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Green Building Resources for Existing Affordable Housing



Mechanical Systems : Equipment : Individual Cooling Systems


Window Unit

Window units sit in a window or wall opening, with controls on the interior. Air moving from outside to inside is cooled as a fan blows it over the evaporator. Hot fluid is is cooled by exhausting hot air to the outside.

Benefits

  • ac-ups-new.gif
    Source: HowStuffWorks.com
    Low up-front costs (but potentially higher operational costs because it is not as efficient as a central system if you are cooling more than a single room). No upfront cost if resident is expected to provide their own; if property owner provides the units then it ensures that a minimum level of efficiency for equipment is being met.

  • Simple system.

  • Allows efficient cooling for a single room.

Challenges

  • Safety concerns for pedestrians below window if not mounted correctly.

  • Maintenance staff needs to establish a removal protocol at the end of each cooling season so that windows do not remain open in cooler months.

  • Storage space, either centrally or within the living space should be allotted for when they are not in use.

  • May not be aesthetically pleasing.

  • Condensate will drip from the unit.

Efficiency


Sizing

  • DOE recommends an output of 20 Btu per square foot of floor space to be cooled.

  • Take into account height of room, shading, window size and glazing and how well the building is insulated.

When to Use a Window Unit

  • If you only need to cool selected rooms.

  • If you would like tenants to pay for their electric use.

  • If you do not have or want ductwork in your building.



Sleeve

wall_air_conditioner.jpg
Source: Best Air Conditioner Reviews
Wall air conditioners are very similar to window air conditioners with the exception they rest in a sleeve in your wall and not through a window opening and they are vented differently than window units. (So don't use a window unit in a sleeve!)

Benefits

  • Low up-front costs (but potentially higher operational costs because it is not as efficient as a central system if you are cooling more than a single room).

  • Simple system.

  • Allows efficient cooling for a single room.

Challenges

  • Penetrates the envelope-may cause air leakage.

  • Safety concerns for pedestrians below window if not mounted correctly.

  • Maintenance staff needs to establish a protocol to cover and seal the sleeves at the end of each cooling season so as to minimize air leakage in the winter months.

  • May not be aesthetically pleasing.

  • Finding an exact fit for your sleeve when replacing units.

Efficiency


Sizing

  • DOE recommends an output of 20 Btu per square foot of floor space to be cooled.

  • Take into account height of room, shading, window size and glazing and how well the building is insulated.

When to Use a Sleeve

  • If you only need to cool selected rooms.

  • If you would like tenants to pay for their electric use.

  • If you do not have or want ductwork in your building.



Mini Split

mini-split-air-conditioner.jpg
Source: Air Conditioning Miami
In ductless systems, refrigerant is piped from the outdoor unit through small-diameter insulated refrigerant lines directly to individual rooms or zones. Cooled air is blown into the room by a fan in the individual evaporator units. The term "mini" is used to describe the small indoor units located in each room or zone.

Benefits

  • Can provide cooling for multiple zones (rooms) off of a single outdoor pump.

  • Much less energy loss during distribution compared with central air ductwork (1-5% instead of up to 30%).

  • One indoor unit can provide between 3/4 and 2-1/2 tons of heating and cooling (range of 8,000-30,000 Btu per hour).

  • Much less time and effort required for retrofit installation than a central air system, however more expensive than a PTAC system.

  • Mini splits require only a small 3” hole in the exterior envelope.

  • No need to remove units during heating season.

Challenges

  • Primary disadvantage of mini splits is the cost. Such systems cost about $1,500–$2,000 per ton (12,000 Btu per hour) of cooling capacity. This is about 30% more than central systems (not including ductwork) and may cost twice as much as PTAC or window units of similar capacity.

  • Requires two main components: an outdoor compressor/condenser, and an indoor air-handling unit. You will need to plan for these elements at each area. As each system requires a separate condenser, this could be a good amount of equipment that you will need space for on the outside of the building or roof depending on how many units there are in your building.

  • Proper sizing to avoid short cycling and inadequate dehumidification.

Efficiency


Sizing

  • To determine the required strength of the mini split needed, first calculate the area of the space to be cooled (length x width). Then multiply that by 25. This will provide you the base BTU rating that you will need.

When to Use a Mini Split

  • If you would like tenants to pay for their electric use.

  • If you do not have or do not want ductwork in your building.

  • If you would like to provide cooling for several zones (rooms) off a single pump.



What kind of feedback have you received from residents after installing mini-splits? Share your story.

PTAC System

ptac-air-path.jpg
Source: Ultimate PTAC
PTAC systems are called wall split air conditioning systems or ductless systems and are frequently used in hotels. They have two separate units (terminal packages): the evaporative unit on the interior and the condensing unit on the exterior, with tubing passing through the wall and connecting them.

PTAC systems may be adapted to provide heating in cold weather by converting the air conditioner into a heat pump. Drain piping is not necessary because the condensate extracted from the air is drawn by the condenser fan onto the condenser coil where it evaporates.

Installation usually requires the following elements:
  • Louver
  • Metal Sleeve
  • Heating Coil
  • The PTAC unit
  • Room enclosure

Benefits

  • Allows efficient cooling for a single room or dwelling unit.

  • Can be used as a air conditioner in cooling months and as a heat pump in the heating months.

  • Allows each space to be adjusted independently thus providing the resident flexibility.

  • Compact; it takes up very little space on the interior.

  • Packaged terminal air conditioners are easy for retrofits.

  • No central systems.

  • No distribution energy losses like those associated with central air ductwork.

Challenges

  • Penetrates the envelope.

  • Not appropriate for open floor plans or very large rooms.

  • Best used in moderate climates; extremes can overtax the units.
    
  • May not be aesthetically pleasing.

  • Can be noisy.

  • Controls can be difficult to operate, especially for the elderly.

  • More efficient options are available.

Efficiency


Sizing

Square Footage
Recommended BTUs
100-150
5,000
150-250
6,000
250-300
7,000
300-350
8,000
350-400
9,000
400-450
10,000
450-550
12,000
550-700
14,000
700-1,000
18,000
1,000-1,400
24,000

When to Use a PTAC System

  • If you would like tenants to pay for their electric use.

  • If you do not have or do not want ductwork in your building.



What challenges did you face installing PTAC systems in your building? Let us know.

Condensing Unit

Heating_Cooling.jpg
Source: Cool Connections Inc
Split systems are made up of two units: the condensing unit and the evaporator coil. The louder condensing unit is placed outside and the quieter evaporator coil is inside, is located inside either a furnace plenum or air handling unit to distribute the cooled air throughout the ducted system into the designated spaces.

Air is drawn in from different parts of the building through the return-air ducts. The air is then pulled through a filter where particles such as dust and lint are removed, depending on your level of filtration, you may also be removing microscopic pollutants. The filtered air is then routed to air supply ductwork that carries it back to rooms. Whenever the air conditioner is running, this cycle repeats continually.

Benefits

  • Can provide cooling for multiple zones (rooms) off of a single outdoor pump.

  • Does not take up floor space within the building or units.

  • If providing central cooling on one master meter, there is less maintenance as you have fewer pieces of equipment to operate.

Challenges

  • This system will not work in an existing building financially unless there is already ductwork in place.

  • Primary disadvantage of the split system is cost. You need to consider the cost of the condensing unit(s) as well as the cooling coils.

  • If you would like tenants to pay their own electric bill, you will need a separate condenser for each unit so if you have a larger building you will need to consider where you will place these units (usually either on the roof or ground level if you do not have vandalism concerns) and factor in the additional maintenance necessary.

Efficiency


Sizing

  • The sizing of your condensing unit depends on the size of the space you are cooling (central system or individual units), the performance of your insulation and windows and the amount of air leakage (the better the performance, the smaller the system you need generally).

  • Use Manuals J, S and D of the Air Conditioning Contractors of America (ACCA). Manual J looks at heating and cooling loads, Manual S looks at system size and Manual D covers duct design. These calculations will help the system run more efficiently, save energy and in turn save money.

  • It is important to size your system correctly because an over-sized system will cycle on and off more frequently, thus reducing its efficiency and wearing out the compressor and other electrical components more rapidly.

When to Use a Condensing Unit

  • When you have existing ductwork in your building.

  • You have a central electric meter so that the owner pays for tenant's electric use and you do not want to install individual meters OR you have the space for and desire to install individual condensers for each unit so that the tenant is responsible for their own electric bill.



Heat Pumps

Learn more about how heat pumps work and when to use them.



ContibuteKnowledge.jpg

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