Filter Change Schedules & the Cost of Energy

The most obvious consequence of an HVAC system breakdown, is usually a fan failure from the increased load. This is very expensive but also hard to not attribute to the deferred maintenance. Harder to catch, but much more common, is the increased cost of energy consumed from a filter that is in use beyond its ideal change point.

In the example below, we look at a common standard capacity pleated filter scheduled for a filter change out at 3, 4 and 6 months respectively. Standard capacity pleated filters should be changed out every 3 months in most applications. You can see in both the 4 month and 6 month filter change intervals, the TCO increases despite fewer dollars spent on filters, changeout labor and disposal. Building management systems that include HVAC typically represent 40% of a building’s cost of energy.

Filter Change Deferred Maintenance

On a narrower product scale-NAFA, the National Air Filtration Association, estimates that 81% of the TCO of a filter is the cost of energy. In the example above, the cost of energy is 79%, 86% and 94% respectively. In other words – the 4 and 6-month filter change intervals reduced non-energy costs by 25% and 50% respectively. The increased cost of energy was 25% and 110% respectively, on a much larger base.

Air Filter Resistance and the Cost of Energy Over its Life

The longer a filter is installed, the faster the DP will increase. This is explained by the fact that a filter is a controlled leak with a very high, but fixed, number of tiny holes in its media. This allows the clean air to pass through while trapping indoor air particulates before they contaminate the air quality. As the filter catches dust and dirt, the filter’s holes plug one by one. Then each incremental unplugged hole is relied on more significantly in terms of the air bypass allowance remaining. This means that a pleated filter loads linearly from their time of installation, but their dust loading to service life relationship is actually curved, as shown here.

Total Pressure Drop

This exponential relationship causes deferred maintenance on air filters to be particularly damaging to a building’s energy bill.

Finding the right air filter and filter change point is critical to managing your total maintenance and energy budget – be careful not to fall into the common trap of deferred maintenance on HVAC filters!

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