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Community and Q&A

Evaporative cooling and attic ventilation fan

steve_smith | Posted in General Questions on

I’ll begin with acknowledging that attic ventilation fans are nigh universally frowned upon in these parts.  With that being said, I am curious if it would make sense in my situation.

My ~1600sf single story house was built in 2004 and is in CZ 3B.  I have 11kw of PV on my roof.  This is enough to offset all of my home electricity use and the electricity use of two EVs.  The design temperature is 99F.  I have a smart thermostat that allows me to easily view AC run time.  Using my thermostat logs I calculated a cooling load on a 105F day of ~14,000 btu/h.  The AC ducts are in unconditioned space (I know, don’t do that).  I don’t typically use the AC.  I typically cool the house using an evaporative cooler located near the living room and kitchen.  The evaporative cooler is connected to a smart plug.  The smart plug is indirectly controlled by my ecobee thermostat (ecobee -> IFTTT -> smart plug -> evaporative cooler).  When the thermostat rises to 75F the evaporative cooler turns on.  When the thermostat goes down to 73 the evaporative cooler turns off.  If the thermostat hits 80 the evaporative cooler turns off and the AC turns on and holds the house at 80.  This works reasonably well, in part because I have generic up-dux in the bedrooms and don’t have to remember to open any windows.  However, the bedrooms are still substantially warmer that the living room.  My ecobee thermostat has two room sensors, one in the master bedroom and one in a child’s bedroom.  The temperature reported by the thermostat is an average of the three data points (thermostat in the hallway and the two room sensors).  When the thermostat reads 74, in actuality the hallway is high 60’s and the bedrooms are high 70s even with bedroom doors open all day.  I would prefer to have more comfortable bedrooms.  Using the current AC is a pretty bad option.  It is a SEER 10 unit with ducts in the attic.  A couple options I am considering are listed below –

1) Add an attic ventilation fan connected to a smart plug.  Whenever the evaporative cooler turns on, so does the attic ventilation fan.  When the evaporative cooler turns off, so does the fan.  In this setup I would not be concerned with the house being under negative pressure when the AC runs a few times a year.  The hope is the negative pressure will aid the flow of cooler air to the bedrooms.  This would likely cost a few hundred dollars.

2) Abandon the evaporative cooler and current AC.  Install a Fujitsu 15RLS3 in the living room.  The cooling and heating loads are within the capabilities of this unit.  The issue I see with this option is if there is already a substantial temperature difference between the living room and bedrooms with the evaporative cooler, I suspect this issue will remain.  This would likely cost several thousand dollars, but would also remove the need to use fossil fuels for heating.

What am I forgetting or what should I be considering?

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  1. Expert Member
    BILL WICHERS | | #1

    If yours is a typical roof-mounted evaporative (“swamp”) cooler, you should be aware that they normally need a second air pathway somewhere else in the house to be able to operate properly. Evaporative coolers work using air circulation (in addition to the evaporative cooling effect), they don’t typically just recirculate and cool indoor air in the way a regular air conditioner does. Is your system setup that way? If not, fixing that may be all you need to do.

    It’s hard to beat an evaporative cooler for the amount of cooling you get per kWh (especially if you’re in a very dry area), so I’d try to optimize that system before looking at other options.


    1. steve_smith | | #2

      The evaporative cooler is a window mounted unit. It's even in a shaded area, maximizing effectiveness in that way.

      The venting of the house is done via spring-mounted vents in the ceilings of the bedrooms. These vents work by a spring closing a baffle if the house is under neutral or negative pressure. When the house is under positive pressure the baffles open, allowing air to flow to the attic.

      The hope with the gable fan would be that only when the evaporative cooler is on will the gable vent turn on. In this situation the conditioned space of the house will be under positive pressure from the evaporative cooler and the attic will be under negative pressure from the gable fan, enhancing the air flow throw the ceiling mounted baffled vents.


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