Rooftop HVAC equipment:

This article describes rooftop mounted HVAC systems - rooftop air conditioners, rooftop heat pumps, and rooftop heating equipment.

We describe types of roof-mounted HVAC equipment and common inspection points, operating problems, and repair approaches. At page top is a roof mounted swamp cooler or evaporative cooling system on a private home in Tucson.

Roof-mounted Air Conditioners, Heat Pumps, Heating Systems

Commercial rooftop-mounted combined air conditioning or heat pump units

Rooftop mounted central air conditioning systems may include both the cooling unit (evaporator coil, blower fan, filters) and the compressor/condenser unit in one package. At above right is a complex of roof-mounted AC/heat pump systems on a Christchurch, New Zealand high rise building roof.

These units may be a bit crowded as well as blown-on by the building's ventilation system openings at the upper right of the image.

At above left is a flat-roof mounted commercial cooling system located in New York.

Rooftop mounted central cooling systems may be smaller packaged systems which blow their cool air down directly into the cooled space through an opening in the roof, drawing return air from a nearby location, or the rooftop cooling system may be connected to duct work which in turn blows down into multiple building areas to deliver cool air, drawing return air from one or more centralized returns.

In many commercial installations, the entire area over a suspended ceiling may serve as one giant return plenum through which pass the supply ducts, delivering air to individual supply registers.

While the list above describes the common components of a typical residential air conditioning system, other configurations and packaged units are also in increased use in both residential and commercial installations.

Alternative HVAC designs may combine all components except for the duct work in a rooftop mounted unit such as the one shown above where it was mounted on a flat roof over offices at a commercial building.

Split-System A/C Rooftop Mounted Air Conditioners & Heat Pumps

The outdoor compressor/condenser unit of split-system air conditioners and heat pumps is also commonly mounted on rooftops, especially flat or low-slope roofs such as the unit shown at left.

Installation requirements for rooftop-mounted split system compressors / condenser units include both a service disconnect (circled in our photo) and often additional measures to assure that the unit is properly padded to avoid damage to the roof membrane, and to assure that the unit is properly levelled.

IN our OPINION, where rooftop mounted split system condenser units are exposed to high temperatures system operating efficiency and costs may be affeted.

Details are at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS

Rooftop HVAC Duct System Leaks & Water, Mold, and Pathogens

These photographs show what happens when a rooftop packaged-terminal air-conditioning air handler

(PTAC) and horizontal runs of air-ducts are installed on a roof surface and when that air duct system is improperly sealed and also is lined with fiberglass insulation.

Water ponded on the old, concave sections of air conditioning ducts on the roof of this commercial office space.

As water leaked into the duct system it saturated fiberglass duct liner which in turn, had its normal coating of organic dust and debris from the building, risking an indoor mold or pathogen problem for some of the occupants.

The cure for these duct leaks was costly: it was determined that it was less costly to completely replace the rooftop ducts with new metal ducts using outside insulation than it would have been to remove the contaminated fiberglass liner, clean the existing ducts, repair the leaks, and insulate their exterior.

The work was combined with other building HVAC cleaning and repairs.

Support Requirements for Rooftop-Mounted HVAC Equipment

Common inspection points & functional issues around rootop-mounted HVAC equipment include:

• adequacy of structural support - weight distribution
• proper mounting and flashing at support base to avoid roof leaks
• provision for working space for maintenance and repair
• leaks into the HVAC equipment or duct system (described above)

Shown above, support system for rooftop air conditioners / heat pumps mounted on a metal roof in Christchurch, New Zealand.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2019-04-08 - by (mod) -

Kanayo

We apologize if our articles have not been clear on this point, but InspectApedia.com

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You are welcome to read articles, print them if they are helpful, and to ask questions about what you have read.

On 2019-04-08 by Kanayo Frederick

Am using this opportunity to inform you I want to enhance my knowledge,by more requirements on to repair Central A/c,in your organization, please detail about the logistics of coming to your country.i will be delighted if my request is granted

On 2018-07-25 - by (mod) -

Alex,

Thank you for the interesting and helpful suggestion and detail.

On 2018-07-25 by Alex Jameson

Advantages:
equipped with reliable spiral or reciprocating compressors
represent a single monoblock construction

The intake and distribution of air into the room can be made both in the horizontal and in the vertical direction
the direction of the airflow can be changed by reinstalling the air conditioning fan
there is the possibility of adjusting the airflow and static pressure of the fan, which provides a certain freedom when adjusting the air conditioning system to specific requirements

The functioning is controlled by a wired microprocessor control system

On 2018-01-22 by Kevin

I have a 20 year old York central heat and air. I turn on the heat and raise the temp on the thermostat and it clicks but it doesn't blow out any air. So I checked to see if the air is working and I tried the fan and the AC and both of them work. But the heat doesn't but you can hear it turning on. Do you think theres a sensor out. I am going to buy a new thermostat thinking theres a sensor out in it.

On 2017-02-18 by Jim

we have a roof-top heat and a/c system. What would be the reason the heat system runs perfectly EXCEPT when it's windy and/or rainy?

On 2016-10-07 by frank

I have a 300 square foot lab with dedicated 3 ton a/c mounted on the roof.

Air supply is further filtered via ceiling mounted HEPA filters, resulting in a positive pressure room. Very modest additional return air is via ambient outdoor ambient environment. Room is kept super cold (63 F).

We cannot get the humidity down to a comfortable level (75%) when the a/c has cycled off. Research is telling me that my a/c unit is too oversized. Any suggestions on how to remedy?

In an effort to support these high positive pressures we need, we upgraded to this larger unit only to find that original source of unacceptable lower pressures was due to clogged pre-filters, NOT inadequate "blower" power from the a/c unit!

On 2015-11-07 - by (mod) -

If it's a heat pump and it's very cold outside you may be calling for backup heat that's not working.

On 2015-11-07 by Mike

My thermostat light turns on but the heater doesn't kick on.

On 2015-06-01 by Sajib

may a 400 TR incapacitated outdoor unit be risky for vibration on 15th floor rooftop?

Question:

(May 18, 2014) :yn said:

Is it wise and/or possible to put the compressor for a multi split unit in the attic of a home?

Reply:

YN

OPINION: I can't say if it's wise or not. It's more stunning. Normally the compressor/condenser is placed outside where it exchanges heat with outdoor air. IN an attic putting the compressor condenser unit in such a location would be very unusual and probably not work.

Question:

(July 28, 2014) DE said:

Is it possible to move a roof mounted HVAC unit from the roof to a platform adjacent to the house? The roof has been leaking where the unit has been mounted & we'd like to know if we can move the unit to prevent further damage.

Reply:

DE

Yes if you are talking about a refrigerant based system, but depending on the type of system you are moving the cost could be considerable. For example if the rooftop unit includes the blower assembly.

Rooftop swamp coolers are usually designed to work just in that location.

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Continue reading at SPLIT SYSTEM Ductless Air Conditioners or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SPLIT SYSTEM Ductless Air Conditioners
• SWAMP COOLERS

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Citations & References

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