Ductless Heat PumpsDuctless Heat PumpsSizing ExampleSizing Example

Gary NordeenGary NordeenBill Kingrey, P.E.Bill Kingrey, P.E.

11/5/08 AFE Webinar11/5/08 AFE Webinarwww.nwBuildings.org/dhp.aspxinfo@nwBuildings.org

IntroductionIntroduction

This presentation is intended to show how a ductless heat pump is sized for a particular space.

The example is a maternity room inside a health facility, that needs supplemental cooling.

Heat Loss CalculationsHeat Loss Calculations

Calculate the heat loss through walls, glass, ceiling/roof, floor and any other building components in the zone to be served, using ASHRAE procedures (chapter 26, Fundamentals)

Heat Gain CalculationsHeat Gain Calculations

• Survey each zone and estimate heat gains from lights, equipment (computers, copiers, etc), people, building envelope and ventilation

• Calculate both sensible and latent (water vapor) loads

• Use ASHRAE Handbook of Fundamentals Chapter 26

Heat Gain Calculation Heat Gain Calculation ExampleExample

Assume we are going to provide cooling to a maternity room that is currently heated and ventilated by a central system with heating and ventilation air. We surveyed the room and found the following existing conditions.

Maternity Room ExampleMaternity Room Example

• 2 fluorescent lights at 32 watts each - ballast heat included

• 2 People @ 275 BTUH sensible, 105 BTUH latent (water vapor)

• Equipment = 1 computer = 65W• Vent. Air = 40 CFM outdoor air at

ambient temp in summer, 70 in winter• Envelope (windows, walls and roof)

calculated gain = 4,800 BTU

Maternity Room Example Maternity Room Example Sensible Cooling LoadsSensible Cooling Loads

• Lights + Equipment + People =

2x32 + 65 + 2 x 275 = 679 watts

679w X 3.412 BTU / watt = 2317 BTUH Sensible

• 40 CFM Ventilation Air at 92dB, 74wB =

40CFM x 1.08 BTUH / CFM x ºF x (92ºF – 75ºF) = 734 BTUH

• Total Sensible Cooling Load =

4,800 BTUH (envelope) + 2,317BTUH + 734 BTUH = 7,850 BTUH Sensible

Maternity Room ExampleMaternity Room ExampleCalculate Latent LoadsCalculate Latent Loads

To calculate latent loads, use a Psychometric Chart or software that will determine enthalpy and density of water/air mixtures

Psychometric ChartPsychometric Chart

Psychometric CalculationsPsychometric Calculations

• Or you can use this free online program that will calculate enthalphy, density, relative humidity, etc.

http://www.gorhamschaffler.com/psy.htm

Latent Cooling LoadsLatent Cooling Loads

Assumptions:• Ventilation air is cooled and dehumidified

to 75°FDb (dry bulb) at 30%RH (relative humidity)

• Cooling design conditions at the site were 86Db and 68 Wb (wet bulb)

Latent Cooling LoadsLatent Cooling Loads• People = 2 x 105 BTUH = 210 BTUH latent• Ventilation Air = 40ft³/min at 86Db/68Wb

• 13.55 ft³ of air at that temp = 1 lb of air• In one hour we will cool 40ft³/min x 60 min/hr/13.55

ft³/lb = 177lbs/hr of air• Air at 86dB/68wB contains 32.21 BTU/lb (of air) • Air at 75dB/30% RH = 24.05 BTU/lb • It will require 32.21- 24.05 = 8.16 BTU for each pound

of air cooled• Total Ventilation Load = 177 lbs/hr x 8.16 BTU/lb

= 1,440 BTU/hr• Latent Load = People + Ventilation Air

= 210 + 1440 = 1,650 BTU/hr (about 1.5 pints of water per hour)

Selecting Ductless Heat PumpSelecting Ductless Heat Pump

• Pick a unit that has a capacity greater than or equal to the total cooling load

• Mini-splits express heating and cooling capacity in kW. To convert to BTUH multiply the kW rating by 3412

More InformationMore Information

Northwest Building Efficiency Center

866-929-6232

Info@nwBuildings.org

www.nwBuildings.org