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After what seemed like a long hot summer I was excited for the temperatures to turn a bit cooler.  Unfortunately, this also means that pollen is thick in the air (at least in Arkansas).  We control these allergens inside our homes and businesses by using a filter in the HVAC system.  If you’ve ever shopped for filters, you know there are many to choose from.  But which one to buy?  In this case, higher efficiency filter isn’t always the best choice.

HVAC filters are rated using a standard called the Minimum Efficiency Reporting Value (MERV).  The higher the MERV value, the finer the filtration.  All filters constrict the airflow (by adding pressure drop), and generally the higher the MERV value the more the airflow is restricted.  Also, as time passes and your filter collects particles out of the air it will restrict the air more and more.  As you can imagine, a more restrictive filter causes your fan to run more or at a higher speed (if the drive is variable) and this can reduce the overall system efficiency.

The first step to finding the right balance between filter efficiency and system efficiency is to select the right filter.  Unless you are operating a cleanroom, laboratory, hospital or a smoking lounge you shouldn’t need a filter with a MERV rating higher than 13.  Filters rated above 4 can be expected to remove pollen from the air as well as dust and textile fibers.  Filters rated above 7 can remove mold spores.  You can see a full breakdown of the MERV ratings here.

You should also purchase a filter that has a rated pressure drop less than what your HVAC unit can handle.  As the air flows through the return ductwork and filters, the air pressure drops from friction.  This is measured using inches of water gauge (w.g.).  For example, a fan blower may be rated to overcome 0.5 inches w.g.  If the return ductwork has a pressure loss of 0.4 inches w.g. and you install a filter that has a pressure drop of 0.3 w.g. then your system will no longer work very well (0.7 > 0.5).  Most systems are designed and installed with return ductwork pressure drops less than 0.05 w.g. so there usually isn’t much to worry about here.  Even so, if you want to install a high efficiency filter it may be worth it to have a professional analyze the system to make sure you aren’t hurting your system efficiency too much by installing a more efficient filter.

As mentioned earlier, the filter pressure drop can also increase as particles build up on the filter.  To combat this, the filter should be replaced or cleaned (if washable) periodically to keep airflow up.  In general, disposable filters should be replaced every three months and washable filters should be cleaned once a month.  Of course, depending on your building type and usage it may require additional filter replacements.

It is important to choose a filter with the minimum acceptable MERV rating that is also selected to work well with the HVAC unit.  Otherwise, the unit efficiency may be impacted and building costs would go up.  If you are having air quality issues with your building or are interested in increasing efficiency, Forward Engineers can perform an energy audit to identity projects that lead to cost savings.  Please contact us and we’ll be happy to take a look.

As a design professional, I spend most of my time putting plans on paper.  It’s always a pleasure to see the plans constructed, but how do you know that the building is operating as designed once complete?  Building commissioning is a process in which all the building components are checked to ensure they are working properly.

There are many levels of commissioning.  Some may be formal processes completed using guides such as ASHRAE Standard 202 – Commissioning Process for Buildings and Systems.  Others may be informal and include just some basic tests such as building controls review, HVAC test and balance check as well as lighting checks.  The scope of commissioning can easily be modified to match the building size and type as well as what the building owner desires.

Commissioning is completed by the commissioning agent.  For large formal commissioning projects, this agent is ideally independent of the contractor and design team and works directly for the building owner.  The earlier the agent is involved in the design process the more input they can have on possible operation, installation, testing or performance issues before they become construction issues.  For smaller informal jobs, the commissioning agent may be the design professional and complete the building checks once construction is complete.

Another aspect of commissioning is called retro-commissioning.  This process is very similar to the initial checks of a new building, but is done after the building as been in service for a number of years.  It verifies that a building is operating as it should and corrects any issues so that they building continues to operate as it was originally designed.

Both commissioning and retro-commissioning are important aspects of the building design and construction process.  There is no benefit to a great design if the building isn’t living up to its potential.  We strive to include some level of commissioning in all of our design jobs.  We also offer retro-commissioning services for building that are already in service.  If you are interested in any of these services, please contact us and we’ll get your building operating as it should.

Air conditioning and heating equipment is rated by manufacturer’s to give the consumer an idea of how efficient it is.  Typically calculated by comparing the energy input versus the energy output, it allows consumers to compare similar equipment.  But what exactly do these numbers mean and how do they relate to actual costs?

For heating, the calculations are generally quite simple.  A natural gas furnace that puts out 104,000 Btu/hr in heat but requires 130,000 Btu/hr to do so would be rated at 80%.  The missing 26,000 Btu/hr are lost up the chimney.  How does this translate to actual costs?  The national average cost of natural gas in 2015 was approximately $0.80 per therm (aka 100,000 Btu).  For every hour your furnace is on, you pay $1.04 to get $0.83 worth of heating (a loss of $0.21 per hour).  Let’s say the same furnace was rated 94%.  Now you only pay $0.89 for the same amount of heat (a loss of only $0.06 per hour).  If our example building was in Arkansas, these savings could add up to about $200 per year for this size of furnace.

On the air conditioning side, the equipment works very differently and requires an alternate approach.  Air conditioners are called heat pumps because they pump the heat out of your home or business.  The efficiency is shown using the Energy Efficiency Ratio (EER) which is a ratio of output cooling energy (in Btu) to input electrical energy (in Wh).  For example, a unit with an EER of 12 would require 5 kWh to move 60,000 Btu (aka 5 tons).  To translate this to costs, the 5 ton unit requires $0.52 to operate for one hour (based on 2015 national average of 10.42 cents/kWh).  The same unit with an EER of 16 would only cost $0.39 cents per hour.  Continuing our example, if this unit was on a typical building in Arkansas the savings between 12 and 16 EER could be around $600 per year.

The trick with energy efficiency is to balance the cost savings from usage (like we calculated above) with the initial cost of the equipment, available incentives, project location, building type and usage.  These are all major factors and we engineers often use computer simulations to help us in calculating the savings.  If you think your building could use some energy upgrades, please consider hiring us to take an in-depth look and determine how you can lower your costs by saving energy.