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Cashins & Associates Blog

How Much Is That Exhaust Ventilation Fan Costing You?

Posted by Mike Cashins on Thu, Jan, 12, 2012 @ 09:01 AM

You can’t get away from it! If you don’t provide exhaust, odors, smoke, haze and other contaminants will build-up and complaints about poor indoor air quality will follow. But running those wall exhausters and roof exhausters each winter gets expensive. What can you do?

Exhaust FansCashins & Associates has found that most exhaust systems are inefficient, ineffective and often both. To control costs as well as improve efficiency and contaminant control effectiveness, we recommend the following:


 1.  Start by calculating the annual unit cost of present dilution exhaust systems.

 Yearly Cost = 0.154 (Q) (dg) (T) (c)


Q = air flow rate, CFM = 10,000 CFM
T = operating time, hours/week  = 60 hours/ week
dg = annual degree days  = 6,950 (Boston @ 68 F. temp.)
c = cost of fuel, $/unit  = $3.50/ gallon oil
q = available heat per unit fuel  = 106,000 BTU/Gal. oil @ 75% efficiency.

The “Yearly Cost” then would be $2.00 per CFM exhausted per heating season with just 60 hours per week of operation.

2.    Add up or roughly estimate the total air being exhausted via dilution exhaust systems.

Just a couple of nominal 10,000 CFM roof or wall exhausted could be costing you $40,000 annually.

3.    Make sure the building has adequate make-up air; that is, assure any air being exhausted has the approximate equivalent of make-up air.

Buildings without adequate make-up air are likely to have safety, comfort and cost inefficiency issues. They will run at a negative pressure. This can be easily determined by placing a wet finger along wall opening cracks on 2 or 3 sides of the building. Evidence of inward air velocities at all cracks/openings will reveal a negative pressure.

4.    Evaluate all possible air contaminant sources within the building.

Wherever feasible, estimate the cost of alternate contaminant controls rather than using dilution exhaust systems. For example, a 20,000 CFM roof exhauster used to control propane-fueled lift truck emissions may cost upwards of $40,000 annually in additional utility costs. Here, battery powered lift trucks may prove a cost saver.

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Cashins & Associates, Inc. can evaluate your ventilation systems and recommend changes that could save you thousands of dollars a year!



Topics: health & safety, indoor air quality, ventilation assessment, exhaust ventilation, energy saving

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