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Selecting HVAC systems using OEM data – Not AHRI: ACCA Manual S equipment sizing.

At Savoy Engineering Group we perform hundreds of residential ACCA Manual J, S & D HVAC designs each year throughout the United States. We also review HVAC designs performed by others. Often it is either the homeowner/contractor/builder/architect that does not trust the results they received or a lawsuit may be involved.

What we see throughout the HVAC industry is people not sizing HVAC equipment with the manufacturers’ performance data (OEM data). We NEVER see a Manual S compliance report included. We look at a lot of both Elite & WrightSoft reports and no one includes. While we see a lot of errors in the Manual J reports, the trend of improperly sized equipment is far more troubling. We ALWAYS use OEM data – not AHRI.

When we send our client the completed ACCA certified Manual J load calculation, we ask them to send us the make/model & OEM data for the indoor coil, outdoor condenser and furnace. More often than not we receive equipment way oversized and AHRI #. For example, we completed a job that required 5 tons of cooling total and the client sent back a quote for two 4-tons and a 5-ton unit.

We also perform many Arizona HVAC designs and we often have HVAC contractors who send us the AHRI certificate. We are happy to walk our clients through the Manual S process but many do not understand or simply don’t care to invest the time needed to master the Manual S sizing theory.

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What happens at the AHRI laboratory? A coil and condenser are paired together and a simulated 80 F indoor temperature and a simulated 95 F outdoor temperature is created. Then they record the resulting BTUh produced. That’s it – there’s your AHRI certificate.

Problem #1:ACCA Manual J, Energy Star and LEED all require a 75 F indoor design temperature.Plus most people would find an 80 F indoor temperature quite uncomfortable.

Problem #2: Most geographical locations have a different outdoor summer design temperature than 95 F. Phoenix, AZ requires a 108 F outdoor design temp while Santa Monica has an 80 F outdoor summer design temperature.

Relying on the results given on an AHRI certificate at 80 F indoor and 95 F outdoor would NEVER, NEVER, NEVER meet the Phoenix AZ conditions of 75 F indoor and 108 F outdoor.

Unfortunately some HVAC manufacturers do not produce OEM data but most do. The OEM data offers performance data at a range of different indoor and outdoor conditions. While each manufacturer may present their data different ways, it should include airflow in CFM, EWB (entering wet bulb), cooling capacities for both total and sensible, indoor temperature and outdoor temperature. Beware, many manufacturer includes capacity correction factors and high altitude adjustments. It takes a lot of time and effort to understand OEM data thoroughly.

Avoid a Goldilocks home - Using OEM data, the HVAC equipment will be properly sized for YOUR specific home!

Please see http://www.iccsafe.org/newsroom/Documents/0109_hvac.pdf for more on how to properly size HVAC equipment using ACCA Manual S.

What is ACCA Manual J load calculations? What is ACCA Manual S equipment selection? What is ACCA Manual D duct design? We are ACCA certified and understand Manual J, S & D theory and WrightSoft theory from the inside out. Please visit our website, www.load-calculations.com, for more information.



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