I have a 2500sqft home with a 5T combo HVAC/Heat Pump system and 2 thermostat controlled exhaust attic fans which run an ave. of 6 hrs/day.  I run advanced oxidation air purifiers constantly for my air quality.  I just read about the ERV's and wondered if they would reduce my power bills considering my outside air is virtually always hotter than the indoor air.  It would have to be dramatic to payback a $1000+ investment.  Sounds good for Northerners but maybe not for here.  Any experience here in my area? Nizhoni

It will not reduce your bill

Unless your house is very tight at the attic/conditioned space boundary and your attic ventilation cross sectional are is large, the thermostatically controlled attic exhaust fans may be increasing your AC load by drawing more humid air into your home, increasing the latent load for the AC (and lowering air quality.) In most instances the impact is neutral to negative if the exhaust fans are powered by the grid, and become more negative with higher R-values. If the fans are powered by solar panels and the R-value in the attic is modest (R19 batts or something), they might reduce the cooling power use by a small amount (but will rarely be cost-effective.)

See: http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/index.htm#fourA

Try not running your attic fans for a full billing period during the cooling season, and compare power use against cooling degree-day data for that period with kwh/CDD for a billing period when the fans were running. Powering two fans driving the air infiltration driving up the latent they could be costing you real money, and lowering the indoor air quality too.

ERVs make PLENTY of sense in humid FL- the higher the outdoor dew-point, the more you get out of them. But unless the house is very tight- tight enough to need active ventilation to maintain indoor air quality you won't notice it much. By reducing the amount of humidity in the ventilation air stream it lowers the latent air conditioning load- the indoor air stays drier. The net decrease on the AC or dehumidifier's duty cycle is small but measurable in very tight & well insulated houses (but probably not in your case.)

To make real dents in your power bill, air-sealing the place, air-sealing the ducts, upgrading the insulation where appropriate/possible, and controlling the solar gain at windows, walls, and roof deck make a bigger difference than any amount of attic ventilation. Leaky &/or uninsulated AC ducts will often be sending 15-30% of the conditioned air somewhere else. Modifying & repairing the ducts to meet California Title 24 2008 standards would be a significant efficiency boost to most homes with central AC. Seal your ducts & air seal your house to at MOST 1.0 air-changes per hour @ 50 pascals pressure before even THINKING about putting money into an ERV, and then only run it at minimum ventilation rates when the outdoor dew point is above 55F (which is most of hte summer), never continuously. At 1 ACH it isn't tight enough to absolutely NEED active ventilation, but being tighter will reduce summertime humidity indoors by keeping more of the drier air-conditioned air inside.

Thank you for your detailed and informative reply. One thing I overlooked in my description was a lack of clarity wth the term"attic exhaust fan". I had forgotten that this phrase also refers to the type of exhaust fan which is inside the conditioned space and exhausts air from the inside to the attic. That is not my case. The only access to my attic is from the garage. These fans remove the overheated hot air from my attic and exhausts it to the outside. I have wondered if the power use of the fans exceeds the energy saved by cooling the attic space. I definitely need to check and repair any duct damage. Nizhoni

Nizoni;

those exhaust fans have been found to cost more money than they save, they create negative pressure in your attic space sucking out cool air from your conditioned space and from leaking ducts.

The best way to save money in a conventional south Florida home is to seal ducts and add insulation

Chris,

The only conditioned air loss I can think of other than duct leaks, would be the minimal leakage from unsealed ceiling penetrations used for high hats and fans. Since air will take the path of least resistance, I felt that the air intake from the eave vents would offset the exhaust air basically eliminating any negative pressure to draw air from the fan and light penetrations. I guess the best test is as you suggested. I could turn them off for a period, match the degree days and compare my bills. Thanks, Don

Posted By nizhoni on 16 Jun 2010 09:28 PM
Thank you for your detailed and informative reply. One thing I overlooked in my description was a lack of clarity wth the term"attic exhaust fan". I had forgotten that this phrase also refers to the type of exhaust fan which is inside the conditioned space and exhausts air from the inside to the attic. That is not my case. The only access to my attic is from the garage. These fans remove the overheated hot air from my attic and exhausts it to the outside. I have wondered if the power use of the fans exceeds the energy saved by cooling the attic space. I definitely need to check and repair any duct damage. Nizhoni

There was no confusion- I was talking about exactly the type of attic ventilation scheme you are using. 

Even when you have a perfect air-seal between conditioned space and the attic the effect on power use is neutral at best.  When there is any air-leakage at all between the attic & conditioned space (which is USUALLY the case), the induced infiltration from the depressurized attic tips it negative (often strongly negative)-  indoor humidity & power bills go up, and your indoor air quality falls. 

Try running without them- the only time they're of any cooling benefit is in un-insulated attics (or sub-code attics with unsulation levels under R15.)  With R19 it's barely break-even on power use in regions with dry summer air, but in places as humid as FL the odds of breaking-even are remote.  Attic ventilation fans as higher-efficiency supplemental cooling is a concept that has been soundly de-bunked by many studies over the past 30-40 years.   The FSEC study I linked to with the solar powered version showed very marginal results (a net 6% reduction) even when they were self-powered, drawing NOTHING from the grid.