Had our furnace and air ducts cleaned today for the first time in years; could have spun an entire dog from all the embedded hair.  The serviceman tried to upsell me with both a new reusable filter (to get 94% of the airborne substances), and a UV light to be installed in the furnace to get the other 6% worth of mold spores, bacteria and no-seeums.  I bought the filter and also a thorough cleaning of the furnace, but am waiting to decide on the UV light--another $400--until I can determine if this will really improve our indoor air quality.  The house is 1000 sq ft, one dog and two dogwalkers live there.  Any opinions?  One dogwalker (me) has asthma.  I couldn't find any research from a casual googling.

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UV lights have their place - but in my opinion are ridiculously oversold.  In order to do ANYTHING worthwhile they MUST shine directly on the evaporator coil / drain pan.  Any other location and they do nothing useful.  Only when the shine on the evaporator coil do they have a long enough exposure time to reduce mold and such.

If the UV light is expected to kill mold spores as they fly past in the return duct - it will NEVER happen.  the total dose in the fraction of a second the mold passes thru the light is way to small to damage the mold in any way.  And as far as "killing" mold spores - those are the toughest, most well protected life stage of mold.  It can survive for decades in harsh environments and still be viable.  Why would spores be 'killed' by a fraction of a second of UV?  If that were to happen, then non-metal ducts would not survive the UV light.

I bought the UV lights and found myself replacing the expensive bulbs way more often than one would expect.  I do not recommend the UV component unless there were a realistic possibility that it "might" (emphasize "MIGHT") be of some benefit to an extremely infirm occupant whose doctor recommends such equipment.  Our houses are not hospital isolation wards . . . not even if there is an asthmatic occupant. 

Still, ducts do need to be cleaned periodically.  It's not clear to me why anybody is surprised to learn this to be the case.  The insides of the ducts are inside your house every bit as much as the living room rug.  Vacuuming the rug the does not involve shining an ultraviolet light on the floor.

I did use the 4" pleated filter but went back to a more conventional "rock catcher." The high-MERV filters collect smaller and smaller particles and thus may disturb the air flow and compromise the life of the furnace by making the fan work more than it may have been engineered for and by allowing the heat exchanger to expand excessively because of overheating. 

Don't do UV.

Grab a MERV 13 from filtersfast or something.

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