have had is the inclusion of motorized dampers to control airflow. The North side of the house does not need as much conditioning as the other sides. The first floor and basement do not need as much during the day. So damping down some of the main floor to push it to the 2nd floor would really help. Crazy thoughts when I should be doing my day job!

      • yeah, I really like the idea. My house currently has a system that runs off compressed air to control the dampers, but it only covers the basement. So when we are not using the basement in the winter the system is off. In the summer the system is off. I really would love to zone my second floor, but my current controller will not handle it and then there are the air lines that I need to run. Seems easier to go electric and control each one with a moteino

  2. Other things to consider:
    – You house orientation, and how to add shading/screening (i.e. or with tree – especially deciduous trees) or white painting sun-exposed parts.
    – I do not have attic, but I wanted to ask if and only if moving air aback down, at the time when the attic is cooler than the below area is possible-and-safe.
    – Did you consider moving air from outside into the attic rather than from rooms below attic. You are trying to cool the attic space which is hotter than outside (regardless from where the cooling is coming) i.e
    – If you have basement, then did you consider moving cool air into the rooms above at the right time.

    Other things individually may not seem involved, but they (collectively) do really affect the main objective (staying cool) and need to be reviewed
    – type of food.
    – clothing.
    – house activities (showering time, washing machine time and cooking time).
    – heat generating equipment in the house.
    – installing a fan inside the house if none are there

  3. Hi Felix,
    In my last job, we did energy efficient retrofits for houses, we used inline booster fans for the long runs, depending on your HVAC system this could help get more conditioned air to the location you want it. on average the ROI is between 1 and 2 years. for the houses we installed them.

  4. Being the son of a carpenter and having done all of the different jobs in house construction (just, not as jobs), I would clean out the bedroom, tear off the drywall or the exterior wall(s), replace the insulation, possibly adding some decoupling for both thermal and acoustic dampening to the wall, and then re-drywall, paint, etc. (Good time to inspect/repair/upgrade the outlets, too! Might even put in outside- and inside-insulation temp sensors to see just what the wall is doing…)

    But if you can’t do that yourself, it would likely be pretty expensive to have done.

    (Sigh… If I only owned a house…)

    The suggestion of a heat-reflecting exterior paint is also a good one, and pretty cost effective. A lattice on that exterior wall about 6″ off the wall, either wide boards providing their own shade or a nice creeping vine for shade would also help lower that evening heat radiation. Exterior shading shutters to close during the day to block the window from direct sun would also be smart (blinds and curtains help a little, but it is usually too late once the sunlight gets through the window).

    However, all of that very much depends on the style of the house and the possible rules laid down by whatever homeowners association or town codes there may be. (I once got a ticket for not mowing my lawn often enough, and a warning about “having” a piece of cardboard wrapped around my mailbox post … that had been blown there by the wind. I don’t live in that town anymore.)

    Personally, I’ve been pondering building a human-sized “hamster wheel” to (literally) run emergency power. Recently, I wondered just how much effort it would take to use this to physically operate the compressor and fans (as in, don’t convert kinetic->electricity->kinetic)… but that’s probably going off-topic. 🙂

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