Sharing our Guidance

Heater Selection in HVAC Units

Basic Formulas

 Sensible (Dry) Heat or Cooling qs (btu/hour) = 1.08 x CFM x delta T Where: q s= rate of heat transfer in btu/hour CFM = volume or quantity of air flow delta T = temperature difference (F)   Latent (Wet) Heat or Cooling ql (btu/hour) = 0.69 x CFM x delta W Where: ql = rate of heat transfer in btu/hour CFM = volume or quantity of air flow delta W = humidity ratio difference (grains/lb)   Total Heat or Cooling qt (btu/hour) = 4.5 x CFM x delta h Where: qt = rate of heat transfer in btu/hour CFM = volume or quantity of air flow delta h = enthalpy difference (btu/lb) 

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Heating Mixed Air Temperature

(% OA x OA Temperature) + (% RA x RA Temperature)

Example:(30% OA x 5) x (70% RA x 68)

= 49.1 degree mixed air temperature.

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Project #1

Heating load in MBH as output by HAP = 39.2 MBH1900 supply air cfm. 86%260 cfm outside air (OA). 14%Return air temperature (RAT) 70 degreesOutside air temperature -14 degrees

Mixed air =(14%x14)+(86%x70)= 58.2 degreesHeater selected output 41 MBH

1900 x(1.08x Delta T) = 41000Delta T = 41000/(1.08 x 1900)19 degree temperature Rise58.2 degrees + 19 degrees

Leaving air temperature (LAT) 77.2 degrees

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1900 x(1.08x Delta T) = 41000Delta T = 41000/(1.08 x 1900)19 degree temperature Rise58.2 degrees + 19 degrees

Leaving air temperature (LAT) 77.2 degrees

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Comments:

• Typically we would like to select 80 – 100 degree leaving air temperature (LAT)

• We should be considering a 10-15% safety factor.• This example was a site in the upper NE. Design outside air

temperature -14 degrees.• We have the ability to catch this in shop drawings but in this

case, wire size/breaker is going to change as the correct heater is a larger KW.

• We would prefer not to catch this at this stage. Which is why we have a process that needs to be adhered to where load calculations, as well as equipment selections are to be reviewed and signed off on.

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CONCLUSION . . .

THANK YOU.