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Make Ceiling Fans Work Even Better!

The ceiling fans in my upstairs rooms work great most of the time but in this recent heatwave with temperatures over 30 °c  my loft was getting unbelievably hot – I measured 47 °c !  Yes, I do have glass-fibre insulation above my ceilings of 4″ or more and I also had sprayed the whole of my loft with isocyanate polyurethane foam to help keep the heat out.  However without any decent ventilation my loft became an oven.  The hot air rose to the eves of my roof was trapped there and just got hotter and hotter throughout the day.  Eventually that heat spread downwards through to my ceilings which became very warm.  The ceiling fans were drawing in the now hot midday air into my rooms and the air got no cooling from my ceiling structure.  So yes the air was moving and much better than no fan at all but no-where as good as I knew it could be.

Putting my chemical engineering hat on and remembering back to my lessons on heat transfer and air-flow I came up with the following solution which worked a treat:   I installed 2 low-noise 80 w German exhaust fans in my loft to remove the excessively hot trapped air thus preventing the roof and whole upstairs getting really hot.   One fan was blowing air in from the outside and other extracting the 47°c air and blowing it out of the loft (the one shown in the picture).  These fans extracted 750 m3/hr of air, nowhere near as much as a ceiling fan but more than sufficient to change the air in my loft space 10 times an hour bringing the temperature down to the max outside air temperature.  I wired them up to a thermostat so that whenever the loft temperature rose above 20°c they kicked in.  I also left them running overnight so that the whole loft and ceilings cooled to the lower air temperature of the early morning ie 3-4 am when it was 15°c.   In the mornings my upstairs was amazingly cool as the air cooled on entering the house despite temperatures rising rapidly outside up to 28°C by midday.  Back inside beneath the foam and glass-fibre insulation my ceilings kept relatively cool, meaning my upstairs rooms continued to feel lovely and cool even in mid-afternoon when the whole house had basked in the sun and the air temperature rose to above 30 °c.  Inside upstairs was around 24°c ambient temperature and after allowing for the 4-5 °c wind chill  effect of the ceiling fans working dropped to 21°c.   Downstairs was even better staying below 20’c with no fans turned on.  Contrast this with my visit to Topps Tiles store in Wokingham on Thurs which is a big shed with high ceilings but no ventilation whatsoever.  It was much hotter inside the shop than out and was well over 30 °c – I felt very sorry for the wilting staff who were suffering!

Overall a great result keeping my upstairs office as cool and pleasant as with air-conditioning but without the expense and health downsides.  Instead of having to work downstairs after 1pm I can now continue working all day upstairs.  Another thumbs up for fans – exhaust and ceiling working together to beat the heat!