1. Ductless Heat Pumps Sizing Example
Gary Nordeen Bill Kingrey, P.E. 11/5/08 AFE Webinar

2. Introduction
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.
BILL – accurate?

3. Heat 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)

4. Heat 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

5. Heat Gain Calculation Example
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.

6. Maternity Room Example
2 fluorescent lights at 32 watts each - ballast heat included
2 275 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
FURTHER NOTES
Ventilation from Variable Volume System:
System must have controls to maintain minimum OA requirements at minimum system CFM
Determine minimum CFM for the zone where you plan to add the DHP
Determine % OA at minimum flow
Calculate OA to zone as for constant volume systems
Ventilation from Constant Volume HVAC System
If zone is supplied by an existing HVAC system, the amount of outside air provided depends on the system design and the minimum outside air setting.
Measure the return air flow (RAcfm ) and the outside air flow (OAcfm) using a flow grid
OA% = OAcfm/(OAcfm + RAcfm)

7. Maternity Room Example Sensible Cooling Loads
Lights + Equipment + People =
2x x 275 = 679 watts
679w X 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 BTUH = 7,850 BTUH Sensible

8. Maternity Room Example Calculate Latent Loads
To calculate latent loads, use a Psychometric Chart or software that will determine enthalpy and density of water/air mixtures

9. Psychometric Chart

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

11. Latent 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 Load
Assumes Ventilation air is cooled and dehumidified to 75oFDb (dry bulb) at 30%RH (relative humidity).
Cooling design conditions at the site were 86Db and 68 Wb (wet bulb)
People = 2 x 105 BTUH = 210 BTUH latent
Ventilation air = 40 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 BTU/lb (of air) ft3 of air at that temp = 1 lb of air.
Air at 75 dB/30% RH = BTU/lb
It will require = 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 = = 1,650 BTU/hr - that's about 1 1/2 pints of water per hour

12. Latent 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 BTU/lb (of air)
Air at 75dB/30% RH = BTU/lb
It will require = 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
= = 1,650 BTU/hr
(about 1.5 pints of water per hour)

13. Selecting 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

14. Northwest Building Efficiency Center
More Information
Northwest Building Efficiency Center