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"Clearances Around
HVAC Units for
Proper Operation & Service"
The Comfort Zone
January / February 2001
by Maury Tiernan, Geary Pacific Corp. |
All mechanical refrigeration systems, whether they are used in water coolers,
refrigerators, soda machines, walk in cooler units, or for air comfort cooling,
have unique clearance requirements for proper operation and service.
Understanding clearance requirements allows us to help catch and correct
changeable site clearance issues in our mobile modular industry before they
impact our customers.
We are probably all familiar with the refrigerators in our homes. We know to
leave 3-4" of space behind the appliance. Providing that space between the back
of the refrigerator and the wall is about all we need to do to provide adequate
clearance to dissipate the heat expelled from the refrigerator. Rolling the
unit away from the wall and removing the panels provides the service clearance
space.
The water cooler (drinking fountain) clearance for venting and service is
usually taken care of by ADA requirements. The slots in the side of the water
cooler allow the heat to escape into the room. Service is accomplished in these
systems by removing the panel on the front.
The heat that is rejected into the room from any of the above mentioned systems
is then absorbed by the comfort cooling system and rejected outside.
Comfort cooling systems usually have a capacity 4-10 times greater than that of
a water cooler or refrigerator. With 4-10 times the heat to dissipate, they are
generally an outside product.
The unit type, (whether a roof mount, split system, or wall mount), the brand
design and engineering all determine how the condenser air is discharged. For
example, roof mount and split system units can discharge the condenser air
either vertically or horizontally. It just depends on the brand, and how the
unit was designed. Each unit will therefore have different operational
clearance requirements. For service clearance, panels can be found on all four
sides to access the units, again depending on the design.
Larger units, like those used to heat and cool our mobile/modular buildings,
have very critical clearance requirements. Let's take a look at just how
critical the clearance can be. For example, if the outdoor unit on a split
system is up-discharge, and there is an overhang above it, the hot air can blow
up, hit the overhang, and be recirculated back into the unit. This causes the
head pressure to increase, and the capacity to be reduced.
If this same split system outdoor unit is installed too close to the wall, it
may not allow a service man enough room to remove panels/parts, or may prevent
adequate ambient air circulation to be drawn through the coil.
The same problems can happen with a roof mount if equipment screens are
required around the unit for aesthetics. If the screen is too close to the
unit, it may obstruct the air being drawn into the coil, or prevent the removal
of panels/parts.
This all seems very logical, and we don't really hear of too many problems on
roof mount or split system units, but the same cannot be said for wall mounts
systems.
Our modular building industry is very mobile. And in many cases, site plans are
not required when the blueprints are drawn. Local jurisdictions usually review
and permit the building without responsibility for the HVAC operation
(clearance). Engineers or draftsman of the buildings frequently draw the plans
before anyone knows where the building will be set.
These scenarios lead to mobile/modular buildings with wall mount HVAC systems
to be set too close to another building, a wall, or even another mobile/modular
building's opposing wall mount. The hot condenser air becomes trapped around
the unit, recycling it into the condenser coil and "fresh" air intake. As the
unit runs, it continues to increase the temperature of the air across the
outdoor coil, decreasing efficiency and increasing operational costs.
This can also happen when units are installed near the inside offset corner of
two floors of a building. Without proper clearance to the side of the wall
mount units, airflow (to the condenser) may be blocked by the offset module,
causing the wall mount to trip on high head pressure.
Mansards and overhangs that "enclose" the top of wall mount units can restrict
service access. The top half of the wall mount houses the filter, blower motor
and heat strips, which all require access for proper servicing.
Wall mounts usually require from 2000-2600 cfm of air to properly dissipate the
heat from the condenser coil. Remember HVAC units are heat transfer machines.
One coil absorbs heat while the other coil rejects heat. The air crossing the
coils facilitates this. The temperature of this air across the outdoor coil
could be from 130-160 deg F, depending on the outdoor temperature and other
load factors. Without proper clearance the HVAC unit will experience high head
pressure, reduced capacity, reduced compressor life expectancy, and customer
complaints of "no cooling."
We can see the many ways that site restrictions affect clearance and why we
need to keep an eye on site changes. But just how much clearance is required
for wall mount HVAC units?
Here are some rules that may help you determine the clearance required for a
wall mount type HVAC unit. For one unit discharging against a wall or another
building: 15' from the coil to the wall. For one unit discharging against
another opposing unit: 20' from the coil of one unit to the coil of the other
unit.
There are many site-specific factors that impact the above distances:
prevailing winds, the height of the obstruction, chain link fences with slates,
overhangs, lattice boards, etc. All of these factors affect the proper
operation of the wall mount.
Knowledge of the HVAC system operation, and good old common sense will help in
deciding how to resolve the site circumstances that confine the condenser air.
Review your HVAC unit specifications sheets and installation instructions, or
contact your mechanical engineer, HVAC contractor, or mechanical supplier
whenever in doubt about the proper service or air discharge clearance.
California had 80 deg F for the Rose Bowl, while my Mom watched from her chair
in Minnesota at -10 deg F (after getting 8" of snow.) Somehow it just doesn't
seem fair.
Take care of yourselves, until the next time we meet in . . . The Comfort Zone.
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