Roof Top Heating / Cooling Units
Sizes and building construction
Roof Top Units (RTU's for future reference) come in sizes rated by their cooling capacity from 1 ½ ton to 75.0 tons. When we refer to tons of cooling we are not talking about the weight of a system, but the BTU capacity of the unit. One BTU, from your high school science class is the amount of heat required to raise the temperature of one pound of water, one degree Fahrenheit at sea level where the pressure or weight of the atmosphere measures 14.7 pounds per square inch. One ton of cooling is 12,000 BTU's. That's enough math for us to understand sizing of a system. The average homeowner knows their system is a 2 ton or 2 ½ ton unit for example, so a 2 ton unit is 2 x 12,000 = 24,000 BTU's. A 3 ton would be 3 x 12,000 = 36,000 BTU's. In commercial applications there is a general rule of thumb for sizing that is actually pretty accurate for office space. Sizing can be roughly estimated based upon square footage of the space to be cooled. The ratio is 400 square feet per ton. Should you have a 2,000 square foot office space you would require 2,000 / 400 = 5 tons of cooling. This same ratio does not work for residential applications and the simple reason is that homes have improved in construction and efficiency in the last 60 years to the point that a newly constructed house of 2,800 square feet could require the same size of unit as a 900 square foot bungalow that was built in the 1950s'. Commercial construction of buildings has not really kept pace with the same improvements to efficiency as the residential market. Most homes by comparison will have 12 to 20" of attic insulation whereas a commercial building with a flat roof has no attic and maybe 2 to 3" of felt board insulation between the roof deck and the tar & gravel covering. The greatest heat loss or heat gain in most structures is through the roof. Homes in particular have become so well sealed from the elements that they are unhealthy to live in. Hence the term "Sick Building Syndrome" was coined. The same term was adopted for office buildings and factories, but more for the pollution generated within the structure rather than the lack of air changes. New home design will almost surely include an "air to air heat exchanger", a "heat recovery ventilator (HRV)" or an "energy recovery ventilator (ERV)". All of these are the same thing with the same purpose of exhausting a portion of stale air and bringing in an equal amount of fresh air. The manufacturers of these units attempt to recover some of the heat lost in the process and filter the air, but will actually cost you energy versus saving energy. This type of unit is discussed fully in the "Learn About" section of our site and under the "General" sub heading.
Accurate sizing of a system from the tonnage of cooling required to the amount of BTU's of heat needed is a calculated figure based upon all the known insulating factors and surface areas of each type of material used in the construction of a building. Every building product has been tested by a recognized authority and given a "K" factor which is a number multiplied by the area of the surface exposed to outdoor conditions. This "K Factor" is a measurement of the resistance to flow of heat through the product. Another factor considered in the design is the local temperature conditions. Sizing a cooling system for an identical building in Toronto versus Las Vegas would result in totally different requirements. The Toronto building would require more heating capacity and less cooling capacity on an annual basis. Permits are required for all new installations and the heat loss /heat gain calculation would be required information on the application. This calculation is best provided by a mechanical engineer or a certified engineering technician.
Equipment Features And Options
It is possible to purchase heating only, cooling only or heat / cool units. In Canada we have basically 3 voltages available for air conditioning which are 230 volt single phase, 208/230 volt, 3 phase and 575/600 volt, 3 phase. In new construction your engineer will specify the voltage to be used, most often by what power is available for the size of system being installed. The United States uses 460 volt ,3 phase power versus our 575 / 600 volt supply. With most systems being manufactured in the USA, it is not uncommon to purchase a unit in Canada with 460 volt motors with a transformer added to reduce our supply voltage to accommodate their components.
Roof top units can be purchased in 2 basic configurations. Down Discharge and Horizontal Discharge. The Horizontal Discharge unit would have exposed ductwork on the side or end of the unit that would enter the building through an independent roof curb and opening. This exposed ductwork would have to be insulated to prevent heat loss and sealed at every connection and seam against the weather. Regular maintenance is required on the ductwork insulation coating to maintain the integrity of the weather seal. Down Discharge units are supported by a curb assembly that is fastened and sealed to the roof. The ductwork exits the base of the unit and enters the building making for a much tidier and more efficient installation.
The most common feature or option on RTU's is the fresh air economizer. This option is described in detail on our site in the "Learn About" section under the General sub heading. This is an energy saver that is regularly specified by Engineers over manual outside air dampers that do not regulate the quality of air introduced into the building.
Normally coupled with a fresh air economizer is a barometric relief damper. This has the specific purpose of maintaining the building pressure at a normal level. If the fresh air economizer is operating it is conceivable that the large amount of fresh air that is introduced could over pressurize the space. The relief damper controls this situation by opening to vent stale air outdoors.
We have had as recently as last summer, 3 customers that experienced severe hail damage to the condenser coils on their RTU's. These coils are much like the radiator on your car and easily damaged as they are made of aluminum. The flattening of the aluminum fins restricts the air flow causing the freon temperatures to increase. This temperature rise can result in compressor failure. Most manufacturers have "hail guards" as an optional accessory that will protect the coil(s) from damage. In two cases last summer for my customers, the damage was covered by insurance, the other was not covered, did nothing and had a compressor failure. The coil and compressor was then replaced after a 3 week wait for the coil to be made and shipped from the USA. Total cost for the repair of this one unit was $4,800.00 + taxes
Most RTU's up to a capacity of 6 tons (72,000 BTU's) will use a single hermetically sealed compressor to provide the cooling. Units from 7 1/2 tons of cooling and above would normally have multiple compressors permitting economical staging of the system based upon the temperature demand at the thermostat. For example a 10 ton unit could have 2 -5 ton compressors. When the thermostat calls for cooling, one compressor starts and provided the one compressor maintains the setting on the thermostat there is a saving in energy plus superior dehumidifying of the air. Should the space temperature increase by more than 2 degrees above the set point, the second compressor will start and the unit will operate at full capacity. Air conditioning compressors up to 10 tons are normally hermetically sealed, meaning they are not serviceable. If a compressor fails, it would be replaced with a new one. A semi-hermetic compressor is serviceable and would appear very similar to a car engine. See our picture gallery for one suspended from our portable lift, ready for craning to a roof. This style of compressor would be found in units 30 tons in capacity or more and would generally have two or more for staging of the system.
The heating section of a roof top unit can use electric heat, natural gas or propane as the fuel source. The heating section operates in a manner very similar to a residential furnace using burners, gas valve, limit sensor, ignition control, combustion blower and a heat exchanger. The room or space thermostat will cycle the furnace section of the system on and off the same way as the cooling is cycled. Unfortunately we cannot have high efficiency on roof top units as high efficiency furnaces require the second heat exchanger that could freeze during winter months. Most motors in the system are high efficiency and on larger systems the motors can be equipped with variable frequency drives (VFD's) to further save on electrical energy. Both the heating and cooling are all in one shell, hence the name "Packaged Roof Top Equipment" whereas a typical residential system has the furnace remote from the cooling and therefore called a "Split System Unit"
The latest features available as options on roof top units will automatically turn on the supply fan and open outside, fresh air economizers when carbon dioxide levels exceed building norms. This is particularly useful on systems feeding cafeterias where there is an inrush of people at one time. Normal respiration of the people can deplete the oxygen levels and increase the carbon dioxide levels.
Another recent option is a method of controlling excess humidity in a space without "sub-cooling" the space. Humidity is removed with the operation of the air conditioning, but there are days when it could be warm enough for just a touch of cooling but really humid. The dehumidification option will close the fresh air economizer stopping high humidity air from entering and cycle on the cooling for dehumidification. The space could become too cold before the humidity is in balance so the heat section is started to maintain space temperature. I have yet to find a unit equipped with this option as there is not much call for it and it is pricey.
Should you have to replace a roof top unit at some time in your occupancy there are a few things that you should know. It is important to match the voltage of the existing unit yet the new unit may be more efficient and require a reduction in the size of fusing or circuit breaker. No matter what make or brand of unit you have, you can replace the unit with almost any brand you wish. This is possibly because regardless of what unit you have, the roof support curb will not accept the new unit. A curb adapter will be required and it will be made by the manufacturer of the new unit out of welded steel and insulation to match the "footprint" of the new with the old. The curb adapter will raise the elevation of the new unit by approximately 14" and require an extension of the gas line and possibly the electrical. The curb adapter saves calling a roof to tear up the old curb and install a new one, uses all the existing support steel on the underside of the roof and permits the installation to be completed in one day.
Your contractor should know specifics about your installation to determine if the fan motor horsepower rating needs to be increased because of a long run of ductwork such as from the roof of a three story building to the first floor ceiling. If you feel you need to increase the capacity of a system because you may have added another office on the same system or increased your heat load with lighting, computers or extra staff it may be possible for a modest increase. You must realize that the ductwork for the original unit was sized for the air flow delivery of the system. A large unit requires more air flow and larger ductwork. Adding another ceiling outlet or two to the existing duct may be all that is required, but the noise level of air flow could also increase.
About all that is left to discuss is the maintenance and warranty. Not all manufacturers have the same warranty, but it is fairly standard that compressors have a 5 year part warranty and heat exchangers have a 10 year part warranty. Check and get it in writing. The labor warranty has to be a minimum of 1 year on a new install by government ruling. Warranties from the manufacturer can be denied for lack of qualified maintenance causing the breakdown. There are some exclusions to just about every warranty such as power outages, fire damaging a unit, vandalism, misuse or abuse, but these conditions are easily understood as not being the fault of the equipment or the installer. Air filters should be replaced a minimum of 4 times per year on a RTU that provides heating and cooling. Some units do not require fan belts, but those that do should be replaced at the start of the heating and start of the cooling season. Lubrication is required to the instructions of the manufacturer and coil cleaning (condenser coil) should be done annually after the second summer to reduce the system freon pressures on hot days. This should be included with any basic preventive maintenance contract, however many will charge a pricey extra if they have excluded this service. A dirty condenser coil or plugged air filters will destroy a compressor in short order on a hot day. Coincidentally the compressor is the most costly part to replace whether in or out of warranty.